25. Fragment Operations

Fragments produced by rasterization go through a number of operations to determine whether or how values produced by fragment shading are written to the framebuffer.

The following fragment operations adhere to rasterization order, and are typically performed in this order:

The coverage mask generated by rasterization describes the initial coverage of each sample covered by the fragment. Fragment operations will update the coverage mask to add or subtract coverage where appropriate. If a fragment operation results in all bits of the coverage mask being 0, the fragment is discarded, and no further operations are performed. Fragments can also be programmatically discarded in a fragment shader by executing OpKill.

If early per-fragment operations are enabled, fragment shading and multisample coverage operations are instead performed after sample counting.

Once all fragment operations have completed, fragment shader outputs for covered color attachment samples pass through framebuffer operations.

25.1. Scissor Test

The scissor test compares the framebuffer coordinates (xf,yf) of each sample covered by a fragment against a scissor rectangle at the index equal to the fragment’s ViewportIndex.

Each scissor rectangle is defined by a VkRect2D. These values are either set by the VkPipelineViewportStateCreateInfo structure during pipeline creation, or dynamically by the vkCmdSetScissor command.

A given sample is considered inside a scissor rectangle if xf is in the range [VkRect2D::offset.x, VkRect2D::offset.x + VkRect2D::extent.x), and yf is in the range [VkRect2D::offset.y, VkRect2D::offset.y + VkRect2D::extent.y). Samples with coordinates outside the scissor rectangle at the corresponding ViewportIndex will have their coverage set to 0.

The scissor rectangles can be set dynamically with the command:

// Provided by VK_VERSION_1_0
void vkCmdSetScissor(
    VkCommandBuffer                             commandBuffer,
    uint32_t                                    firstScissor,
    uint32_t                                    scissorCount,
    const VkRect2D*                             pScissors);
  • commandBuffer is the command buffer into which the command will be recorded.

  • firstScissor is the index of the first scissor whose state is updated by the command.

  • scissorCount is the number of scissors whose rectangles are updated by the command.

  • pScissors is a pointer to an array of VkRect2D structures defining scissor rectangles.

The scissor rectangles taken from element i of pScissors replace the current state for the scissor index firstScissor + i, for i in [0, scissorCount).

This command sets the state for a given draw when the graphics pipeline is created with VK_DYNAMIC_STATE_SCISSOR set in VkPipelineDynamicStateCreateInfo::pDynamicStates.

Valid Usage
  • firstScissor must be less than VkPhysicalDeviceLimits::maxViewports

  • The sum of firstScissor and scissorCount must be between 1 and VkPhysicalDeviceLimits::maxViewports, inclusive

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

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

  • The x and y members of offset member of any element of pScissors must be greater than or equal to 0

  • Evaluation of (offset.x + extent.width) must not cause a signed integer addition overflow for any element of pScissors

  • Evaluation of (offset.y + extent.height) must not cause a signed integer addition overflow for any element of pScissors

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

  • pScissors must be a valid pointer to an array of scissorCount VkRect2D structures

  • commandBuffer must be in the recording state

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

  • scissorCount 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

25.2. Sample Mask Test

The sample mask test compares the coverage mask for a fragment with the sample mask defined by VkPipelineMultisampleStateCreateInfo::pSampleMask.

Each bit of the coverage mask is associated with a sample index as described in the rasterization chapter. If the bit in VkPipelineMultisampleStateCreateInfo::pSampleMask which is associated with that same sample index is set to 0, the coverage mask bit is set to 0.

25.3. Multisample Coverage

If a fragment shader is active and its entry point’s interface includes a built-in output variable decorated with SampleMask, the coverage mask is ANDed with the bits of the SampleMask built-in to generate a new coverage mask. If sample shading is enabled, bits written to SampleMask corresponding to samples that are not being shaded by the fragment shader invocation are ignored. If no fragment shader is active, or if the active fragment shader does not include SampleMask in its interface, the coverage mask is not modified.

Next, the fragment alpha value and coverage mask are modified based on the alphaToCoverageEnable and alphaToOneEnable members of the VkPipelineMultisampleStateCreateInfo structure.

