25. Rasterization

Rasterization is the process by which a primitive is converted to a two-dimensional image. Each point of this image contains associated data such as depth, color, or other attributes.

Rasterizing a primitive begins by determining which squares of an integer grid in framebuffer coordinates are occupied by the primitive, and assigning one or more depth values to each such square. This process is described below for points, lines, and polygons.

A grid square, including its (x,y) framebuffer coordinates, z (depth), and associated data added by fragment shaders, is called a fragment. A fragment is located by its upper left corner, which lies on integer grid coordinates.

Rasterization operations also refer to a fragment’s sample locations, which are offset by subpixel fractional values from its upper left corner. The rasterization rules for points, lines, and triangles involve testing whether each sample location is inside the primitive. Fragments need not actually be square, and rasterization rules are not affected by the aspect ratio of fragments. Display of non-square grids, however, will cause rasterized points and line segments to appear fatter in one direction than the other.

We assume that fragments are square, since it simplifies antialiasing and texturing. After rasterization, fragments are processed by the early per-fragment tests, if enabled.

Several factors affect rasterization, including the members of VkPipelineRasterizationStateCreateInfo and VkPipelineMultisampleStateCreateInfo.

The VkPipelineRasterizationStateCreateInfo structure is defined as:

typedef struct VkPipelineRasterizationStateCreateInfo {
    VkStructureType                            sType;
    const void*                                pNext;
    VkPipelineRasterizationStateCreateFlags    flags;
    VkBool32                                   depthClampEnable;
    VkBool32                                   rasterizerDiscardEnable;
    VkPolygonMode                              polygonMode;
    VkCullModeFlags                            cullMode;
    VkFrontFace                                frontFace;
    VkBool32                                   depthBiasEnable;
    float                                      depthBiasConstantFactor;
    float                                      depthBiasClamp;
    float                                      depthBiasSlopeFactor;
    float                                      lineWidth;
} VkPipelineRasterizationStateCreateInfo;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • flags is reserved for future use.

  • depthClampEnable controls whether to clamp the fragment’s depth values instead of clipping primitives to the z planes of the frustum, as described in Primitive Clipping.

  • rasterizerDiscardEnable controls whether primitives are discarded immediately before the rasterization stage.

  • polygonMode is the triangle rendering mode. See VkPolygonMode.

  • cullMode is the triangle facing direction used for primitive culling. See VkCullModeFlagBits.

  • frontFace is a VkFrontFace value specifying the front-facing triangle orientation to be used for culling.

  • depthBiasEnable controls whether to bias fragment depth values.

  • depthBiasConstantFactor is a scalar factor controlling the constant depth value added to each fragment.

  • depthBiasClamp is the maximum (or minimum) depth bias of a fragment.

  • depthBiasSlopeFactor is a scalar factor applied to a fragment’s slope in depth bias calculations.

  • lineWidth is the width of rasterized line segments.

The application can also add a VkPipelineRasterizationStateRasterizationOrderAMD structure to the pNext chain of a VkPipelineRasterizationStateCreateInfo structure. This structure enables selecting the rasterization order to use when rendering with the corresponding graphics pipeline as described in Rasterization Order.

Valid Usage
  • If the depth clamping feature is not enabled, depthClampEnable must be VK_FALSE

  • If the non-solid fill modes feature is not enabled, polygonMode must be VK_POLYGON_MODE_FILL or VK_POLYGON_MODE_FILL_RECTANGLE_NV

  • If the VK_NV_fill_rectangle extension is not enabled, polygonMode must not be VK_POLYGON_MODE_FILL_RECTANGLE_NV

Valid Usage (Implicit)
typedef VkFlags VkPipelineRasterizationStateCreateFlags;

VkPipelineRasterizationStateCreateFlags is a bitmask type for setting a mask, but is currently reserved for future use.

The VkPipelineMultisampleStateCreateInfo structure is defined as:

typedef struct VkPipelineMultisampleStateCreateInfo {
    VkStructureType                          sType;
    const void*                              pNext;
    VkPipelineMultisampleStateCreateFlags    flags;
    VkSampleCountFlagBits                    rasterizationSamples;
    VkBool32                                 sampleShadingEnable;
    float                                    minSampleShading;
    const VkSampleMask*                      pSampleMask;
    VkBool32                                 alphaToCoverageEnable;
    VkBool32                                 alphaToOneEnable;
} VkPipelineMultisampleStateCreateInfo;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • flags is reserved for future use.

  • rasterizationSamples is a VkSampleCountFlagBits specifying the number of samples per pixel used in rasterization.

  • sampleShadingEnable can be used to enable Sample Shading.

  • minSampleShading specifies a minimum fraction of sample shading if sampleShadingEnable is set to VK_TRUE.

  • pSampleMask is a bitmask of static coverage information that is ANDed with the coverage information generated during rasterization, as described in Sample Mask.

  • alphaToCoverageEnable controls whether a temporary coverage value is generated based on the alpha component of the fragment’s first color output as specified in the Multisample Coverage section.

  • alphaToOneEnable controls whether the alpha component of the fragment’s first color output is replaced with one as described in Multisample Coverage.

Valid Usage
  • If the sample rate shading feature is not enabled, sampleShadingEnable must be VK_FALSE

  • If the alpha to one feature is not enabled, alphaToOneEnable must be VK_FALSE

  • minSampleShading must be in the range [0,1]

  • If the subpass has any color attachments and rasterizationSamples is greater than the number of color samples, then sampleShadingEnable must be VK_FALSE

Valid Usage (Implicit)
typedef VkFlags VkPipelineMultisampleStateCreateFlags;

VkPipelineMultisampleStateCreateFlags is a bitmask type for setting a mask, but is currently reserved for future use.

Rasterization only produces fragments corresponding to pixels in the framebuffer. Fragments which would be produced by application of any of the primitive rasterization rules described below but which lie outside the framebuffer are not produced, nor are they processed by any later stage of the pipeline, including any of the early per-fragment tests described in Early Per-Fragment Tests.

Surviving fragments are processed by fragment shaders. Fragment shaders determine associated data for fragments, and can also modify or replace their assigned depth values.

If the subpass for which this pipeline is being created uses color and/or depth/stencil attachments, then rasterizationSamples must be the same as the maximum of the sample counts of those subpass attachments.

If the subpass for which this pipeline is being created does not use color or depth/stencil attachments, rasterizationSamples must follow the rules for a zero-attachment subpass.

25.1. Discarding Primitives Before Rasterization

Primitives are discarded before rasterization if the rasterizerDiscardEnable member of VkPipelineRasterizationStateCreateInfo is enabled. When enabled, primitives are discarded after they are processed by the last active shader stage in the pipeline before rasterization.

25.2. Rasterization Order

Within a subpass of a render pass instance, for a given (x,y,layer,sample) sample location, the following operations are guaranteed to execute in rasterization order, for each separate primitive that includes that sample location:

Each of these operations is atomically executed for each primitive and sample location.

Execution of these operations for each primitive in a subpass occurs in an order determined by the application.

The rasterization order to use for a graphics pipeline is specified by adding a VkPipelineRasterizationStateRasterizationOrderAMD structure to the pNext chain of a VkPipelineRasterizationStateCreateInfo structure.

The VkPipelineRasterizationStateRasterizationOrderAMD structure is defined as:

typedef struct VkPipelineRasterizationStateRasterizationOrderAMD {
    VkStructureType            sType;
    const void*                pNext;
    VkRasterizationOrderAMD    rasterizationOrder;
} VkPipelineRasterizationStateRasterizationOrderAMD;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • rasterizationOrder is a VkRasterizationOrderAMD value specifying the primitive rasterization order to use.

Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_PIPELINE_RASTERIZATION_STATE_RASTERIZATION_ORDER_AMD

  • rasterizationOrder must be a valid VkRasterizationOrderAMD value

If the VK_AMD_rasterization_order device extension is not enabled or the application does not request a particular rasterization order through specifying a VkPipelineRasterizationStateRasterizationOrderAMD structure then the rasterization order used by the graphics pipeline defaults to VK_RASTERIZATION_ORDER_STRICT_AMD.

Possible values of VkPipelineRasterizationStateRasterizationOrderAMD::rasterizationOrder, specifying the primitive rasterization order, are:

typedef enum VkRasterizationOrderAMD {
    VK_RASTERIZATION_ORDER_STRICT_AMD = 0,
    VK_RASTERIZATION_ORDER_RELAXED_AMD = 1,
} VkRasterizationOrderAMD;
  • VK_RASTERIZATION_ORDER_STRICT_AMD specifies that operations for each primitive in a subpass must occur in primitive order.

  • VK_RASTERIZATION_ORDER_RELAXED_AMD specifies that operations for each primitive in a subpass may not occur in primitive order.

25.3. Multisampling

Multisampling is a mechanism to antialias all Vulkan primitives: points, lines, and polygons. The technique is to sample all primitives multiple times at each pixel. Each sample in each framebuffer attachment has storage for a color, depth, and/or stencil value, such that per-fragment operations apply to each sample independently. The color sample values can be later resolved to a single color (see Resolving Multisample Images and the Render Pass chapter for more details on how to resolve multisample images to non-multisample images).

Vulkan defines rasterization rules for single-sample modes in a way that is equivalent to a multisample mode with a single sample in the center of each pixel.

Each fragment includes a coverage value with rasterizationSamples bits (see Sample Mask). Each fragment includes rasterizationSamples depth values and sets of associated data. An implementation may choose to assign the same associated data to more than one sample. The location for evaluating such associated data may be anywhere within the pixel including the pixel center or any of the sample locations. When rasterizationSamples is VK_SAMPLE_COUNT_1_BIT, the pixel center must be used. The different associated data values need not all be evaluated at the same location. Each pixel fragment thus consists of integer x and y grid coordinates, rasterizationSamples depth values and sets of associated data, and a coverage value with rasterizationSamples bits.

