Vulkan-Docs/chapters/fragops.txt

// Copyright (c) 2015-2018 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

[[fragops]]
= Fragment Operations

Fragment operations execute on a per-fragment or per-sample basis, affecting
whether or how a fragment or sample is written to the framebuffer.
Some operations execute <<fragops-early, before fragment shading>>, and
others <<fragops-late, after>>.
Fragment operations always adhere to <<primrast-order,rasterization order>>.


[[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.

ifndef::VK_NV_scissor_exclusive[]
The <<fragops-scissor, scissor test>> and
endif::VK_NV_scissor_exclusive[]
ifdef::VK_NV_scissor_exclusive[]
The <<fragops-scissor, scissor test>>, <<fragops-exclusive-scissor,
exclusive scissor test>>, and
endif::VK_NV_scissor_exclusive[]
<<fragops-samplemask, sample mask generation>> are always performed during
early fragment tests.

Fragment operations are performed in the following order:

ifdef::VK_EXT_discard_rectangles[]
  * the discard rectangles test (see <<fragops-discard-rectangles,Discard
    Rectangles Test>>)
endif::VK_EXT_discard_rectangles[]
  * the scissor test (see <<fragops-scissor,Scissor Test>>)
ifdef::VK_NV_scissor_exclusive[]
  * the exclusive scissor test (see <<fragops-exclusive-scissor,Exclusive
    Scissor Test>>)
endif::VK_NV_scissor_exclusive[]
  * multisample fragment operations (see <<fragops-samplemask,Sample Mask>>)

If early per-fragment operations are <<shaders-fragment-earlytest,enabled by
the fragment shader>>, these operations are also performed:

  * <<fragops-dbt, Depth bounds test>>
  * <<fragops-stencil, Stencil test>>
  * <<fragops-depth, Depth test>>
ifdef::VK_NV_representative_fragment_test[]
  * <<fragops-rep-frag-test, Representative fragment test>>
endif::VK_NV_representative_fragment_test[]
  * <<fragops-samplecount, Sample counting>> for <<queries-occlusion,
     occlusion queries>>

ifdef::VK_EXT_post_depth_coverage[]
If post-depth coverage operation is
<<shaders-fragment-earlytest-postdepthcoverage,enabled by the fragment
shader>>, the <<interfaces-builtin-variables-samplemask,code:SampleMask>>
coverage is determined after the early stencil and depth tests.
endif::VK_EXT_post_depth_coverage[]

ifdef::VK_EXT_discard_rectangles[]
[[fragops-discard-rectangles]]
== Discard Rectangles Test

[open,refpage='VkPipelineDiscardRectangleStateCreateInfoEXT',desc='Structure specifying discard rectangle',type='structs']
--

The discard rectangles test determines if fragment's framebuffer coordinates
[eq]#(x~f~,y~f~)# are inclusive or exclusive to a set of discard-space
rectangles.
The discard rectangles are set with the
sname:VkPipelineDiscardRectangleStateCreateInfoEXT pipeline state, which is
defined as:

include::../api/structs/VkPipelineDiscardRectangleStateCreateInfoEXT.txt[]

  * 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:discardRectangleMode is the mode used to determine whether
    fragments that lie within the discard rectangle are discarded or not.
  * pname:discardRectangleCount is the number of discard rectangles used by
    the pipeline.
  * pname:pDiscardRectangles is a pointer to an array of slink:VkRect2D
    structures, defining the discard rectangles.
    If the discard rectangle state is dynamic, this member is ignored.

.Valid Usage
****
  * [[VUID-VkPipelineDiscardRectangleStateCreateInfoEXT-discardRectangleCount-00582]]
    pname:discardRectangleCount must: be between `0` and
    sname:VkPhysicalDeviceDiscardRectanglePropertiesEXT::pname:maxDiscardRectangles,
    inclusive
****

include::../validity/structs/VkPipelineDiscardRectangleStateCreateInfoEXT.txt[]
--

[open,refpage='VkPipelineDiscardRectangleStateCreateFlagsEXT',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkPipelineDiscardRectangleStateCreateFlagsEXT.txt[]

sname:VkPipelineDiscardRectangleStateCreateFlagsEXT is a bitmask type for
setting a mask, but is currently reserved for future use.
--

The sname:VkPipelineDiscardRectangleStateCreateInfoEXT state is set by
adding an instance of this structure to the pname:pNext chain of an instance
of the sname:VkGraphicsPipelineCreateInfo structure and setting the graphics
pipeline state with flink:vkCreateGraphicsPipelines.