All alpha values in this section refer only to the alpha component of the fragment shader output that has a Location and Index decoration of zero (see the Fragment Output Interface section). If that shader output has an integer or unsigned integer type, then these operations are skipped.

If alphaToCoverageEnable is enabled, a temporary coverage mask is generated where each bit is determined by the fragment’s alpha value, which is ANDed with the fragment coverage mask.

No specific algorithm is specified for converting the alpha value to a temporary coverage mask. It is intended that the number of 1’s in this value be proportional to the alpha value (clamped to [0,1]), with all 1’s corresponding to a value of 1.0 and all 0’s corresponding to 0.0. The algorithm may be different at different framebuffer coordinates.

Note

Using different algorithms at different framebuffer coordinates may help to avoid artifacts caused by regular coverage sample locations.

Next, if alphaToOneEnable is enabled, each alpha value is replaced by the maximum representable alpha value for fixed-point color buffers, or by 1.0 for floating-point buffers. Otherwise, the alpha values are not changed.

25.4. Depth and Stencil Operations

Pipeline state controlling the depth bounds tests, stencil test, and depth test is specified through the members of the VkPipelineDepthStencilStateCreateInfo structure.

The VkPipelineDepthStencilStateCreateInfo structure is defined as:

// Provided by VK_VERSION_1_0
typedef struct VkPipelineDepthStencilStateCreateInfo {
    VkStructureType                           sType;
    const void*                               pNext;
    VkPipelineDepthStencilStateCreateFlags    flags;
    VkBool32                                  depthTestEnable;
    VkBool32                                  depthWriteEnable;
    VkCompareOp                               depthCompareOp;
    VkBool32                                  depthBoundsTestEnable;
    VkBool32                                  stencilTestEnable;
    VkStencilOpState                          front;
    VkStencilOpState                          back;
    float                                     minDepthBounds;
    float                                     maxDepthBounds;
} VkPipelineDepthStencilStateCreateInfo;
  • sType is the type of this structure.

  • pNext is NULL or a pointer to a structure extending this structure.

  • flags is reserved for future use.

  • depthTestEnable controls whether depth testing is enabled.

  • depthWriteEnable controls whether depth writes are enabled when depthTestEnable is VK_TRUE. Depth writes are always disabled when depthTestEnable is VK_FALSE.

  • depthCompareOp is the comparison operator used in the depth test.

  • depthBoundsTestEnable controls whether depth bounds testing is enabled.

  • stencilTestEnable controls whether stencil testing is enabled.

  • front and back control the parameters of the stencil test.

  • minDepthBounds is the minimum depth bound used in the depth bounds test.

  • maxDepthBounds is the maximum depth bound used in the depth bounds test.

Valid Usage
Valid Usage (Implicit)
  • sType must be VK_STRUCTURE_TYPE_PIPELINE_DEPTH_STENCIL_STATE_CREATE_INFO

  • pNext must be NULL

  • flags must be 0

  • depthCompareOp must be a valid VkCompareOp value

  • front must be a valid VkStencilOpState structure

  • back must be a valid VkStencilOpState structure

// Provided by VK_VERSION_1_0
typedef VkFlags VkPipelineDepthStencilStateCreateFlags;

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

25.5. Depth Bounds Test

The depth bounds test compares the depth value za in the depth/stencil attachment at each sample’s framebuffer coordinates (xf,yf) and sample index i against a set of depth bounds.

The depth bounds are determined by two floating point values defining a minimum (minDepthBounds) and maximum (maxDepthBounds) depth value. These values are either set by the VkPipelineDepthStencilStateCreateInfo structure during pipeline creation, or dynamically by vkCmdSetDepthBounds.

A given sample is considered within the depth bounds if za is in the range [minDepthBounds,maxDepthBounds]. Samples with depth attachment values outside of the depth bounds will have their coverage set to 0.

If the depth bounds test is disabled, or if there is no depth attachment, the coverage mask is unmodified by this operation.

To dynamically set the depth bounds range values call:

// Provided by VK_VERSION_1_0
void vkCmdSetDepthBounds(
    VkCommandBuffer                             commandBuffer,
    float                                       minDepthBounds,
    float                                       maxDepthBounds);
  • commandBuffer is the command buffer into which the command will be recorded.

