// Copyright (c) 2015-2016 The Khronos Group Inc. // Copyright notice at https://www.khronos.org/registry/speccopyright.html [[fragops]] = Fragment Operations [[fragops-early]] == Early Per-Fragment Tests Once fragments are produced by rasterization, a number of per-fragment operations are performed prior to fragment shader execution. If a fragment is discarded during any of these operations, it will not be processed by any subsequent stage, including fragment shader execution. Two fragment operations are performed in the following order: * the scissor test (see <>) * multisample fragment operations (see <>) If early per-fragment operations are <>, these tests are also performed in the following order: * the depth bounds tests (see <>) * the stencil test (see <>) * the depth test (see <>) * sample counting (see <>) [[fragops-scissor]] == Scissor Test The scissor test determines if a fragment's framebuffer coordinates latexmath:[$(x_f,y_f)$] lie within the scissor rectangle corresponding to the viewport index (see <>) used by the primitive that generated the fragment. If the pipeline state object is created without ename:VK_DYNAMIC_STATE_SCISSOR enabled then the scissor rectangles are set by the slink:VkPipelineViewportStateCreateInfo state of the pipeline state object. Otherwise, to dynamically set the scissor rectangles call: include::../protos/vkCmdSetScissor.txt[] * pname:commandBuffer is the command buffer into which the command will be recorded. * pname:firstScissor is the index of the first scissor whose state is updated by the command. * pname:scissorCount is the number of scissors whose rectangles are updated by the command. * pname:pScissors is a pointer to an array of slink:VkRect2D structures defining scissor rectangles. The scissor rectangles taken from element latexmath:[$i$] of pname:pScissors replace the current state for the scissor index latexmath:[$\mathit{firstScissor}+i$], for latexmath:[$i$] in latexmath:[$[0, scissorCount)$]. Each scissor rectangle is described by a slink:VkRect2D structure, with the pname:offset.x and pname:offset.y values determining the upper left corner of the scissor rectangle, and the pname:extent.width and pname:extent.height values determining the size in pixels. include::../validity/protos/vkCmdSetScissor.txt[] If latexmath:[$\mathit{offset.x} \le x_f \lt \mathit{offset.x} + \mathit{extent.width}$] and latexmath:[$\mathit{offset.y} \le y_f \lt \mathit{offset.y} + \mathit{extent.height}$] for the selected scissor rectangle, then the scissor test passes. Otherwise, the test fails and the fragment is discarded. For points, lines, and polygons, the scissor rectangle for a primitive is selected in the same manner as the viewport (see <>). The scissor rectangles only apply to drawing commands, not to other commands like clears or copies. It is legal for latexmath:[$\mathit{offset.x} + \mathit{extent.width}$] or latexmath:[$\mathit{offset.y} + \mathit{extent.height}$] to exceed the dimensions of the framebuffer - the scissor test still applies as defined above. Rasterization does not produce fragments outside of the framebuffer, so such fragments never have the scissor test performed on them. The scissor test is always performed. Applications can: effectively disable the scissor test by specifying a scissor rectangle that encompasses the entire framebuffer. [[fragops-samplemask]] == Sample Mask This step modifies fragment coverage values based on the values in the pname:pSampleMask array member of slink:VkPipelineMultisampleStateCreateInfo, as described previously in section <>. pname:pSampleMask contains a bitmask of static coverage information that is code:ANDed with the coverage information generated during rasterization. Bits that are zero disable coverage for the corresponding sample. Bit B of mask word M corresponds to sample latexmath:[$32 \times M + B$]. The array is sized to a length of latexmath:[$\lceil{rasterizationSamples / 32}\rceil$] words. If pname:pSampleMask is `NULL`, it is treated as if the mask has all bits enabled, i.e. no coverage is removed from fragments. [[fragops-early-mode]] == Early Fragment Test Mode The depth bounds test, stencil test, depth test, and occlusion query sample counting are performed before fragment shading if and only if early fragment tests are enabled by the fragment shader (see <>). When early per-fragment operations are enabled, these operations are performed prior to fragment shader execution, and the stencil buffer, depth buffer, and occlusion query sample counts will be updated accordingly; these operations will not be performed again after fragment shader execution. If