It is understood that each pixel has rasterizationSamples locations associated with it. These locations are exact positions, rather than regions or areas, and each is referred to as a sample point. The sample points associated with a pixel must be located inside or on the boundary of the unit square that is considered to bound the pixel. Furthermore, the relative locations of sample points may be identical for each pixel in the framebuffer, or they may differ. If the current pipeline includes a fragment shader with one or more variables in its interface decorated with Sample and Input, the data associated with those variables will be assigned independently for each sample. The values for each sample must be evaluated at the location of the sample. The data associated with any other variables not decorated with Sample and Input need not be evaluated independently for each sample.

If the standardSampleLocations member of VkPhysicalDeviceLimits is VK_TRUE, then the sample counts VK_SAMPLE_COUNT_1_BIT, VK_SAMPLE_COUNT_2_BIT, VK_SAMPLE_COUNT_4_BIT, VK_SAMPLE_COUNT_8_BIT, and VK_SAMPLE_COUNT_16_BIT have sample locations as listed in the following table, with the ith entry in the table corresponding to bit i in the sample masks. VK_SAMPLE_COUNT_32_BIT and VK_SAMPLE_COUNT_64_BIT do not have standard sample locations. Locations are defined relative to an origin in the upper left corner of the pixel.

Table 34. Standard sample locations

VK_SAMPLE_COUNT_1_BIT

VK_SAMPLE_COUNT_2_BIT

VK_SAMPLE_COUNT_4_BIT

VK_SAMPLE_COUNT_8_BIT

VK_SAMPLE_COUNT_16_BIT

(0.5,0.5)

(0.75,0.75)
(0.25,0.25)

(0.375, 0.125)
(0.875, 0.375)
(0.125, 0.625)
(0.625, 0.875)

(0.5625, 0.3125)
(0.4375, 0.6875)
(0.8125, 0.5625)
(0.3125, 0.1875)
(0.1875, 0.8125)
(0.0625, 0.4375)
(0.6875, 0.9375)
(0.9375, 0.0625)

(0.5625, 0.5625)
(0.4375, 0.3125)
(0.3125, 0.625)
(0.75, 0.4375)
(0.1875, 0.375)
(0.625, 0.8125)
(0.8125, 0.6875)
(0.6875, 0.1875)
(0.375, 0.875)
(0.5, 0.0625)
(0.25, 0.125)
(0.125, 0.75)
(0.0, 0.5)
(0.9375, 0.25)
(0.875, 0.9375)
(0.0625, 0.0)

sample count 1

sample count 2

sample count 4

sample count 8

sample count 16

Color images created with multiple samples per pixel use a compression technique where there are two arrays of data associated with each pixel. The first array contains one element per sample where each element stores an index to the second array defining the fragment mask of the pixel. The second array contains one element per color fragment and each element stores a unique color value in the format of the image. With this compression technique it’s not always necessary to actually use unique storage locations for each color sample: when multiple samples share the same color value the fragment mask may have two samples referring to the same color fragment. The number of color fragments is determined by the samples member of the VkImageCreateInfo structure used to create the image. The VK_AMD_shader_fragment_mask device extension provides shader instructions enabling the application to get direct access to the fragment mask and the individual color fragment values.

image/svg+xml ColorSamples 0 0 1 1 1 2 2 3 ColorFragments 0 1 2 3 Unused Unused 0 1 2 3
Figure 26. Fragment Mask

25.4. Custom Sample Locations

Applications can also control the sample locations used for rasterization.

If the pNext chain of the VkPipelineMultisampleStateCreateInfo structure specified at pipeline creation time includes an instance of the VkPipelineSampleLocationsStateCreateInfoEXT structure, then that structure controls the sample locations used when rasterizing primitives with the pipeline.

The VkPipelineSampleLocationsStateCreateInfoEXT structure is defined as:

typedef struct VkPipelineSampleLocationsStateCreateInfoEXT {
    VkStructureType             sType;
    const void*                 pNext;
    VkBool32                    sampleLocationsEnable;
    VkSampleLocationsInfoEXT    sampleLocationsInfo;
} VkPipelineSampleLocationsStateCreateInfoEXT;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • sampleLocationsEnable controls whether custom sample locations are used. If sampleLocationsEnable is VK_FALSE, the default sample locations are used and the values specified in sampleLocationsInfo are ignored.

  • sampleLocationsInfo is the sample locations to use during rasterization if sampleLocationsEnable is VK_TRUE and the graphics pipeline is not created with VK_DYNAMIC_STATE_SAMPLE_LOCATIONS_EXT.

Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_PIPELINE_SAMPLE_LOCATIONS_STATE_CREATE_INFO_EXT

  • sampleLocationsInfo must be a valid VkSampleLocationsInfoEXT structure

The VkSampleLocationsInfoEXT structure is defined as:

typedef struct VkSampleLocationsInfoEXT {
    VkStructureType               sType;
    const void*                   pNext;
    VkSampleCountFlagBits         sampleLocationsPerPixel;
    VkExtent2D                    sampleLocationGridSize;
    uint32_t                      sampleLocationsCount;
    const VkSampleLocationEXT*    pSampleLocations;
} VkSampleLocationsInfoEXT;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • sampleLocationsPerPixel is a VkSampleCountFlagBits specifying the number of sample locations per pixel.

  • sampleLocationGridSize is the size of the sample location grid to select custom sample locations for.

  • sampleLocationsCount is the number of sample locations in pSampleLocations.

  • pSampleLocations is an array of sampleLocationsCount VkSampleLocationEXT structures.

This structure can be used either to specify the sample locations to be used for rendering or to specify the set of sample locations an image subresource has been last rendered with for the purposes of layout transitions of depth/stencil images created with VK_IMAGE_CREATE_SAMPLE_LOCATIONS_COMPATIBLE_DEPTH_BIT_EXT.

The sample locations in pSampleLocations specify sampleLocationsPerPixel number of sample locations for each pixel in the grid of the size specified in sampleLocationGridSize. The sample location for sample i at the pixel grid location (x,y) is taken from pSampleLocations[(x + y * sampleLocationGridSize.width) * sampleLocationsPerPixel + i].

Valid Usage
  • sampleLocationsPerPixel must be a bit value that is set in VkPhysicalDeviceSampleLocationsPropertiesEXT::sampleLocationSampleCounts

  • sampleLocationsCount must equal sampleLocationsPerPixel × sampleLocationGridSize.width × sampleLocationGridSize.height

Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_SAMPLE_LOCATIONS_INFO_EXT

  • sampleLocationsPerPixel must be a valid VkSampleCountFlagBits value

  • pSampleLocations must be a valid pointer to an array of sampleLocationsCount VkSampleLocationEXT structures

  • sampleLocationsCount must be greater than 0

The VkSampleLocationEXT structure is defined as:

typedef struct VkSampleLocationEXT {
    float    x;
    float    y;
} VkSampleLocationEXT;
  • x is the horizontal coordinate of the sample’s location.

  • y is the vertical coordinate of the sample’s location.

The domain space of the sample location coordinates has an upper-left origin within the pixel in framebuffer space.

The values specified in a VkSampleLocationEXT structure are always clamped to the implementation-dependent sample location coordinate range [sampleLocationCoordinateRange[0],sampleLocationCoordinateRange[1]] that can be queried by chaining the VkPhysicalDeviceSampleLocationsPropertiesEXT structure to the pNext chain of VkPhysicalDeviceProperties2.

The custom sample locations used for rasterization when VkPipelineSampleLocationsStateCreateInfoEXT::sampleLocationsEnable is VK_TRUE are specified by the VkPipelineSampleLocationsStateCreateInfoEXT::sampleLocationsInfo property of the bound graphics pipeline, if the pipeline was not created with VK_DYNAMIC_STATE_SAMPLE_LOCATIONS_EXT enabled.

Otherwise, the sample locations used for rasterization are set by calling vkCmdSetSampleLocationsEXT:

void vkCmdSetSampleLocationsEXT(
    VkCommandBuffer                             commandBuffer,
    const VkSampleLocationsInfoEXT*             pSampleLocationsInfo);
  • commandBuffer is the command buffer into which the command will be recorded.

  • pSampleLocationsInfo is the sample locations state to set.

Valid Usage
  • The bound graphics pipeline must have been created with the VK_DYNAMIC_STATE_SAMPLE_LOCATIONS_EXT dynamic state enabled

  • The sampleLocationsPerPixel member of pSampleLocationsInfo must equal the rasterizationSamples member of the VkPipelineMultisampleStateCreateInfo structure the bound graphics pipeline has been created with

  • If VkPhysicalDeviceSampleLocationsPropertiesEXT::variableSampleLocations is VK_FALSE then the current render pass must have been begun by specifying a VkRenderPassSampleLocationsBeginInfoEXT structure whose pPostSubpassSampleLocations member contains an element with a subpassIndex matching the current subpass index and the sampleLocationsInfo member of that element must match the sample locations state pointed to by pSampleLocationsInfo

Valid Usage (Implicit)
  • commandBuffer must be a valid VkCommandBuffer handle

  • pSampleLocationsInfo must be a valid pointer to a valid VkSampleLocationsInfoEXT structure

  • commandBuffer must be in the recording state

  • The VkCommandPool that commandBuffer was allocated from must support graphics operations

Host Synchronization
  • Host access to commandBuffer must be externally synchronized

  • Host access to the VkCommandPool that commandBuffer was allocated from must be externally synchronized

Command Properties
Command Buffer Levels Render Pass Scope Supported Queue Types Pipeline Type

Primary
Secondary

Both

Graphics

25.5. Shading Rate Image

The shading rate image feature allows pipelines to use a shading rate image to control the fragment area and the minimum number of fragment shader invocations launched for each fragment. When the shading rate image is enabled, the rasterizer determines a base shading rate for each region of the framebuffer covered by a primitive by fetching a value from the shading rate image and translating it to a shading rate using a per-viewport shading rate palette. This base shading rate is then adjusted to derive a final shading rate, which specifies the fragment area and fragment shader invocation count to use for fragments generated in the region.