  • minDepthBounds is the minimum depth bound.

  • maxDepthBounds is the maximum depth bound.

This command sets the state for a given draw when the graphics pipeline is created with VK_DYNAMIC_STATE_DEPTH_BOUNDS set in VkPipelineDynamicStateCreateInfo::pDynamicStates.

Valid Usage
  • minDepthBounds must be between 0.0 and 1.0, inclusive

  • maxDepthBounds must be between 0.0 and 1.0, inclusive

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

25.6. Stencil Test

The stencil test compares the stencil attachment value sa in the depth/stencil attachment at each sample’s framebuffer coordinates (xf,yf) and sample index i against a stencil reference value.

If the stencil test is not enabled, as specified by VkPipelineDepthStencilStateCreateInfo::stencilTestEnable, or if there is no stencil attachment, the coverage mask is unmodified by this operation.

The stencil test is controlled by one of two sets of stencil-related state, the front stencil state and the back stencil state. Stencil tests and writes use the back stencil state when processing fragments generated by back-facing polygons, and the front stencil state when processing fragments generated by front-facing polygons or any other primitives.

The comparison performed is based on the VkCompareOp, compare mask sc , and stencil reference value sr of the relevant state set. The compare mask and stencil reference value are set by either the VkPipelineDepthStencilStateCreateInfo structure during pipeline creation, or by the vkCmdSetStencilCompareMask and vkCmdSetStencilReference commands respectively. The compare operation is set by VkStencilOpState::compareOp during pipeline creation.

The stencil reference and attachment values sr and sa are each independently combined with the compare mask sc using a logical AND operation to create masked reference and attachment values s'r and s'a. s'r and s'a are used as A and B, respectively, in the operation specified by VkCompareOp.

If the comparison evaluates to false, the coverage for the sample is set to 0.

A new stencil value sg is generated according to a stencil operation defined by VkStencilOp parameters set by VkPipelineDepthStencilStateCreateInfo. If the stencil test fails, failOp defines the stencil operation used. If the stencil test passes however, the stencil op used is based on the depth test - if it passes, VkPipelineDepthStencilStateCreateInfo::passOp is used, otherwise VkPipelineDepthStencilStateCreateInfo::depthFailOp is used.

The stencil attachment value sa is then updated with the generated stencil value sg according to the write mask sw defined by VkPipelineDepthStencilStateCreateInfo::writeMask as:

sa = (sa & ¬sw) | (sg & sw)

The VkStencilOpState structure is defined as:

// Provided by VK_VERSION_1_0
typedef struct VkStencilOpState {
    VkStencilOp    failOp;
    VkStencilOp    passOp;
    VkStencilOp    depthFailOp;
    VkCompareOp    compareOp;
    uint32_t       compareMask;
    uint32_t       writeMask;
    uint32_t       reference;
} VkStencilOpState;
  • failOp is a VkStencilOp value specifying the action performed on samples that fail the stencil test.

  • passOp is a VkStencilOp value specifying the action performed on samples that pass both the depth and stencil tests.

  • depthFailOp is a VkStencilOp value specifying the action performed on samples that pass the stencil test and fail the depth test.

  • compareOp is a VkCompareOp value specifying the comparison operator used in the stencil test.

  • compareMask selects the bits of the unsigned integer stencil values participating in the stencil test.

  • writeMask selects the bits of the unsigned integer stencil values updated by the stencil test in the stencil framebuffer attachment.

  • reference is an integer reference value that is used in the unsigned stencil comparison.

Valid Usage (Implicit)

To dynamically set the stencil compare mask call:

// Provided by VK_VERSION_1_0
void vkCmdSetStencilCompareMask(
    VkCommandBuffer                             commandBuffer,
    VkStencilFaceFlags                          faceMask,
    uint32_t                                    compareMask);
  • commandBuffer is the command buffer into which the command will be recorded.

  • faceMask is a bitmask of VkStencilFaceFlagBits specifying the set of stencil state for which to update the compare mask.

  • compareMask is the new value to use as the stencil compare mask.