a pipeline's fragment shader has early fragment tests disabled, these operations are performed only after fragment program execution, in the order described below. If a pipeline does not contain a fragment shader, these operations are performed only once. If early fragment tests are enabled, any depth value computed by the fragment shader has no effect. Additionally, the depth test (including depth writes), stencil test (including stencil writes) and sample counting operations are performed even for fragments or samples that would be discarded after fragment shader execution due to per-fragment operations such as alpha-to-coverage tests, or due to the fragment being discarded by the shader itself. [[fragops-late]] == Late Per-Fragment Tests After programmable fragment processing, per-fragment operations are performed before blending and color output to the framebuffer. A fragment is produced by rasterization with framebuffer coordinates of latexmath:[$(x_f,y_f)$] and depth latexmath:[$z$], as described in <>. The fragment is then modified by programmable fragment processing, which adds associated data as described in <>. The fragment is then further modified, and possibly discarded by the late per-fragment operations described in this chapter. These operations are diagrammed in figure <>, in the order in which they are performed. Finally, if the fragment was not discarded, it is used to update the framebuffer at the fragment's framebuffer coordinates for any samples that remain covered. The depth bounds test, stencil test, and depth test are performed for each pixel sample, rather than just once for each fragment. Stencil and depth operations are performed for a pixel sample only if that sample's fragment coverage bit is a value of 1 when the fragment executes the corresponding stage of the graphics pipeline. If the corresponding coverage bit is 0, no operations are performed for that sample. Failure of the depth bounds, stencil, or depth test results in termination of the processing of that sample by means of disabling coverage for that sample, rather than discarding of the fragment. If, at any point, a fragment's coverage becomes zero for all samples, then the fragment is discarded. All operations are performed on the depth and stencil values stored in the depth/stencil attachment of the framebuffer. The contents of the color attachments are not modified at this point. The depth bounds test, stencil test, depth test, and occlusion query operations described in <>, <>, <>, <> are instead performed prior to fragment processing, as described in <>, if requested by the fragment shader. [[fragops-covg]] == Multisample Coverage If a fragment shader is active and its entry point's interface includes a built-in output variable decorated with code:SampleMask, the fragment coverage is code:ANDed with the bits of the sample mask to generate a new fragment coverage value. If such a fragment shader did not assign a value to code:SampleMask due to flow of control, the value code:ANDed with the fragment coverage is undefined. If no fragment shader is active, or if the active fragment shader does not include code:SampleMask in its interface, the fragment coverage is not modified. Next, the fragment alpha and coverage values are modified based on the pname:alphaToCoverageEnable and pname:alphaToOneEnable members of the slink:VkPipelineMultisampleStateCreateInfo structure. All alpha values in this section refer only to the alpha component of the fragment shader output that has a code:Location and code:Index decoration of zero (see <>). If that shader output has an integer or unsigned integer type, then these operations are skipped. If pname:alphaToCoverageEnable is enabled, a temporary coverage value is generated where each bit is determined by the fragment's alpha value. The temporary coverage value is then ANDed with the fragment coverage value to generate a new fragment coverage value. 