If the pNext chain of VkPipelineViewportStateCreateInfo includes a VkPipelineViewportShadingRateImageStateCreateInfoNV structure, then that structure includes parameters that control the shading rate.

The VkPipelineViewportShadingRateImageStateCreateInfoNV structure is defined as:

typedef struct VkPipelineViewportShadingRateImageStateCreateInfoNV {
    VkStructureType                  sType;
    const void*                      pNext;
    VkBool32                         shadingRateImageEnable;
    uint32_t                         viewportCount;
    const VkShadingRatePaletteNV*    pShadingRatePalettes;
} VkPipelineViewportShadingRateImageStateCreateInfoNV;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • shadingRateImageEnable specifies whether shading rate image and palettes are used during rasterization.

  • viewportCount specifies the number of per-viewport palettes used to translate values stored in shading rate images.

  • pShadingRatePalettes is a pointer to an array of VkShadingRatePaletteNV structures defining the palette for each viewport. If the shading rate palette state is dynamic, this member is ignored.

If this structure is not present, shadingRateImageEnable is considered to be VK_FALSE, and the shading rate image and palettes are not used.

Valid Usage
  • If the multiple viewports feature is not enabled, viewportCount must be 0 or 1

  • viewportCount must be less than or equal to VkPhysicalDeviceLimits::maxViewports

  • If shadingRateImageEnable is VK_TRUE, viewportCount must be equal to the viewportCount member of VkPipelineViewportStateCreateInfo

  • If no element of the pDynamicStates member of pDynamicState is VK_DYNAMIC_STATE_VIEWPORT_SHADING_RATE_PALETTE_NV, pShadingRatePalettes must be a valid pointer to an array of viewportCount VkShadingRatePaletteNV structures

Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_SHADING_RATE_IMAGE_STATE_CREATE_INFO_NV

  • If viewportCount is not 0, and pShadingRatePalettes is not NULL, pShadingRatePalettes must be a valid pointer to an array of viewportCount valid VkShadingRatePaletteNV structures

When shading rate image usage is enabled in the bound pipeline, the pipeline uses a shading rate image specified by the command:

void vkCmdBindShadingRateImageNV(
    VkCommandBuffer                             commandBuffer,
    VkImageView                                 imageView,
    VkImageLayout                               imageLayout);
  • commandBuffer is the command buffer into which the command will be recorded.

  • imageView is an image view handle that specifies the shading rate image. imageView may be set to VK_NULL_HANDLE, which is equivalent to specifying a view of an image filled with zero values.

  • imageLayout is the layout that the image subresources accessible from imageView will be in when the shading rate image is accessed.

Valid Usage
  • The shading rate image feature must be enabled.

  • If imageView is not VK_NULL_HANDLE, it must be a valid VkImageView handle of type VK_IMAGE_VIEW_TYPE_2D or VK_IMAGE_VIEW_TYPE_2D_ARRAY.

  • If imageView is not VK_NULL_HANDLE, it must have a format of VK_FORMAT_R8_UINT.

  • If imageView is not VK_NULL_HANDLE, the image must have been created with VK_IMAGE_USAGE_SHADING_RATE_IMAGE_BIT_NV set

  • If imageView is not VK_NULL_HANDLE, imageLayout must match the actual VkImageLayout of each subresource accessible from imageView at the time the subresource is accessed.

  • If imageView is not VK_NULL_HANDLE, imageLayout must be VK_IMAGE_LAYOUT_SHADING_RATE_OPTIMAL_NV or VK_IMAGE_LAYOUT_GENERAL.

Valid Usage (Implicit)
  • commandBuffer must be a valid VkCommandBuffer handle

  • imageView must be a valid VkImageView handle

  • imageLayout must be a valid VkImageLayout value

  • commandBuffer must be in the recording state

  • The VkCommandPool that commandBuffer was allocated from must support graphics operations

  • Both of commandBuffer, and imageView must have been created, allocated, or retrieved from the same VkDevice

Host Synchronization
  • Host access to commandBuffer must be externally synchronized

  • Host access to the VkCommandPool that commandBuffer was allocated from must be externally synchronized

Command Properties
Command Buffer Levels Render Pass Scope Supported Queue Types Pipeline Type

Primary
Secondary

Both

Graphics

When the shading rate image is enabled in the current pipeline, rasterizing a primitive covering the pixel with coordinates (x,y) will fetch a shading rate index value from the shading rate image bound by vkCmdBindShadingRateImageNV. If the shading rate image view has a type of VK_IMAGE_VIEW_2D, the lookup will use texel coordinates (u,v) where \(u = \lfloor \frac{x}{twidth} \rfloor\), \(v = \lfloor \frac{y}{theight} \rfloor\), and \(twidth\) and \(theight\) are the width and height of the implementation-dependent shading rate texel size. If the shading rate image view has a type of VK_IMAGE_VIEW_2D_ARRAY, the lookup will use texel coordinates (u,v) to extract a texel from the layer l, where l is the layer of the framebuffer being rendered to. If l is greater than or equal to the number of layers in the image view, layer zero will be used.

If the bound shading rate image view is not VK_NULL_HANDLE and contains a texel with coordinates (u,v) in layer l (if applicable), the single unsigned integer component for that texel will be used as the shading rate index. If the (u,v) coordinate is outside the extents of the subresource used by the shading rate image view, or if the image view is VK_NULL_HANDLE, the shading rate index is zero. If the shading rate image view has multiple mipmap levels, the base level identified by VkImageSubresourceRange::baseMipLevel will be used.

A shading rate index is mapped to a base shading rate using a lookup table called the shading rate image palette. There is a separate palette for each viewport. The number of entries in each palette is given by the implementation-dependent shading rate image palette size.

If a pipeline state object is created with VK_DYNAMIC_STATE_VIEWPORT_SHADING_RATE_PALETTE_NV enabled, the per-viewport shading rate image palettes are set by the command:

void vkCmdSetViewportShadingRatePaletteNV(
    VkCommandBuffer                             commandBuffer,
    uint32_t                                    firstViewport,
    uint32_t                                    viewportCount,
    const VkShadingRatePaletteNV*               pShadingRatePalettes);
  • commandBuffer is the command buffer into which the command will be recorded.

  • firstViewport is the index of the first viewport whose shading rate palette is updated by the command.

  • viewportCount is the number of viewports whose shading rate palettes are updated by the command.

  • pShadingRatePalettes is a pointer to an array of VkShadingRatePaletteNV structures defining the palette for each viewport.

Valid Usage
  • The shading rate image feature must be enabled.

  • The bound graphics pipeline must have been created with the VK_DYNAMIC_STATE_VIEWPORT_SHADING_RATE_PALETTE_NV dynamic state enabled

  • firstViewport must be less than VkPhysicalDeviceLimits::maxViewports

  • The sum of firstViewport and viewportCount must be between 1 and VkPhysicalDeviceLimits::maxViewports, inclusive

  • If the multiple viewports feature is not enabled, firstViewport must be 0

  • If the multiple viewports feature is not enabled, viewportCount must be 1

Valid Usage (Implicit)
  • commandBuffer must be a valid VkCommandBuffer handle

  • pShadingRatePalettes must be a valid pointer to an array of viewportCount valid VkShadingRatePaletteNV structures

  • commandBuffer must be in the recording state

  • The VkCommandPool that commandBuffer was allocated from must support graphics operations

  • viewportCount must be greater than 0

Host Synchronization
  • Host access to commandBuffer must be externally synchronized

  • Host access to the VkCommandPool that commandBuffer was allocated from must be externally synchronized

Command Properties
Command Buffer Levels Render Pass Scope Supported Queue Types Pipeline Type

Primary
Secondary

Both

Graphics

The VkShadingRatePaletteNV structure specifies to contents of a single shading rate image palette and is defined as:

typedef struct VkShadingRatePaletteNV {
    uint32_t                              shadingRatePaletteEntryCount;
    const VkShadingRatePaletteEntryNV*    pShadingRatePaletteEntries;
} VkShadingRatePaletteNV;
  • shadingRatePaletteEntryCount specifies the number of entries in the shading rate image palette.

  • pShadingRatePaletteEntries is a pointer to an array of VkShadingRatePaletteEntryNV enums defining the shading rate for each palette entry.

Valid Usage
  • shadingRatePaletteEntryCount must be between 1 and VkPhysicalDeviceShadingRateImagePropertiesNV::shadingRatePaletteSize, inclusive

Valid Usage (Implicit)
  • pShadingRatePaletteEntries must be a valid pointer to an array of shadingRatePaletteEntryCount valid VkShadingRatePaletteEntryNV values

  • shadingRatePaletteEntryCount must be greater than 0

To determine the base shading rate image, a shading rate index i is mapped to array element i in the array pShadingRatePaletteEntries for the palette corresponding to the viewport used for the fragment. If i is greater than or equal to the palette size shadingRatePaletteEntryCount, the base shading rate is undefined.

The supported shading rate image palette entries are defined by VkShadingRatePaletteEntryNV:

typedef enum VkShadingRatePaletteEntryNV {
    VK_SHADING_RATE_PALETTE_ENTRY_NO_INVOCATIONS_NV = 0,
    VK_SHADING_RATE_PALETTE_ENTRY_16_INVOCATIONS_PER_PIXEL_NV = 1,
    VK_SHADING_RATE_PALETTE_ENTRY_8_INVOCATIONS_PER_PIXEL_NV = 2,
    VK_SHADING_RATE_PALETTE_ENTRY_4_INVOCATIONS_PER_PIXEL_NV = 3,
    VK_SHADING_RATE_PALETTE_ENTRY_2_INVOCATIONS_PER_PIXEL_NV = 4,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_PIXEL_NV = 5,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_2X1_PIXELS_NV = 6,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_1X2_PIXELS_NV = 7,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_2X2_PIXELS_NV = 8,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_4X2_PIXELS_NV = 9,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_2X4_PIXELS_NV = 10,
    VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_4X4_PIXELS_NV = 11,
} VkShadingRatePaletteEntryNV;

The following table indicates the width and height (in pixels) of each fragment generated using the indicated shading rate, as well as the maximum number of fragment shader invocations launched for each fragment. When processing regions of a primitive that have a shading rate of VK_SHADING_RATE_PALETTE_ENTRY_NO_PIXELS_NV, no fragments will be generated in that region.