This command sets the state for a given draw when the graphics pipeline is created with VK_DYNAMIC_STATE_STENCIL_COMPARE_MASK set in VkPipelineDynamicStateCreateInfo::pDynamicStates.

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

  • faceMask must be a valid combination of VkStencilFaceFlagBits values

  • faceMask must not be 0

  • 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

VkStencilFaceFlagBits values are:

// Provided by VK_VERSION_1_0
typedef enum VkStencilFaceFlagBits {
    VK_STENCIL_FACE_FRONT_BIT = 0x00000001,
    VK_STENCIL_FACE_BACK_BIT = 0x00000002,
    VK_STENCIL_FACE_FRONT_AND_BACK = 0x00000003,
    VK_STENCIL_FRONT_AND_BACK = VK_STENCIL_FACE_FRONT_AND_BACK,
} VkStencilFaceFlagBits;
  • VK_STENCIL_FACE_FRONT_BIT specifies that only the front set of stencil state is updated.

  • VK_STENCIL_FACE_BACK_BIT specifies that only the back set of stencil state is updated.

  • VK_STENCIL_FACE_FRONT_AND_BACK is the combination of VK_STENCIL_FACE_FRONT_BIT and VK_STENCIL_FACE_BACK_BIT, and specifies that both sets of stencil state are updated.

// Provided by VK_VERSION_1_0
typedef VkFlags VkStencilFaceFlags;

VkStencilFaceFlags is a bitmask type for setting a mask of zero or more VkStencilFaceFlagBits.

To dynamically set the stencil write mask call:

// Provided by VK_VERSION_1_0
void vkCmdSetStencilWriteMask(
    VkCommandBuffer                             commandBuffer,
    VkStencilFaceFlags                          faceMask,
    uint32_t                                    writeMask);
  • commandBuffer is the command buffer into which the command will be recorded.

  • faceMask is a bitmask of VkStencilFaceFlagBits specifying the set of stencil state for which to update the write mask, as described above for vkCmdSetStencilCompareMask.

  • writeMask is the new value to use as the stencil write mask.

This command sets the state for a given draw when the graphics pipeline is created with VK_DYNAMIC_STATE_STENCIL_WRITE_MASK set in VkPipelineDynamicStateCreateInfo::pDynamicStates.

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

  • faceMask must be a valid combination of VkStencilFaceFlagBits values

  • faceMask must not be 0

  • 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

To dynamically set the stencil reference value call:

// Provided by VK_VERSION_1_0
void vkCmdSetStencilReference(
    VkCommandBuffer                             commandBuffer,
    VkStencilFaceFlags                          faceMask,
    uint32_t                                    reference);
  • commandBuffer is the command buffer into which the command will be recorded.

  • faceMask is a bitmask of VkStencilFaceFlagBits specifying the set of stencil state for which to update the reference value, as described above for vkCmdSetStencilCompareMask.

  • reference is the new value to use as the stencil reference value.

This command sets the state for a given draw when the graphics pipeline is created with VK_DYNAMIC_STATE_STENCIL_REFERENCE set in VkPipelineDynamicStateCreateInfo::pDynamicStates.

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

  • faceMask must be a valid combination of VkStencilFaceFlagBits values

  • faceMask must not be 0

  • 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

Possible values of VkStencilOpState::compareOp, specifying the stencil comparison function, are:

// Provided by VK_VERSION_1_0
typedef enum VkCompareOp {
    VK_COMPARE_OP_NEVER = 0,
    VK_COMPARE_OP_LESS = 1,
    VK_COMPARE_OP_EQUAL = 2,
    VK_COMPARE_OP_LESS_OR_EQUAL = 3,
    VK_COMPARE_OP_GREATER = 4,
    VK_COMPARE_OP_NOT_EQUAL = 5,
    VK_COMPARE_OP_GREATER_OR_EQUAL = 6,
    VK_COMPARE_OP_ALWAYS = 7,
} VkCompareOp;
  • VK_COMPARE_OP_NEVER specifies that the test evaluates to false.

  • VK_COMPARE_OP_LESS specifies that the test evaluates A < B.

  • VK_COMPARE_OP_EQUAL specifies that the test evaluates A = B.