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 latexmath:[$[0,1\]$]), with all 1's corresponding to a value of 1.0 and all 0's corresponding to 0.0. The algorithm may: be different at different pixel locations. [NOTE] .Note ==== Using different algorithms at different pixel location may: help to avoid artifacts caused by regular coverage sample locations. ==== Next, if pname:alphaToOneEnable is enabled, each alpha value is replaced by the maximum representable alpha value for fixed-point color buffers, or by 1.0 for floating-point buffers. Otherwise, the alpha values are not changed. [[fragops-ds-state]] == Depth and Stencil Operations Pipeline state controlling the <>, <>, and <> is specified through the members of sname:VkPipelineDepthStencilStateCreateInfo: include::../structs/VkPipelineDepthStencilStateCreateInfo.txt[] The members of sname:VkPipelineDepthStencilStateCreateInfo structure are as follows: * pname:sType is the type of this structure. * pname:pNext is `NULL` or a pointer to an extension-specific structure. * pname:flags is reserved for future use. * pname:depthTestEnable controls whether <> is enabled. * pname:depthWriteEnable controls whether <> are enabled. * pname:depthCompareOp is the comparison operator used in the <>. * pname:depthBoundsTestEnable controls whether <> is enabled. * pname:stencilTestEnable controls whether <> is enabled. * pname:front and pname:back control the parameters of the <>. * pname:minDepthBounds and pname:maxDepthBounds define the range of values used in the <>. include::../validity/structs/VkPipelineDepthStencilStateCreateInfo.txt[] [[fragops-dbt]] == Depth Bounds Test The depth bounds test conditionally disables coverage of a sample based on the outcome of a comparison between the value latexmath:[$z_a$] in the depth attachment at location latexmath:[$(x_f,y_f)$] (for the appropriate sample) and a range of values. The test is enabled or disabled by the pname:depthBoundsTestEnable member of slink:VkPipelineDepthStencilStateCreateInfo: If the pipeline state object is created without the ename:VK_DYNAMIC_STATE_DEPTH_BOUNDS dynamic state enabled then the range of values used in the depth bounds test are defined by the pname:minDepthBounds and pname:maxDepthBounds members of the slink:VkPipelineDepthStencilStateCreateInfo structure. Otherwise, to dynamically set the depth bounds range values call: include::../protos/vkCmdSetDepthBounds.txt[] * pname:commandBuffer is the command buffer into which the command will be recorded. * pname:minDepthBounds is the lower bound of the range of depth values used in the depth bounds test. * pname:maxDepthBounds is the upper bound of the range. include::../validity/protos/vkCmdSetDepthBounds.txt[] If latexmath:[$\mathit{minDepthBounds} \leq z_a \leq \mathit{maxDepthBounds}$], then the depth bounds test passes. Otherwise, the test fails and the sample's coverage bit is cleared in the fragment. If there is no depth framebuffer attachment or if the depth bounds test is disabled, it is as if the depth bounds test always passes. [[fragops-stencil]] == Stencil Test The stencil test conditionally disables coverage of a sample based on the outcome of a comparison between the stencil value in the depth/stencil attachment at location latexmath:[$(x_f,y_f)$] (for the appropriate sample) and a reference value. The stencil test also updates the value in the stencil attachment, depending on the test state, the stencil value and the stencil write masks. The test is enabled or disabled by the pname:stencilTestEnable member of slink:VkPipelineDepthStencilStateCreateInfo. When disabled, the stencil test and associated modifications are not made, and the sample's coverage is not modified. The stencil test is controlled with the pname:front and pname:back members of sname:VkPipelineDepthStencilStateCreateInfo which are of type sname:VkStencilOpState. The definition of sname:VkStencilOpState is: include::../structs/VkStencilOpState.txt[] The members of sname:VkStencilOpState structure are as follows: * pname:failOp is the action performed on samples that fail the stencil test. * pname:passOp is the action performed on samples that pass both the depth and stencil tests. * pname:depthFailOp is the action performed on samples that pass the stencil test and fail the depth test. * pname:compareOp is the comparison operator used in the stencil test. * pname:compareMask selects the bits of the unsigned integer stencil values participating in the stencil test. * pname:writeMask selects the bits of the unsigned integer stencil values updated by the stencil test in the stencil framebuffer attachment. * pname:reference is an integer reference value that is used in the unsigned stencil comparison. include::../validity/structs/VkStencilOpState.txt[] There are two sets of stencil-related state, the front stencil state set and the back stencil state set. Stencil tests and writes use the front set of stencil state when processing fragments rasterized from non-polygon primitives (points and lines) and front-facing polygon primitives while the back set of stencil state is used when processing fragments rasterized from back-facing polygon primitives. For the purposes of stencil testing, a primitive is still considered a polygon even if the polygon is to be rasterized as points or lines due to