Shading Rate Width Height Invocations

VK_SHADING_RATE_PALETTE_ENTRY_NO_INVOCATIONS_NV

0

0

0

VK_SHADING_RATE_PALETTE_ENTRY_16_INVOCATIONS_PER_PIXEL_NV

1

1

16

VK_SHADING_RATE_PALETTE_ENTRY_8_INVOCATIONS_PER_PIXEL_NV

1

1

8

VK_SHADING_RATE_PALETTE_ENTRY_4_INVOCATIONS_PER_PIXEL_NV

1

1

4

VK_SHADING_RATE_PALETTE_ENTRY_2_INVOCATIONS_PER_PIXEL_NV

1

1

2

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_PIXEL_NV

1

1

1

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_2X1_PIXELS_NV

2

1

1

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_1X2_PIXELS_NV

1

2

1

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_2X2_PIXELS_NV

2

2

1

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_4X2_PIXELS_NV

4

2

1

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_2X4_PIXELS_NV

2

4

1

VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_4X4_PIXELS_NV

4

4

1

When the shading rate image is disabled, a shading rate of VK_SHADING_RATE_PALETTE_ENTRY_1_INVOCATION_PER_PIXEL_NV will be used as the base shading rate.

Once a base shading rate has been established, it is adjusted to produce a final shading rate. First, if the base shading rate uses multiple pixels for each fragment, the implementation may reduce the fragment area to ensure that the total number of coverage samples for all pixels in a fragment does not exceed an implementation-dependent maximum.

If sample shading is active in the current pipeline and would result in processing n (n > 1) unique samples per fragment when the shading rate image is disabled, the shading rate is adjusted in an implementation-dependent manner to increase the number of fragment shader invocations spawned by the primitive. If the shading rate indicates fs pixels per fragment and fs is greater than n, the fragment area is adjusted so each fragment has approximately \(fs \over n\) pixels. Otherwise, if the shading rate indicates ipf invocations per fragment, the fragment area will be adjusted to a single pixel with approximately \(ipf \times n \over fs\) invocations per fragment.

If sample shading occurs due to the use of a fragment shader input variable decorated with SampleId or SamplePosition, the shading rate is ignored. Each fragment will have a single pixel and will spawn up to totalSamples fragment shader invocations, as when using sample shading without a shading rate image.

Finally, if the shading rate specifies multiple fragment shader invocations per fragment, the total number of invocations in the shading rate is clamped to be no larger than the value of totalSamples used for sample shading.

When the final shading rate for a primitive covering pixel (x,y) has a fragment area of \(fw \times fh\), the fragment for that pixel will cover all pixels with coordinates (x',y') that satisfy the equations:

\[\begin{aligned} \lfloor \frac{x}{fw} \rfloor == \lfloor \frac{x'}{fw} \rfloor \end{aligned}\]
\[\begin{aligned} \lfloor \frac{y}{fh} \rfloor == \lfloor \frac{y'}{fh} \rfloor \end{aligned}\]

This combined fragment is considered to have multiple coverage samples; the total number of samples in this fragment is given by \(samples = fw \times fh \times rs\) where rs indicates the value of VkPipelineMultisampleStateCreateInfo::rasterizationSamples specified at pipeline creation time. The set of coverage samples in the fragment is the union of the per-pixel coverage samples in each of the fragment’s pixels The location and order of coverage samples within each pixel in the combined fragment are assigned as described in Multisampling and Custom Sample Locations. Each coverage sample in the set of pixels belonging to the combined fragment is assigned a unique sample number in the range [0,samples-1]. If the shadingRateCoarseSampleOrder feature is supported, the order of coverage samples can be specified for each combination of fragment area and coverage sample count. If this feature is not supported, the sample order is implementation-dependent.

If the pNext chain of VkPipelineViewportStateCreateInfo includes a VkPipelineViewportCoarseSampleOrderStateCreateInfoNV structure, then that structure includes parameters that control the order of coverage samples in fragments larger than one pixel.

The VkPipelineViewportCoarseSampleOrderStateCreateInfoNV structure is defined as:

typedef struct VkPipelineViewportCoarseSampleOrderStateCreateInfoNV {
    VkStructureType                       sType;
    const void*                           pNext;
    VkCoarseSampleOrderTypeNV             sampleOrderType;
    uint32_t                              customSampleOrderCount;
    const VkCoarseSampleOrderCustomNV*    pCustomSampleOrders;
} VkPipelineViewportCoarseSampleOrderStateCreateInfoNV;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • sampleOrderType specifies the mechanism used to order coverage samples in fragments larger than one pixel.

  • customSampleOrderCount specifies the number of custom sample orderings to use when ordering coverage samples.

  • pCustomSampleOrders is a pointer to an array of VkCoarseSampleOrderCustomNV structures, each of which specifies the coverage sample order for a single combination of fragment area and coverage sample count.

If this structure is not present, sampleOrderType is considered to be VK_COARSE_SAMPLE_ORDER_TYPE_DEFAULT_NV.

If sampleOrderType is VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV, the coverage sample order used for any combination of fragment area and coverage sample count not enumerated in pCustomSampleOrders will be identical to that used for VK_COARSE_SAMPLE_ORDER_TYPE_DEFAULT_NV.

If the pipeline was created with VK_DYNAMIC_STATE_VIEWPORT_COARSE_SAMPLE_ORDER_NV, the contents of this structure (if present) are ignored, and the coverage sample order is instead specified by vkCmdSetCoarseSampleOrderNV.

Valid Usage
  • If sampleOrderType is not VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV, customSamplerOrderCount must be 0

  • The array pCustomSampleOrders must not contain two structures with matching values for both the shadingRate and sampleCount members.

Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_COARSE_SAMPLE_ORDER_STATE_CREATE_INFO_NV

  • sampleOrderType must be a valid VkCoarseSampleOrderTypeNV value

  • If customSampleOrderCount is not 0, pCustomSampleOrders must be a valid pointer to an array of customSampleOrderCount valid VkCoarseSampleOrderCustomNV structures

The type VkCoarseSampleOrderTypeNV specifies the technique used to order coverage samples in fragments larger than one pixel, and is defined as:

typedef enum VkCoarseSampleOrderTypeNV {
    VK_COARSE_SAMPLE_ORDER_TYPE_DEFAULT_NV = 0,
    VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV = 1,
    VK_COARSE_SAMPLE_ORDER_TYPE_PIXEL_MAJOR_NV = 2,
    VK_COARSE_SAMPLE_ORDER_TYPE_SAMPLE_MAJOR_NV = 3,
} VkCoarseSampleOrderTypeNV;
  • VK_COARSE_SAMPLE_ORDER_TYPE_DEFAULT_NV specifies that coverage samples will be ordered in an implementation-dependent manner.

  • VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV specifies that coverage samples will be ordered according to the array of custom orderings provided in either the pCustomSampleOrders member of VkPipelineViewportCoarseSampleOrderStateCreateInfoNV or the pCustomSampleOrders member of vkCmdSetCoarseSampleOrderNV.

  • VK_COARSE_SAMPLE_ORDER_TYPE_PIXEL_MAJOR_NV specifies that coverage samples will be ordered sequentially, sorted first by pixel coordinate (in row-major order) and then by coverage sample number.

  • VK_COARSE_SAMPLE_ORDER_TYPE_SAMPLE_MAJOR_NV specifies that coverage samples will be ordered sequentially, sorted first by coverage sample number and then by pixel coordinate (in row-major order).

When using a coarse sample order of VK_COARSE_SAMPLE_ORDER_TYPE_PIXEL_MAJOR_NV for a fragment with an upper-left corner of \((fx,fy)\) with a width of \(fw \times fh\) and \(fsc\) coverage samples per pixel, sample \(cs\) of the fragment will be assigned to sample \(fs\) of pixel \((px,py)\) will be assigned as follows:

\[\begin{aligned} px = & fx + (\lfloor {cs \over fsc} \rfloor \text{ \% } fw) \\ py = & fy + \lfloor {cs \over {fsc \times fw}} \rfloor \\ fs = & cs \text{ \% } fsc \end{aligned}\]

When using a coarse sample order of VK_COARSE_SAMPLE_ORDER_TYPE_SAMPLE_MAJOR_NV, sample \(cs\) will be assigned as follows:

\[\begin{aligned} px = & fx + cs \text{ \% } fw \\ py = & (fy + \lfloor {cs \over fw} \rfloor \text{ \% } fh) \\ fs = & \lfloor {cs \over {fw \times fh}} \rfloor \end{aligned}\]

The VkCoarseSampleOrderCustomNV structure is used with a coverage sample ordering type of VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV to specify the order of coverage samples for one combination of fragment width, fragment height, and coverage sample count. The structure is defined as:

typedef struct VkCoarseSampleOrderCustomNV {
    VkShadingRatePaletteEntryNV        shadingRate;
    uint32_t                           sampleCount;
    uint32_t                           sampleLocationCount;
    const VkCoarseSampleLocationNV*    pSampleLocations;
} VkCoarseSampleOrderCustomNV;
  • shadingRate is a shading rate palette entry that identifies the fragment width and height for the combination of fragment area and per-pixel coverage sample count to control.