  • VK_COMPARE_OP_LESS_OR_EQUAL specifies that the test evaluates A ≤ B.

  • VK_COMPARE_OP_GREATER specifies that the test evaluates A > B.

  • VK_COMPARE_OP_NOT_EQUAL specifies that the test evaluates A ≠ B.

  • VK_COMPARE_OP_GREATER_OR_EQUAL specifies that the test evaluates A ≥ B.

  • VK_COMPARE_OP_ALWAYS specifies that the test evaluates to true.

Possible values of the failOp, passOp, and depthFailOp members of VkStencilOpState, specifying what happens to the stored stencil value if this or certain subsequent tests fail or pass, are:

// Provided by VK_VERSION_1_0
typedef enum VkStencilOp {
    VK_STENCIL_OP_KEEP = 0,
    VK_STENCIL_OP_ZERO = 1,
    VK_STENCIL_OP_REPLACE = 2,
    VK_STENCIL_OP_INCREMENT_AND_CLAMP = 3,
    VK_STENCIL_OP_DECREMENT_AND_CLAMP = 4,
    VK_STENCIL_OP_INVERT = 5,
    VK_STENCIL_OP_INCREMENT_AND_WRAP = 6,
    VK_STENCIL_OP_DECREMENT_AND_WRAP = 7,
} VkStencilOp;
  • VK_STENCIL_OP_KEEP keeps the current value.

  • VK_STENCIL_OP_ZERO sets the value to 0.

  • VK_STENCIL_OP_REPLACE sets the value to reference.

  • VK_STENCIL_OP_INCREMENT_AND_CLAMP increments the current value and clamps to the maximum representable unsigned value.

  • VK_STENCIL_OP_DECREMENT_AND_CLAMP decrements the current value and clamps to 0.

  • VK_STENCIL_OP_INVERT bitwise-inverts the current value.

  • VK_STENCIL_OP_INCREMENT_AND_WRAP increments the current value and wraps to 0 when the maximum value would have been exceeded.

  • VK_STENCIL_OP_DECREMENT_AND_WRAP decrements the current value and wraps to the maximum possible value when the value would go below 0.

For purposes of increment and decrement, the stencil bits are considered as an unsigned integer.

25.7. Depth Test

The depth test compares the depth value za in the depth/stencil attachment at each sample’s framebuffer coordinates (xf,yf) and sample index i against the sample’s depth value zf.

If the depth test is not enabled, as specified by VkPipelineDepthStencilStateCreateInfo::depthTestEnable, or if there is no depth attachment, the coverage mask is unmodified by this operation.

The comparison performed is based on the VkCompareOp, set by VkPipelineDepthStencilStateCreateInfo::depthCompareOp during pipeline creation. zf and za are used as A and B, respectively, in the operation specified by the VkCompareOp.

If VkPipelineRasterizationStateCreateInfo::depthClampEnable is enabled, before the sample’s zf is compared to za, zf is clamped to [min(n,f),max(n,f)], where n and f are the minDepth and maxDepth depth range values of the viewport used by this fragment, respectively.

If the comparison evaluates to false, the coverage for the sample is set to 0.

If depth writes are enabled, as specified by VkPipelineDepthStencilStateCreateInfo::depthWriteEnable, and the comparison evaluated to true, the depth attachment value za is set to the sample’s depth value zf.

25.8. Sample Counting

Occlusion queries use query pool entries to track the number of samples that pass all the per-fragment tests. The mechanism of collecting an occlusion query value is described in Occlusion Queries.

The occlusion query sample counter increments by one for each sample with a coverage value of 1 in each fragment that survives all the per-fragment tests, including scissor, sample mask, alpha to coverage, stencil, and depth tests.

25.9. Coverage Reduction

Coverage reduction generates a color sample mask from the coverage mask, with one bit for each sample in the color attachment(s) for the subpass. If a bit in the color sample mask is 0, then blending and writing to the framebuffer are not performed for that sample.

If the fragment covers a single pixel with a number of samples equal to the number of samples in each color attachment, each color sample is associated with a unique rasterization sample, and the value of the coverage mask is assigned to the color sample mask.