If the bound pipeline state object was not created with the
ename:VK_DYNAMIC_STATE_DISCARD_RECTANGLE_EXT dynamic state enabled, discard
rectangles are specified using the pname:pDiscardRectangles member of
sname:VkPipelineDiscardRectangleStateCreateInfoEXT linked to the pipeline
state object.

[open,refpage='vkCmdSetDiscardRectangleEXT',desc='Set discard rectangles dynamically',type='protos']
--

If the pipeline state object was created with the
ename:VK_DYNAMIC_STATE_DISCARD_RECTANGLE_EXT dynamic state enabled, the
discard rectangles are dynamically set and changed with the command:

include::../api/protos/vkCmdSetDiscardRectangleEXT.txt[]

  * pname:commandBuffer is the command buffer into which the command will be
    recorded.
  * pname:firstDiscardRectangle is the index of the first discard rectangle
    whose state is updated by the command.
  * pname:discardRectangleCount is the number of discard rectangles whose
    state are updated by the command.
  * pname:pDiscardRectangles is a pointer to an array of slink:VkRect2D
    structures specifying discard rectangles.

The discard rectangle taken from element [eq]#i# of pname:pDiscardRectangles
replace the current state for the discard rectangle index
[eq]#pname:firstDiscardRectangle {plus} i#, for [eq]#i# in [eq]#[0,
pname:discardRectangleCount)#.

.Valid Usage
****
  * [[VUID-vkCmdSetDiscardRectangleEXT-None-00583]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_DISCARD_RECTANGLE_EXT dynamic state enabled
  * [[VUID-vkCmdSetDiscardRectangleEXT-firstDiscardRectangle-00585]]
    The sum of pname:firstDiscardRectangle and pname:discardRectangleCount
    must: be less than or equal to
    slink:VkPhysicalDeviceDiscardRectanglePropertiesEXT::pname:maxDiscardRectangles
  * [[VUID-vkCmdSetDiscardRectangleEXT-x-00587]]
    The pname:x and pname:y member of pname:offset in each slink:VkRect2D
    element of pname:pDiscardRectangles must: be greater than or equal to
    `0`
  * [[VUID-vkCmdSetDiscardRectangleEXT-offset-00588]]
    Evaluation of [eq]#(pname:offset.x {plus} pname:extent.width)# in each
    slink:VkRect2D element of pname:pDiscardRectangles must: not cause a
    signed integer addition overflow
  * [[VUID-vkCmdSetDiscardRectangleEXT-offset-00589]]
    Evaluation of [eq]#(pname:offset.y {plus} pname:extent.height)# in each
    slink:VkRect2D element of pname:pDiscardRectangles must: not cause a
    signed integer addition overflow
****

include::../validity/protos/vkCmdSetDiscardRectangleEXT.txt[]
--

The sname:VkOffset2D::pname:x and sname:VkOffset2D::pname:y values of the
discard rectangle sname:VkRect2D specify the upper-left origin of the
discard rectangle box.
The lower-right corner of the discard rectangle box is specified as the
sname:VkExtent2D::pname:width and sname:VkExtent2D::pname:height from the
upper-left origin.