  • sampleCount identifies the per-pixel coverage sample count for the combination of fragment area and coverage sample count to control.

  • sampleLocationCount specifies the number of sample locations in the custom ordering.

  • pSampleLocations is a pointer to an array of VkCoarseSampleOrderCustomNV structures that specifies the location of each sample in the custom ordering.

When using a custom sample ordering, element i in pSampleLocations specifies a specific pixel and per-pixel coverage sample number that corresponds to the coverage sample numbered i in the multi-pixel fragment.

Valid Usage
  • shadingRate must be a shading rate that generates fragments with more than one pixel.

  • sampleCount must correspond to a sample count enumerated in VkSampleCountFlags whose corresponding bit is set in VkPhysicalDeviceLimits::framebufferNoAttachmentsSampleCounts.

  • sampleLocationCount must be equal to the product of sampleCount, the fragment width for shadingRate, and the fragment height for shadingRate.

  • sampleLocationCount must be less than or equal to the value of VkPhysicalDeviceShadingRateImagePropertiesNV::shadingRateMaxCoarseSamples.

  • The array pSampleLocations must contain exactly one entry for every combination of valid values for pixelX, pixelY, and sample in the structure VkCoarseSampleOrderCustomNV.

Valid Usage (Implicit)
  • shadingRate must be a valid VkShadingRatePaletteEntryNV value

  • pSampleLocations must be a valid pointer to an array of sampleLocationCount VkCoarseSampleLocationNV structures

  • sampleLocationCount must be greater than 0

The VkCoarseSampleLocationNV structure identifies a specific pixel and sample number for one of the coverage samples in a fragment that is larger than one pixel. This structure is defined as:

typedef struct VkCoarseSampleLocationNV {
    uint32_t    pixelX;
    uint32_t    pixelY;
    uint32_t    sample;
} VkCoarseSampleLocationNV;
  • pixelX is added to the x coordinate of the upper-leftmost pixel of each fragment to identify the pixel containing the coverage sample.

  • pixelY is added to the y coordinate of the upper-leftmost pixel of each fragment to identify the pixel containing the coverage sample.

  • sample is the number of the coverage sample in the pixel identified by pixelX and pixelY.

Valid Usage
  • pixelX must be less than the width (in pixels) of the fragment.

  • pixelY must be less than the height (in pixels) of the fragment.

  • sample must be less than the number of coverage samples in each pixel belonging to the fragment.

If a pipeline state object is created with VK_DYNAMIC_STATE_VIEWPORT_COARSE_SAMPLE_ORDER_NV enabled, the order of coverage samples in fragments larger than one pixel is set by the command:

void vkCmdSetCoarseSampleOrderNV(
    VkCommandBuffer                             commandBuffer,
    VkCoarseSampleOrderTypeNV                   sampleOrderType,
    uint32_t                                    customSampleOrderCount,
    const VkCoarseSampleOrderCustomNV*          pCustomSampleOrders);
  • commandBuffer is the command buffer into which the command will be recorded.

  • sampleOrderType specifies the mechanism used to order coverage samples in fragments larger than one pixel.

  • customSampleOrderCount specifies the number of custom sample orderings to use when ordering coverage samples.

  • pCustomSampleOrders is a pointer to an array of VkCoarseSampleOrderCustomNV structures, each of which specifies the coverage sample order for a single combination of fragment area and coverage sample count.

If sampleOrderType is VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV, the coverage sample order used for any combination of fragment area and coverage sample count not enumerated in pCustomSampleOrders will be identical to that used for VK_COARSE_SAMPLE_ORDER_TYPE_DEFAULT_NV.

Valid Usage
  • If sampleOrderType is not VK_COARSE_SAMPLE_ORDER_TYPE_CUSTOM_NV, customSamplerOrderCount must be 0

  • The array pCustomSampleOrders must not contain two structures with matching values for both the shadingRate and sampleCount members.

Valid Usage (Implicit)
  • commandBuffer must be a valid VkCommandBuffer handle

  • sampleOrderType must be a valid VkCoarseSampleOrderTypeNV value

  • If customSampleOrderCount is not 0, pCustomSampleOrders must be a valid pointer to an array of customSampleOrderCount valid VkCoarseSampleOrderCustomNV structures

  • commandBuffer must be in the recording state

  • The VkCommandPool that commandBuffer was allocated from must support graphics operations

Host Synchronization
  • Host access to commandBuffer must be externally synchronized

  • Host access to the VkCommandPool that commandBuffer was allocated from must be externally synchronized

Command Properties
Command Buffer Levels Render Pass Scope Supported Queue Types Pipeline Type

Primary
Secondary

Both

Graphics

If the final shading rate for a primitive covering pixel (x,y) results in n invocations per pixel (n > 1), n separate fragment shader invocations will be generated for the fragment. Each coverage sample in the fragment will be assigned to one of the n fragment shader invocations in an implementation-dependent manner. The outputs from the fragment output interface of each shader invocation will be broadcast to all of the framebuffer samples associated with the invocation. If none of the coverage samples associated with a fragment shader invocation is covered by a primitive, the implementation may discard the fragment shader invocation for those samples.

If the final shading rate for a primitive covering pixel (x,y) results in a fragment containing multiple pixels, a single set of fragment shader invocations will be generated for all pixels in the combined fragment. Outputs from the fragment output interface will be broadcast to all covered framebuffer samples belonging to the fragment. If the fragment shader executes code discarding the fragment, none of the samples of the fragment will be updated.

25.6. Sample Shading

Sample shading can be used to specify a minimum number of unique samples to process for each fragment. If sample shading is enabled an implementation must provide a minimum of max(⌈ minSampleShadingFactor × totalSamples ⌉, 1) unique associated data for each fragment, where minSampleShadingFactor is the minimum fraction of sample shading and totalSamples is the number of samples of the color attachments used in the subpass or, if the subpass does not use any color attachments, the value of VkPipelineMultisampleStateCreateInfo::rasterizationSamples specified at pipeline creation time. These are associated with the samples in an implementation-dependent manner. When minSampleShadingFactor is 1.0, a separate set of associated data are evaluated for each sample, and each set of values is evaluated at the sample location.

Sample shading is enabled for a graphics pipeline:

  • If the interface of the fragment shader entry point of the graphics pipeline includes an input variable decorated with SampleId or SamplePosition. In this case minSampleShadingFactor takes the value 1.0.

  • Else if the sampleShadingEnable member of the VkPipelineMultisampleStateCreateInfo structure specified when creating the graphics pipeline is set to VK_TRUE. In this case minSampleShadingFactor takes the value of VkPipelineMultisampleStateCreateInfo::minSampleShading.

Otherwise, sample shading is considered disabled.

25.7. Barycentric Interpolation

When the fragmentShaderBarycentric feature is enabled, the PerVertexNV interpolation decoration can be used with fragment shader inputs to indicate that the decorated inputs do not have associated data in the fragment. Such inputs can only be accessed in a fragment shader using an array index whose value (0, 1, or 2) identifies one of the vertices of the primitive that produced the fragment.

When tessellation, geometry shading, and mesh shading are not active, fragment shader inputs decorated with PerVertexNV will take values from one of the vertices of the primitive that produced the fragment, identified by the extra index provided in SPIR-V code accessing the input. If the n vertices passed to a draw call are numbered 0 through n-1, and the point, line, and triangle primitives produced by the draw call are numbered with consecutive integers beginning with zero, the following table indicates the original vertex numbers used for index values of 0, 1, and 2. If an input decorated with PerVertexNV is accessed with any other vertex index value, the value obtained is undefined.

Primitive Topology Vertex 0 Vertex 1 Vertex 2

VK_PRIMITIVE_TOPOLOGY_POINT_LIST

i

-

-

VK_PRIMITIVE_TOPOLOGY_LINE_LIST

2i

2i+1

-

VK_PRIMITIVE_TOPOLOGY_LINE_STRIP

i

i+1

-

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST

3i

3i+1

3i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP (even)

i

i+1

i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP (odd)

i

i+2

i+1

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN

i+1

i+2

0

VK_PRIMITIVE_TOPOLOGY_LINE_LIST_ADJACENCY

4i+1

4i+2

-

VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_ADJACENCY

i+1

i+2

-

VK_PRIMITIVE_TOPOLOGY_TRIANGLES_ADJACENCY

6i

6i+2

6i+4

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_ADJACENCY (even)

2i

2i+2

2i+4

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_ADJACENCY (odd)

2i

2i+4

2i+2

When geometry or mesh shading is active, primitives processed by fragment shaders are assembled from the vertices emitted by the geometry or mesh shader. In this case, the vertices used for fragment shader inputs decorated with PerVertexNV are derived by treating the primitives produced by the shader as though they were specified by a draw call and consulting the table above.

When using tessellation without geometry shading, the tessellator produces primitives in an implementation-dependent manner. While there is no defined vertex ordering for inputs decorated with PerVertexNV, the vertex ordering used in this case will be consistent with the ordering used to derive the values of inputs decorated with code::BaryCoordNV or code::BaryCoordNoPerspNV.

Fragment shader inputs decorated with BaryCoordNV or BaryCoordNoPerspNV hold three-component vectors with barycentric weights that indicate the location of the fragment relative to the screen-space locations of vertices of its primitive. For point primitives, such variables are always assigned the value (1,0,0). For line primitives, the built-ins are obtained by interpolating an attribute whose values for the vertices numbered 0 and 1 are (1,0,0) and (0,1,0), respectively. For polygon primitives, the built-ins are obtained by interpolating an attribute whose values for the vertices numbered 0, 1, and 2 are (1,0,0), (0,1,0), and (0,0,1), respectively. For BaryCoordNV, the values are obtained using perspective interpolation. For BaryCoordNoPerspNV, the values are obtained using linear interpolation.