the current elink:VkPolygonMode. Whether a polygon is front- or back-facing is determined in the same manner used for face culling (see <>). The operation of the stencil test is also affected by the pname:compareMask, pname:writeMask, and pname:reference members of sname:VkStencilOpState set in the pipeline state object if the pipeline state object is created without the ename:VK_DYNAMIC_STATE_STENCIL_COMPARE_MASK, ename:VK_DYNAMIC_STATE_STENCIL_WRITE_MASK, and ename:VK_DYNAMIC_STATE_STENCIL_REFERENCE dynamic states enabled, respectively. If the pipeline state object is created with the ename:VK_DYNAMIC_STATE_STENCIL_COMPARE_MASK dynamic state enabled, then to dynamically set the stencil compare mask call: include::../protos/vkCmdSetStencilCompareMask.txt[] * pname:commandBuffer is the command buffer into which the command will be recorded. * pname:faceMask is a bitmask specifying the set of stencil state for which to update the compare mask, and may include the bits: + -- include::../enums/VkStencilFaceFlagBits.txt[] ** ename:VK_STENCIL_FACE_FRONT_BIT indicates that only the front set of stencil state is updated. ** ename:VK_STENCIL_FACE_BACK_BIT indicates that only the back set of stencil state is updated. ** ename:VK_STENCIL_FRONT_AND_BACK is the combination of ename:VK_STENCIL_FACE_FRONT_BIT and ename:VK_STENCIL_FACE_BACK_BIT and indicates that both sets of stencil state are updated. -- * pname:compareMask is the new value to use as the stencil compare mask. include::../validity/protos/vkCmdSetStencilCompareMask.txt[] If the pipeline state object is created with the ename:VK_DYNAMIC_STATE_STENCIL_WRITE_MASK dynamic state enabled, then to dynamically set the stencil write mask call: include::../protos/vkCmdSetStencilWriteMask.txt[] * pname:commandBuffer is the command buffer into which the command will be recorded. * pname:faceMask is a bitmask of elink:VkStencilFaceFlagBits specifying the set of stencil state for which to update the write mask, as described above for flink:vkCmdSetStencilCompareMask. * pname:writeMask is the new value to use as the stencil write mask. include::../validity/protos/vkCmdSetStencilWriteMask.txt[] If the pipeline state object is created with the ename:VK_DYNAMIC_STATE_STENCIL_REFERENCE dynamic state enabled, then to dynamically set the stencil reference value call: include::../protos/vkCmdSetStencilReference.txt[] * pname:commandBuffer is the command buffer into which the command will be recorded. * pname:faceMask is a bitmask of elink:VkStencilFaceFlagBits specifying the set of stencil state for which to update the reference value, as described above for flink:vkCmdSetStencilCompareMask. * pname:reference is the new value to use as the stencil reference value. include::../validity/protos/vkCmdSetStencilReference.txt[] pname:reference is an integer reference value that is used in the unsigned stencil comparison. Stencil comparison clamps the reference value to latexmath:[$[0,2^s-1\]$], where latexmath:[$s$] is the number of bits in the stencil framebuffer attachment. The latexmath:[$s$] least significant bits of pname:compareMask are bitwise code:ANDed with both the reference and the stored stencil value, and the resulting masked values are those that participate in the comparison controlled by pname:compareOp. Let latexmath:[$R$] be the masked reference value and latexmath:[$S$] be the masked stored stencil value. pname:compareOp is a symbolic constant that determines the stencil comparison function: include::../enums/VkCompareOp.txt[] * ename:VK_COMPARE_OP_NEVER: the test never passes. * ename:VK_COMPARE_OP_LESS: the test passes when latexmath:[$R \lt S$]. * ename:VK_COMPARE_OP_EQUAL: the test passes when latexmath:[$R = S$]. * ename:VK_COMPARE_OP_LESS_OR_EQUAL: the test passes when latexmath:[$R \leq S$]. * ename:VK_COMPARE_OP_GREATER: the test passes when latexmath:[$R \gt S$]. * ename:VK_COMPARE_OP_NOT_EQUAL: the test passes when latexmath:[$R \neq S$]. * ename:VK_COMPARE_OP_GREATER_OR_EQUAL: the test passes when latexmath:[$R \geq S$]. * ename:VK_COMPARE_OP_ALWAYS: the test always passes. As described earlier, the pname:failOp, pname:passOp, and pname:depthFailOp members of pname:VkStencilOpState indicate what happens to the stored stencil value if this or certain subsequent tests fail or pass. Each enum is of type ename:VkStencilOp, which is defined as: include::../enums/VkStencilOp.txt[] The possible values are: * ename:VK_STENCIL_OP_KEEP keeps the current value. * ename:VK_STENCIL_OP_ZERO sets the value to 0. * ename:VK_STENCIL_OP_REPLACE sets the value to pname:reference. * ename:VK_STENCIL_OP_INCREMENT_AND_CLAMP