If [eq]#pname:offset.x {leq} x~f~ < pname:offset.x {plus}
pname:extent.width# and [eq]#pname:offset.y {leq} y~f~ < pname:offset.y
{plus} pname:extent.height# for the selected discard rectangle, then the
fragment is within the discard rectangle box.
When the discard rectangle mode is
ename:VK_DISCARD_RECTANGLE_MODE_INCLUSIVE_EXT a fragment within at least one
of the active discard rectangle boxes passes the discard rectangle test;
otherwise the fragment fails the discard rectangle test and is discarded.
When the discard rectangle mode is
ename:VK_DISCARD_RECTANGLE_MODE_EXCLUSIVE_EXT a fragment within at least one
of the active discard rectangle boxes fails the discard rectangle test, and
the fragment is discarded; otherwise the fragment passes the discard
rectangles test.
The discard rectangles test only applies to <<drawing, drawing commands>>,
not to other commands like clears or copies.

[open,refpage='VkDiscardRectangleModeEXT',desc='Specify the discard rectangle mode',type='enums']
--

Possible values of
slink:VkPipelineDiscardRectangleStateCreateInfoEXT::pname:discardRectangleMode,
specifying the behavior of the discard rectangle test, are:

include::../api/enums/VkDiscardRectangleModeEXT.txt[]

  * ename:VK_DISCARD_RECTANGLE_MODE_INCLUSIVE_EXT specifies that a fragment
    within any discard rectangle satisfies the test.
  * ename:VK_DISCARD_RECTANGLE_MODE_EXCLUSIVE_EXT specifies that a fragment
    not within any of the discard rectangles satisfies the test.

--

ifdef::VK_NV_shading_rate_image[]

When the use of a shading rate image results in a fragment covering multiple
pixels, the discard rectangle test is performed independently for each pixel
in the fragment.
If a pixel covered by a fragment fails the discard rectangle test, all
samples in the fragment associated with that pixel are treated as not
covered.
If the discard rectangle test results in a fragment with no samples covered,
that fragment is discarded.

endif::VK_NV_shading_rate_image[]

endif::VK_EXT_discard_rectangles[]


[[fragops-scissor]]
== Scissor Test

[open,refpage='vkCmdSetScissor',desc='Set the dynamic scissor rectangles on a command buffer',type='protos']
--

The scissor test determines if a fragment's framebuffer coordinates
[eq]#(x~f~,y~f~)# lie within the scissor rectangle corresponding to the
viewport index (see <<vertexpostproc-viewport,Controlling the Viewport>>)
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::../api/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 [eq]#i# of pname:pScissors replace
the current state for the scissor index [eq]#pname:firstScissor {plus} i#,
for [eq]#i# in [eq]#[0, pname: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.

.Valid Usage
****
  * [[VUID-vkCmdSetScissor-None-00590]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_SCISSOR dynamic state enabled
  * [[VUID-vkCmdSetScissor-firstScissor-00591]]
    pname:firstScissor must: be less than
    sname:VkPhysicalDeviceLimits::pname:maxViewports
  * [[VUID-vkCmdSetScissor-firstScissor-00592]]
    The sum of pname:firstScissor and pname:scissorCount must: be between
    `1` and sname:VkPhysicalDeviceLimits::pname:maxViewports, inclusive
  * [[VUID-vkCmdSetScissor-firstScissor-00593]]
    If the <<features-features-multiViewport,multiple viewports>> feature is
    not enabled, pname:firstScissor must: be `0`
  * [[VUID-vkCmdSetScissor-scissorCount-00594]]
    If the <<features-features-multiViewport,multiple viewports>> feature is
    not enabled, pname:scissorCount must: be `1`
  * [[VUID-vkCmdSetScissor-x-00595]]
    The pname:x and pname:y members of pname:offset must: be greater than or
    equal to `0`
  * [[VUID-vkCmdSetScissor-offset-00596]]
    Evaluation of [eq]#(pname:offset.x {plus} pname:extent.width)# must: not
    cause a signed integer addition overflow
  * [[VUID-vkCmdSetScissor-offset-00597]]
    Evaluation of [eq]#(pname:offset.y {plus} pname:extent.height)# must:
    not cause a signed integer addition overflow
****

include::../validity/protos/vkCmdSetScissor.txt[]
--

If [eq]#pname:offset.x {leq} x~f~ < pname:offset.x {plus}
pname:extent.width# and [eq]#pname:offset.y {leq} y~f~ < pname:offset.y
{plus} pname: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
<<vertexpostproc-viewport,Controlling the Viewport>>).
The scissor rectangles test only applies to <<drawing, drawing commands>>,
not to other commands like clears or copies.