25.8. Points

A point is drawn by generating a set of fragments in the shape of a square centered around the vertex of the point. Each vertex has an associated point size that controls the width/height of that square. The point size is taken from the (potentially clipped) shader built-in PointSize written by:

  • the geometry shader, if active;

  • the tessellation evaluation shader, if active and no geometry shader is active;

  • the vertex shader, otherwise

and clamped to the implementation-dependent point size range [pointSizeRange[0],pointSizeRange[1]]. If the value written to PointSize is less than or equal to zero, or if no value was written to PointSize, results are undefined.

Not all point sizes need be supported, but the size 1.0 must be supported. The range of supported sizes and the size of evenly-spaced gradations within that range are implementation-dependent. The range and gradations are obtained from the pointSizeRange and pointSizeGranularity members of VkPhysicalDeviceLimits. If, for instance, the size range is from 0.1 to 2.0 and the gradation size is 0.1, then the size 0.1, 0.2, …​, 1.9, 2.0 are supported. Additional point sizes may also be supported. There is no requirement that these sizes be equally spaced. If an unsupported size is requested, the nearest supported size is used instead.

25.8.1. Basic Point Rasterization

Point rasterization produces a fragment for each framebuffer pixel with one or more sample points that intersect a region centered at the point’s (xf,yf). This region is a square with side equal to the current point size. Coverage bits that correspond to sample points that intersect the region are 1, other coverage bits are 0.

All fragments produced in rasterizing a point are assigned the same associated data, which are those of the vertex corresponding to the point. However, the fragment shader built-in PointCoord contains point sprite texture coordinates. The s and t point sprite texture coordinates vary from zero to one across the point horizontally left-to-right and top-to-bottom, respectively. The following formulas are used to evaluate s and t:

\[s = {1 \over 2} + { \left( x_p - x_f \right) \over \text{size} }\]
\[t = {1 \over 2} + { \left( y_p - y_f \right) \over \text{size} }\]

where size is the point’s size, (xp,yp) is the location at which the point sprite coordinates are evaluated - this may be the framebuffer coordinates of the pixel center (i.e. at the half-integer) or the location of a sample, and (xf,yf) is the exact, unrounded framebuffer coordinate of the vertex for the point. When rasterizationSamples is VK_SAMPLE_COUNT_1_BIT, the pixel center must be used.

25.9. Line Segments

A line is drawn by generating a set of fragments overlapping a rectangle centered on the line segment. Each line segment has an associated width that controls the width of that rectangle.

The line width is specified by the VkPipelineRasterizationStateCreateInfo::lineWidth property of the currently active pipeline, if the pipeline was not created with VK_DYNAMIC_STATE_LINE_WIDTH enabled.

Otherwise, the line width is set by calling vkCmdSetLineWidth:

void vkCmdSetLineWidth(
    VkCommandBuffer                             commandBuffer,
    float                                       lineWidth);
  • commandBuffer is the command buffer into which the command will be recorded.

  • lineWidth is the width of rasterized line segments.

Valid Usage
  • The bound graphics pipeline must have been created with the VK_DYNAMIC_STATE_LINE_WIDTH dynamic state enabled

  • If the wide lines feature is not enabled, lineWidth must be 1.0

Valid Usage (Implicit)
  • commandBuffer must be a valid VkCommandBuffer handle

  • commandBuffer must be in the recording state

  • The VkCommandPool that commandBuffer was allocated from must support graphics operations

Host Synchronization
  • Host access to commandBuffer must be externally synchronized

  • Host access to the VkCommandPool that commandBuffer was allocated from must be externally synchronized

Command Properties
Command Buffer Levels Render Pass Scope Supported Queue Types Pipeline Type

Primary
Secondary

Both

Graphics

Not all line widths need be supported for line segment rasterization, but width 1.0 antialiased segments must be provided. The range and gradations are obtained from the lineWidthRange and lineWidthGranularity members of VkPhysicalDeviceLimits. If, for instance, the size range is from 0.1 to 2.0 and the gradation size is 0.1, then the size 0.1, 0.2, …​, 1.9, 2.0 are supported. Additional line widths may also be supported. There is no requirement that these widths be equally spaced. If an unsupported width is requested, the nearest supported width is used instead.

25.9.1. Basic Line Segment Rasterization

Rasterized line segments produce fragments which intersect a rectangle centered on the line segment. Two of the edges are parallel to the specified line segment; each is at a distance of one-half the current width from that segment in directions perpendicular to the direction of the line. The other two edges pass through the line endpoints and are perpendicular to the direction of the specified line segment. Coverage bits that correspond to sample points that intersect the rectangle are 1, other coverage bits are 0.

Next we specify how the data associated with each rasterized fragment are obtained. Let pr = (xd, yd) be the framebuffer coordinates at which associated data are evaluated. This may be the pixel center of a fragment or the location of a sample within the fragment. When rasterizationSamples is VK_SAMPLE_COUNT_1_BIT, the pixel center must be used. Let pa = (xa, ya) and pb = (xb,yb) be initial and final endpoints of the line segment, respectively. Set

\[t = {{( \mathbf{p}_r - \mathbf{p}_a ) \cdot ( \mathbf{p}_b - \mathbf{p}_a )} \over {\| \mathbf{p}_b - \mathbf{p}_a \|^2 }}\]

(Note that t = 0 at pa and t = 1 at pb. Also note that this calculation projects the vector from pa to pr onto the line, and thus computes the normalized distance of the fragment along the line.)

The value of an associated datum f for the fragment, whether it be a shader output or the clip w coordinate, must be determined using perspective interpolation:

\[f = {{ (1-t) {f_a / w_a} + t { f_b / w_b} } \over {(1-t) / w_a + t / w_b }}\]

where fa and fb are the data associated with the starting and ending endpoints of the segment, respectively; wa and wb are the clip w coordinates of the starting and ending endpoints of the segments, respectively.

Depth values for lines must be determined using linear interpolation:

z = (1 - t) za + t zb

where za and zb are the depth values of the starting and ending endpoints of the segment, respectively.

The NoPerspective and Flat interpolation decorations can be used with fragment shader inputs to declare how they are interpolated. When neither decoration is applied, perspective interpolation is performed as described above. When the NoPerspective decoration is used, linear interpolation is performed in the same fashion as for depth values, as described above. When the Flat decoration is used, no interpolation is performed, and outputs are taken from the corresponding input value of the provoking vertex corresponding to that primitive.

When the fragmentShaderBarycentric feature is enabled, the PerVertexNV interpolation decoration can also be used with fragment shader inputs which indicate that the decorated inputs are not interpolated and can only be accessed using an extra array dimension, where the extra index identifies one of the vertices of the primitive that produced the fragment.

The above description documents the preferred method of line rasterization, and must be used when the implementation advertises the strictLines limit in VkPhysicalDeviceLimits as VK_TRUE.

When strictLines is VK_FALSE, the edges of the lines are generated as a parallelogram surrounding the original line. The major axis is chosen by noting the axis in which there is the greatest distance between the line start and end points. If the difference is equal in both directions then the X axis is chosen as the major axis. Edges 2 and 3 are aligned to the minor axis and are centered on the endpoints of the line as in Non strict lines, and each is lineWidth long. Edges 0 and 1 are parallel to the line and connect the endpoints of edges 2 and 3. Coverage bits that correspond to sample points that intersect the parallelogram are 1, other coverage bits are 0.

Samples that fall exactly on the edge of the parallelogram follow the polygon rasterization rules.

Interpolation occurs as if the parallelogram was decomposed into two triangles where each pair of vertices at each end of the line has identical attributes.

image/svg+xml Edge 0 Edge 1 Edge 3 Edge 2 OriginalLine (Xb,Yb,Zb) (Xa,Ya,Za) LineWidth
Figure 27. Non strict lines

25.10. Polygons

A polygon results from the decomposition of a triangle strip, triangle fan or a series of independent triangles. Like points and line segments, polygon rasterization is controlled by several variables in the VkPipelineRasterizationStateCreateInfo structure.

25.10.1. Basic Polygon Rasterization

The first step of polygon rasterization is to determine whether the triangle is back-facing or front-facing. This determination is made based on the sign of the (clipped or unclipped) polygon’s area computed in framebuffer coordinates. One way to compute this area is:

\[a = -{1 \over 2}\sum_{i=0}^{n-1} x_f^i y_f^{i \oplus 1} - x_f^{i \oplus 1} y_f^i\]

where \(x_f^i\) and \(y_f^i\) are the x and y framebuffer coordinates of the ith vertex of the n-vertex polygon (vertices are numbered starting at zero for the purposes of this computation) and i ⊕ 1 is (i + 1) mod n.

The interpretation of the sign of a is determined by the VkPipelineRasterizationStateCreateInfo::frontFace property of the currently active pipeline. Possible values are:

typedef enum VkFrontFace {
    VK_FRONT_FACE_COUNTER_CLOCKWISE = 0,
    VK_FRONT_FACE_CLOCKWISE = 1,
} VkFrontFace;
  • VK_FRONT_FACE_COUNTER_CLOCKWISE specifies that a triangle with positive area is considered front-facing.

  • VK_FRONT_FACE_CLOCKWISE specifies that a triangle with negative area is considered front-facing.

Any triangle which is not front-facing is back-facing, including zero-area triangles.

Once the orientation of triangles is determined, they are culled according to the VkPipelineRasterizationStateCreateInfo::cullMode property of the currently active pipeline. Possible values are:

typedef enum VkCullModeFlagBits {
    VK_CULL_MODE_NONE = 0,
    VK_CULL_MODE_FRONT_BIT = 0x00000001,
    VK_CULL_MODE_BACK_BIT = 0x00000002,
    VK_CULL_MODE_FRONT_AND_BACK = 0x00000003,
} VkCullModeFlagBits;
  • VK_CULL_MODE_NONE specifies that no triangles are discarded

  • VK_CULL_MODE_FRONT_BIT specifies that front-facing triangles are discarded

  • VK_CULL_MODE_BACK_BIT specifies that back-facing triangles are discarded

  • VK_CULL_MODE_FRONT_AND_BACK specifies that all triangles are discarded.