increments the current value and clamps to the maximum representable unsigned value. * ename:VK_STENCIL_OP_DECREMENT_AND_CLAMP decrements the current value and clamps to 0. * ename:VK_STENCIL_OP_INVERT bitwise-inverts the current value. * ename:VK_STENCIL_OP_INCREMENT_AND_WRAP increments the current value and wraps to 0 when the maximum value would have been exceeded. * ename:VK_STENCIL_OP_DECREMENT_AND_WRAP decrements the current value and wraps to the maximum possible value when the value would go below 0. For purposes of increment and decrement, the stencil bits are considered as an unsigned integer. If the stencil test fails, the sample's coverage bit is cleared in the fragment. If there is no stencil framebuffer attachment, stencil modification cannot: occur, and it is as if the stencil tests always pass. If the stencil test passes, the pname:writeMask member of the slink:VkStencilOpState structures controls how the updated stencil value is written to the stencil framebuffer attachment. The least significant latexmath:[$s$] bits of pname:writeMask, where latexmath:[$s$] is the number of bits in the stencil framebuffer attachment, specify an integer mask. Where a latexmath:[$1$] appears in this mask, the corresponding bit in the stencil value in the depth/stencil attachment is written; where a latexmath:[$0$] appears, the bit is not written. The pname:writeMask value uses either the front-facing or back-facing state based on the facing-ness of the fragment. Fragments generated by front-facing primitives use the front mask and fragments generated by back-facing primitives use the back mask. [[fragops-depth]] == Depth Test The depth test conditionally disables coverage of a sample based on the outcome of a comparison between the fragment's depth value at the sample location and the sample's depth value in the depth/stencil attachment at location latexmath:[$(x_f,y_f)$]. The comparison is enabled or disabled with the pname:depthTestEnable member of the slink:VkPipelineDepthStencilStateCreateInfo structure. When disabled, the depth comparison and subsequent possible updates to the value of the depth component of the depth/stencil attachment are bypassed and the fragment is passed to the next operation. The stencil value, however, can: be modified as indicated above as if the depth test passed. If enabled, the comparison takes place and the depth/stencil attachment value can: subsequently be modified. The comparison is specified with the pname:depthCompareOp member of slink:VkPipelineDepthStencilStateCreateInfo. Let latexmath:[$z_f$] be the incoming fragment's depth value for a sample, and let latexmath:[$z_a$] be the depth/stencil attachment value in memory for that sample. The depth test passes under the following conditions: * ename:VK_COMPARE_OP_NEVER: the test never passes. * ename:VK_COMPARE_OP_LESS: the test passes when latexmath:[$z_f \lt z_a$]. * ename:VK_COMPARE_OP_EQUAL: the test passes when latexmath:[$z_f = z_a$]. * ename:VK_COMPARE_OP_LESS_OR_EQUAL: the test passes when latexmath:[$z_f \leq z_a$]. * ename:VK_COMPARE_OP_GREATER: the test passes when latexmath:[$z_f \gt z_a$]. * ename:VK_COMPARE_OP_NOT_EQUAL: the test passes when latexmath:[$z_f \neq z_a$]. * ename:VK_COMPARE_OP_GREATER_OR_EQUAL: the test passes when latexmath:[$z_f \geq z_a$]. * ename:VK_COMPARE_OP_ALWAYS: the test always passes. If depth clamping (see <>) is enabled, before the incoming fragment's latexmath:[$z_f$] is compared to latexmath:[$z_a$], latexmath:[$z_f$] is clamped to latexmath:[$[\min(n,f), \max(n,f)\]$], where latexmath:[$n$] and latexmath:[$f$] are the pname:minDepth and pname:maxDepth depth range values of the viewport used by this fragment, respectively. If the depth test fails, the sample's coverage bit is cleared in the fragment. The stencil value at the sample's location is updated according to the function currently in effect for depth test failure. [[fragops-depth-write]] If the depth test passes, the sample's (possibly clamped) latexmath:[$z_f$] value is conditionally written to the depth framebuffer attachment based on the pname:depthWriteEnable member of slink:VkPipelineDepthStencilStateCreateInfo. If pname:depthWriteEnable is ename:VK_TRUE the value is written, and if it is ename:VK_FALSE the value is not written. The stencil value at the sample's location is updated according to the function currently in effect for depth test success. If there is no depth framebuffer attachment, it is as if the depth test always passes. [[fragops-samplecount]] == 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 <>. 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.