It is legal for [eq]#pname:offset.x {plus} pname:extent.width# or
[eq]#pname:offset.y {plus} pname: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.

ifdef::VK_NV_shading_rate_image[]

When the use of a shading rate image results in a fragment covering multiple
pixels, the scissor test is performed independently for each pixel in the
fragment.
If a pixel covered by a fragment fails the scissor test, all samples in the
fragment associated with that pixel are treated as not covered.
If the scissor test results in a fragment with no samples covered, that
fragment is discarded.

endif::VK_NV_shading_rate_image[]



ifdef::VK_NV_scissor_exclusive[]
[[fragops-exclusive-scissor]]
== Exclusive Scissor Test

The exclusive scissor test determines if a pixel's framebuffer coordinates
[eq]#(x~f~,y~f~)# lie outside the exclusive scissor rectangle corresponding
to the viewport index (see <<vertexpostproc-viewport,Controlling the
Viewport>>) used by the primitive that generated the fragment.
The exclusive scissor test behaves identically to the
<<fragops-scissor,scissor test>>, except that it passes only if the pixel is
outside the rectangle instead of passing if the pixel is inside the
rectangle.

[open,refpage='VkPipelineViewportExclusiveScissorStateCreateInfoNV',desc='Structure specifying parameters controlling exclusive scissor testing',type='structs']
--

If the pname:pNext chain of sname:VkPipelineViewportStateCreateInfo includes
a sname:VkPipelineViewportExclusiveScissorStateCreateInfoNV structure, then
that structure includes parameters that affect the exclusive scissor test.

The sname:VkPipelineViewportExclusiveScissorStateCreateInfoNV structure is
defined as:

include::../api/structs/VkPipelineViewportExclusiveScissorStateCreateInfoNV.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:exclusiveScissorCount is the number of exclusive scissor
    rectangles used by the pipeline.
  * pname:pExclusiveScissors is a pointer to an array of slink:VkRect2D
    structures defining exclusive scissor rectangles.
    If the exclusive scissor state is dynamic, this member is ignored.

If this structure is not present, pname:exclusiveScissorCount is considered
to be `0` and the exclusive scissor test is disabled.

.Valid Usage
****
  * [[VUID-VkPipelineViewportExclusiveScissorStateCreateInfoNV-exclusiveScissorCount-02027]]
    If the <<features-features-multiViewport,multiple viewports>> feature is
    not enabled, pname:exclusiveScissorCount must: be `0` or `1`
  * [[VUID-VkPipelineViewportExclusiveScissorStateCreateInfoNV-exclusiveScissorCount-02028]]
    pname:exclusiveScissorCount must: be less than or equal to
    sname:VkPhysicalDeviceLimits::pname:maxViewports
  * [[VUID-VkPipelineViewportExclusiveScissorStateCreateInfoNV-exclusiveScissorCount-02029]]
    pname:exclusiveScissorCount must: be `0` or identical to the
    pname:viewportCount member of sname:VkPipelineViewportStateCreateInfo
  * [[VUID-VkPipelineViewportExclusiveScissorStateCreateInfoNV-pDynamicStates-02030]]
    If no element of the pname:pDynamicStates member of pname:pDynamicState
    is ename:VK_DYNAMIC_STATE_EXCLUSIVE_SCISSOR_NV and
    pname:exclusiveScissorCount is not `0`, pname:pExclusiveScissors must:
    be a valid pointer to an array of pname:exclusiveScissorCount
    sname:VkRect2D structures

****
include::../validity/structs/VkPipelineViewportExclusiveScissorStateCreateInfoNV.txt[]
--

[open,refpage='vkCmdSetExclusiveScissorNV',desc='Set the dynamic exclusive scissor rectangles on a command buffer',type='protos']
--

If the pipeline state object is created with
ename:VK_DYNAMIC_STATE_EXCLUSIVE_SCISSOR_NV enabled, then the exclusive
scissor rectangles are set by:

include::../api/protos/vkCmdSetExclusiveScissorNV.txt[]

  * pname:commandBuffer is the command buffer into which the command will be
    recorded.
  * pname:firstExclusiveScissor is the index of the first exclusive scissor
    rectangle whose state is updated by the command.
  * pname:exclusiveScissorCount is the number of exclusive scissor
    rectangles updated by the command.
  * pname:pExclusiveScissors is a pointer to an array of slink:VkRect2D
    structures defining exclusive scissor rectangles.