Following culling, fragments are produced for any triangles which have not been discarded.

typedef VkFlags VkCullModeFlags;

VkCullModeFlags is a bitmask type for setting a mask of zero or more VkCullModeFlagBits.

The rule for determining which fragments are produced by polygon rasterization is called point sampling. The two-dimensional projection obtained by taking the x and y framebuffer coordinates of the polygon’s vertices is formed. Fragments are produced for any pixels for which any sample points lie inside of this polygon. Coverage bits that correspond to sample points that satisfy the point sampling criteria are 1, other coverage bits are 0. Special treatment is given to a sample whose sample location lies on a polygon edge. In such a case, if two polygons lie on either side of a common edge (with identical endpoints) on which a sample point lies, then exactly one of the polygons must result in a covered sample for that fragment during rasterization. As for the data associated with each fragment produced by rasterizing a polygon, we begin by specifying how these values are produced for fragments in a triangle. Define barycentric coordinates for a triangle. Barycentric coordinates are a set of three numbers, a, b, and c, each in the range [0,1], with a + b + c = 1. These coordinates uniquely specify any point p within the triangle or on the triangle’s boundary as

p = a pa + b pb + c pc

where pa, pb, and pc are the vertices of the triangle. a, b, and c are determined by:

\[a = {{\mathrm{A}(p p_b p_c)} \over {\mathrm{A}(p_a p_b p_c)}}, \quad b = {{\mathrm{A}(p p_a p_c)} \over {\mathrm{A}(p_a p_b p_c)}}, \quad c = {{\mathrm{A}(p p_a p_b)} \over {\mathrm{A}(p_a p_b p_c)}},\]

where A(lmn) denotes the area in framebuffer coordinates of the triangle with vertices l, m, and n.

Denote an associated datum at pa, pb, or pc as fa, fb, or fc, respectively.

The value of an associated datum f for a fragment produced by rasterizing a triangle, whether it be a shader output or the clip w coordinate, must be determined using perspective interpolation:

\[f = { a {f_a / w_a} + b {f_b / w_b} + c {f_c / w_c} } \over { {a / w_a} + {b / w_b} + {c / w_c} }\]

where wa, wb, and wc are the clip w coordinates of pa, pb, and pc, respectively. a, b, and c are the barycentric coordinates of the location at which the data are produced - this must be a pixel center or the location of a sample. When rasterizationSamples is VK_SAMPLE_COUNT_1_BIT, the pixel center must be used.

Depth values for triangles must be determined using linear interpolation:

z = a za + b zb + c zc

where za, zb, and zc are the depth values of pa, pb, and pc, respectively.

The NoPerspective and Flat interpolation decorations can be used with fragment shader inputs to declare how they are interpolated. When neither decoration is applied, perspective interpolation is performed as described above. When the NoPerspective decoration is used, linear interpolation is performed in the same fashion as for depth values, as described above. When the Flat decoration is used, no interpolation is performed, and outputs are taken from the corresponding input value of the provoking vertex corresponding to that primitive.

When the VK_AMD_shader_explicit_vertex_parameter device extension is enabled the CustomInterpAMD interpolation decoration can also be used with fragment shader inputs which indicate that the decorated inputs can only be accessed by the extended instruction InterpolateAtVertexAMD and allows accessing the value of the inputs for individual vertices of the primitive.

When the fragmentShaderBarycentric feature is enabled, the PerVertexNV interpolation decoration can also be used with fragment shader inputs which indicate that the decorated inputs are not interpolated and can only be accessed using an extra array dimension, where the extra index identifies one of the vertices of the primitive that produced the fragment.

For a polygon with more than three edges, such as are produced by clipping a triangle, a convex combination of the values of the datum at the polygon’s vertices must be used to obtain the value assigned to each fragment produced by the rasterization algorithm. That is, it must be the case that at every fragment

\[f = \sum_{i=1}^{n} a_i f_i\]

where n is the number of vertices in the polygon and fi is the value of f at vertex i. For each i, 0 ≤ ai ≤ 1 and \(\sum_{i=1}^{n}a_i = 1\). The values of ai may differ from fragment to fragment, but at vertex i, ai = 1 and aj = 0 for j ≠ i.

Note

One algorithm that achieves the required behavior is to triangulate a polygon (without adding any vertices) and then treat each triangle individually as already discussed. A scan-line rasterizer that linearly interpolates data along each edge and then linearly interpolates data across each horizontal span from edge to edge also satisfies the restrictions (in this case, the numerator and denominator of equation [triangle_perspective_interpolation] are iterated independently and a division performed for each fragment).

25.10.2. Polygon Mode

Possible values of the VkPipelineRasterizationStateCreateInfo::polygonMode property of the currently active pipeline, specifying the method of rasterization for polygons, are:

typedef enum VkPolygonMode {
    VK_POLYGON_MODE_FILL = 0,
    VK_POLYGON_MODE_LINE = 1,
    VK_POLYGON_MODE_POINT = 2,
    VK_POLYGON_MODE_FILL_RECTANGLE_NV = 1000153000,
} VkPolygonMode;
  • VK_POLYGON_MODE_POINT specifies that polygon vertices are drawn as points.

  • VK_POLYGON_MODE_LINE specifies that polygon edges are drawn as line segments.

  • VK_POLYGON_MODE_FILL specifies that polygons are rendered using the polygon rasterization rules in this section.

  • VK_POLYGON_MODE_FILL_RECTANGLE_NV specifies that polygons are rendered using polygon rasterization rules, modified to consider a sample within the primitive if the sample location is inside the axis-aligned bounding box of the triangle after projection. Note that the barycentric weights used in attribute interpolation can extend outside the range [0,1] when these primitives are shaded. Special treatment is given to a sample position on the boundary edge of the bounding box. In such a case, if two rectangles lie on either side of a common edge (with identical endpoints) on which a sample position lies, then exactly one of the triangles must produce a fragment that covers that sample during rasterization.

    Polygons rendered in VK_POLYGON_MODE_FILL_RECTANGLE_NV mode may be clipped by the frustum or by user clip planes. If clipping is applied, the triangle is culled rather than clipped.

    Area calculation and facingness are determined for VK_POLYGON_MODE_FILL_RECTANGLE_NV mode using the triangle’s vertices.

These modes affect only the final rasterization of polygons: in particular, a polygon’s vertices are shaded and the polygon is clipped and possibly culled before these modes are applied.

25.10.3. Depth Bias

The depth values of all fragments generated by the rasterization of a polygon can be offset by a single value that is computed for that polygon. This behavior is controlled by the depthBiasEnable, depthBiasConstantFactor, depthBiasClamp, and depthBiasSlopeFactor members of VkPipelineRasterizationStateCreateInfo, or by the corresponding parameters to the vkCmdSetDepthBias command if depth bias state is dynamic.

void vkCmdSetDepthBias(
    VkCommandBuffer                             commandBuffer,
    float                                       depthBiasConstantFactor,
    float                                       depthBiasClamp,
    float                                       depthBiasSlopeFactor);
  • commandBuffer is the command buffer into which the command will be recorded.

  • depthBiasConstantFactor is a scalar factor controlling the constant depth value added to each fragment.

  • depthBiasClamp is the maximum (or minimum) depth bias of a fragment.

  • depthBiasSlopeFactor is a scalar factor applied to a fragment’s slope in depth bias calculations.

If depthBiasEnable is VK_FALSE, no depth bias is applied and the fragment’s depth values are unchanged.

depthBiasSlopeFactor scales the maximum depth slope of the polygon, and depthBiasConstantFactor scales an implementation-dependent constant that relates to the usable resolution of the depth buffer. The resulting values are summed to produce the depth bias value which is then clamped to a minimum or maximum value specified by depthBiasClamp. depthBiasSlopeFactor, depthBiasConstantFactor, and depthBiasClamp can each be positive, negative, or zero.

The maximum depth slope m of a triangle is

\[m = \sqrt{ \left({{\partial z_f} \over {\partial x_f}}\right)^2 + \left({{\partial z_f} \over {\partial y_f}}\right)^2}\]

where (xf, yf, zf) is a point on the triangle. m may be approximated as

\[m = \max\left( \left| { {\partial z_f} \over {\partial x_f} } \right|, \left| { {\partial z_f} \over {\partial y_f} } \right| \right).\]

The minimum resolvable difference r is an implementation-dependent parameter that depends on the depth buffer representation. It is the smallest difference in framebuffer coordinate z values that is guaranteed to remain distinct throughout polygon rasterization and in the depth buffer. All pairs of fragments generated by the rasterization of two polygons with otherwise identical vertices, but zf values that differ by r, will have distinct depth values.

For fixed-point depth buffer representations, r is constant throughout the range of the entire depth buffer. For floating-point depth buffers, there is no single minimum resolvable difference. In this case, the minimum resolvable difference for a given polygon is dependent on the maximum exponent, e, in the range of z values spanned by the primitive. If n is the number of bits in the floating-point mantissa, the minimum resolvable difference, r, for the given primitive is defined as

r = 2e-n

If a triangle is rasterized using the VK_POLYGON_MODE_FILL_RECTANGLE_NV polygon mode, then this minimum resolvable difference may not be resolvable for samples outside of the triangle, where the depth is extrapolated.

If no depth buffer is present, r is undefined.

The bias value o for a polygon is

\[o = \begin{cases} m \times depthBiasSlopeFactor + r \times depthBiasConstantFactor & depthBiasClamp = 0\ or\ NaN \\ \min(m \times depthBiasSlopeFactor + r \times depthBiasConstantFactor, depthBiasClamp) & depthBiasClamp > 0 \\ \max(m \times depthBiasSlopeFactor + r \times depthBiasConstantFactor, depthBiasClamp) & depthBiasClamp < 0 \\ \end{cases}\]

m is computed as described above. If the depth buffer uses a fixed-point representation, m is a function of depth values in the range [0,1], and o is applied to depth values in the same range.

