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