For fixed-point depth buffers, fragment depth values are always limited to the range [0,1] by clamping after depth bias addition is performed. Unless the VK_EXT_depth_range_unrestricted extension is enabled, fragment depth values are clamped even when the depth buffer uses a floating-point representation.

Valid Usage
  • The bound graphics pipeline must have been created with the VK_DYNAMIC_STATE_DEPTH_BIAS dynamic state enabled

  • If the depth bias clamping feature is not enabled, depthBiasClamp must be 0.0

Valid Usage (Implicit)
  • commandBuffer must be a valid VkCommandBuffer handle

  • commandBuffer must be in the recording state

  • The VkCommandPool that commandBuffer was allocated from must support graphics operations

Host Synchronization
  • Host access to commandBuffer must be externally synchronized

  • Host access to the VkCommandPool that commandBuffer was allocated from must be externally synchronized

Command Properties
Command Buffer Levels Render Pass Scope Supported Queue Types Pipeline Type

Primary
Secondary

Both

Graphics

25.10.4. Conservative Rasterization

Polygon rasterization can be made conservative by setting conservativeRasterizationMode to VK_CONSERVATIVE_RASTERIZATION_MODE_OVERESTIMATE_EXT or VK_CONSERVATIVE_RASTERIZATION_MODE_UNDERESTIMATE_EXT in VkPipelineRasterizationConservativeStateCreateInfoEXT. The VkPipelineRasterizationConservativeStateCreateInfoEXT state is set by adding an instance of this structure to the pNext chain of an instance of the VkPipelineRasterizationStateCreateInfo structure when creating the graphics pipeline. Enabling these modes also affects line and point rasterization if the implementation sets VkPhysicalDeviceConservativeRasterizationPropertiesEXT::conservativePointAndLineRasterization to VK_TRUE.

VkPipelineRasterizationConservativeStateCreateInfoEXT is defined as:

typedef struct VkPipelineRasterizationConservativeStateCreateInfoEXT {
    VkStructureType                                           sType;
    const void*                                               pNext;
    VkPipelineRasterizationConservativeStateCreateFlagsEXT    flags;
    VkConservativeRasterizationModeEXT                        conservativeRasterizationMode;
    float                                                     extraPrimitiveOverestimationSize;
} VkPipelineRasterizationConservativeStateCreateInfoEXT;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to an extension-specific structure.

  • flags is reserved for future use.

  • conservativeRasterizationMode is the conservative rasterization mode to use.

  • extraPrimitiveOverestimationSize is the extra size in pixels to increase the generating primitive during conservative rasterization at each of its edges in X and Y equally in screen space beyond the base overestimation specified in VkPhysicalDeviceConservativeRasterizationPropertiesEXT::primitiveOverestimationSize.

Valid Usage
  • extraPrimitiveOverestimationSize must be in the range of 0.0 to VkPhysicalDeviceConservativeRasterizationPropertiesEXT::maxExtraPrimitiveOverestimationSize inclusive

Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_PIPELINE_RASTERIZATION_CONSERVATIVE_STATE_CREATE_INFO_EXT

  • flags must be 0

  • conservativeRasterizationMode must be a valid VkConservativeRasterizationModeEXT value

typedef VkFlags VkPipelineRasterizationConservativeStateCreateFlagsEXT;

VkPipelineRasterizationConservativeStateCreateFlagsEXT is a bitmask type for setting a mask, but is currently reserved for future use.

Possible values of VkPipelineRasterizationConservativeStateCreateInfoEXT::conservativeRasterizationMode, specifying the conservative rasterization mode are:

typedef enum VkConservativeRasterizationModeEXT {
    VK_CONSERVATIVE_RASTERIZATION_MODE_DISABLED_EXT = 0,
    VK_CONSERVATIVE_RASTERIZATION_MODE_OVERESTIMATE_EXT = 1,
    VK_CONSERVATIVE_RASTERIZATION_MODE_UNDERESTIMATE_EXT = 2,
} VkConservativeRasterizationModeEXT;
  • VK_CONSERVATIVE_RASTERIZATION_MODE_DISABLED_EXT specifies that conservative rasterization is disabled and rasterization proceeds as normal.

  • VK_CONSERVATIVE_RASTERIZATION_MODE_OVERESTIMATE_EXT specifies that conservative rasterization is enabled in overestimation mode.

  • VK_CONSERVATIVE_RASTERIZATION_MODE_UNDERESTIMATE_EXT specifies that conservative rasterization is enabled in underestimation mode.

When overestimate conservative rasterization is enabled, rather than evaluating coverage at individual sample locations, a determination is made of whether any portion of the pixel (including its edges and corners) is covered by the primitive. If any portion of the pixel is covered, then all bits of the coverage sample mask for the fragment are enabled. If the implementation supports VkPhysicalDeviceConservativeRasterizationPropertiesEXT::conservativeRasterizationPostDepthCoverage and the PostDepthCoverage execution mode is specified the SampleMask built-in input variable will reflect the coverage after the early per-fragment depth and stencil tests are applied.

For the purposes of evaluating which pixels are covered by the primitive, implementations can increase the size of the primitive by up to VkPhysicalDeviceConservativeRasterizationPropertiesEXT::primitiveOverestimationSize pixels at each of the primitive edges. This may increase the number of fragments generated by this primitive and represents an overestimation of the pixel coverage.

This overestimation size can be increased further by setting the extraPrimitiveOverestimationSize value above 0.0 in steps of VkPhysicalDeviceConservativeRasterizationPropertiesEXT::extraPrimitiveOverestimationSizeGranularity up to and including VkPhysicalDeviceConservativeRasterizationPropertiesEXT::extraPrimitiveOverestimationSize. This will: further increase the number of fragments generated by this primitive.

The actual precision of the overestimation size used for conservative rasterization may vary between implementations and produce results that only approximate the primitiveOverestimationSize and extraPrimitiveOverestimationSizeGranularity properties.

For triangles if VK_CONSERVATIVE_RASTERIZATION_MODE_OVERESTIMATE_EXT is enabled, fragments will be generated if the primitive area covers any portion of the pixel, including its edges or corners. The tie-breaking rule described in Basic Polygon Rasterization does not apply during conservative rasterization and coverage is set for all fragments generated from shared edges of polygons. Degenerate triangles that evaluate to zero area after rasterization, even for pixels that contain a vertex or edge of the zero-area polygon, will be culled if VkPhysicalDeviceConservativeRasterizationPropertiesEXT::degenerateTrianglesRasterized is VK_FALSE or will generate fragments if degenerateTrianglesRasterized is VK_TRUE. The fragment input values for these degenerate triangles take their attribute and depth values from the provoking vertex. Degenerate triangles are considered backfacing and the application can enable backface culling if desired. Triangles that are zero area before rasterization may be culled regardless.

For lines if VK_CONSERVATIVE_RASTERIZATION_MODE_OVERESTIMATE_EXT is enabled, and the implementation sets VkPhysicalDeviceConservativeRasterizationPropertiesEXT::conservativePointAndLineRasterization to VK_TRUE, fragments will be generated if the line covers any portion of the pixel, including its edges or corners. Degenerate lines that evaluate to zero length after rasterization will be culled if VkPhysicalDeviceConservativeRasterizationPropertiesEXT::degenerateLinesRasterized is VK_FALSE or will generate fragments if degenerateLinesRasterized is VK_TRUE. The fragments input values for these degenerate lines take their attribute and depth values from the provoking vertex. Lines that are zero length before rasterization may be culled regardless.

For points if VK_CONSERVATIVE_RASTERIZATION_MODE_OVERESTIMATE_EXT is enabled, and the implementation sets VkPhysicalDeviceConservativeRasterizationPropertiesEXT::conservativePointAndLineRasterization to VK_TRUE, fragments will be generated if the point square covers any portion of the pixel square, including its edges or corners.

When underestimate conservative rasterization is enabled, rather than evaluating coverage at individual sample locations, a determination is made of whether all of the pixel (including its edges and corners) is covered by the primitive. If the entire pixel is covered, then a fragment is generated with all bits of its coverage sample mask enabled, otherwise the fragment is discarded even if some portion of the pixel is covered. If the implementation supports VkPhysicalDeviceConservativeRasterizationPropertiesEXT::conservativeRasterizationPostDepthCoverage and the PostDepthCoverage execution mode is specified the SampleMask built-in input variable will reflect the coverage after the early per-fragment depth and stencil tests are applied.

For triangles, if VK_CONSERVATIVE_RASTERIZATION_MODE_UNDERESTIMATE_EXT is enabled, fragments will only be generated if they are fully covered by the generating primitive, including its edges and corners.

For lines, if VK_CONSERVATIVE_RASTERIZATION_MODE_UNDERESTIMATE_EXT is enabled, fragments will be generated if the entire pixel, including its edges and corners is covered by the line.

For points, if VK_CONSERVATIVE_RASTERIZATION_MODE_UNDERESTIMATE_EXT is enabled, fragments will only be generated for pixel squares that are fully covered by the point square.

For both overestimate and underestimate conservative rasterization modes a pixel square that is fully covered by the generating primitive must set FullyCoveredEXT to VK_TRUE if the implementation enables the VkPhysicalDeviceConservativeRasterizationPropertiesEXT::fullyCoveredFragmentShaderInputVariable feature.

When the use of a shading rate image results in fragments covering multiple pixels, coverage for conservative rasterization is still evaluated on a per-pixel basis and may result in fragments with partial coverage. For fragment shader inputs decorated with FullyCoveredEXT, a fragment is considered fully covered if and only if all pixels in the fragment are fully covered by the generating primitive.