The scissor rectangles taken from element [eq]#i# of
pname:pExclusiveScissors replace the current state for the scissor index
[eq]#pname:firstExclusiveScissor {plus} i#, for [eq]#i# in [eq]#[0,
pname:exclusiveScissorCount)#.

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.

.Valid Usage
****
  * [[VUID-vkCmdSetExclusiveScissorNV-None-02031]]
    The <<features-features-exclusiveScissor,exclusive scissor>> feature
    must: be enabled.
  * [[VUID-vkCmdSetExclusiveScissorNV-None-02032]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_EXCLUSIVE_SCISSOR_NV dynamic state enabled
  * [[VUID-vkCmdSetExclusiveScissorNV-firstExclusiveScissor-02033]]
    pname:firstExclusiveScissor must: be less than
    sname:VkPhysicalDeviceLimits::pname:maxViewports
  * [[VUID-vkCmdSetExclusiveScissorNV-firstExclusiveScissor-02034]]
    The sum of pname:firstExclusiveScissor and pname:exclusiveScissorCount
    must: be between `1` and
    sname:VkPhysicalDeviceLimits::pname:maxViewports, inclusive
  * [[VUID-vkCmdSetExclusiveScissorNV-firstExclusiveScissor-02035]]
    If the <<features-features-multiViewport,multiple viewports>> feature is
    not enabled, pname:firstExclusiveScissor must: be `0`
  * [[VUID-vkCmdSetExclusiveScissorNV-exclusiveScissorCount-02036]]
    If the <<features-features-multiViewport,multiple viewports>> feature is
    not enabled, pname:exclusiveScissorCount must: be `1`
  * [[VUID-vkCmdSetExclusiveScissorNV-x-02037]]
    The pname:x and pname:y members of pname:offset in each member of
    pname:pExclusiveScissors must: be greater than or equal to `0`
  * [[VUID-vkCmdSetExclusiveScissorNV-offset-02038]]
    Evaluation of [eq]#(pname:offset.x {plus} pname:extent.width)# for each
    member of pname:pExclusiveScissors must: not cause a signed integer
    addition overflow
  * [[VUID-vkCmdSetExclusiveScissorNV-offset-02039]]
    Evaluation of [eq]#(pname:offset.y {plus} pname:extent.height)# for each
    member of pname:pExclusiveScissors must: not cause a signed integer
    addition overflow
****

include::../validity/protos/vkCmdSetExclusiveScissorNV.txt[]
--

If [eq]#pname:offset.x {leq} x~f~ < pname:offset.x {plus}
pname:extent.width# and [eq]#pname:offset.y {leq} y~f~ < pname:offset.y
{plus} pname:extent.height# for the selected exclusive scissor rectangle,
then the exclusive scissor test fails and the fragment is discarded.
Otherwise, the exclusive scissor test passes.
For points, lines, and polygons, the exclusive scissor rectangle for a
primitive is selected in the same manner as the viewport (see
<<vertexpostproc-viewport,Controlling the Viewport>>).
The exclusive scissor test only applies to <<drawing, drawing commands>>,
not to other commands like clears or copies.

It is legal for [eq]#pname:offset.x {plus} pname:extent.width# or
[eq]#pname:offset.y {plus} pname:extent.height# to exceed the dimensions of
the framebuffer - the exclusive scissor test still applies as defined above.
Rasterization does not produce fragments outside of the framebuffer, so such
fragments never have the exclusive scissor test performed on them.

The exclusive scissor test is performed if and only if the current pipeline
was created with a non-zero pname:exclusiveScissorCount.
Applications can: effectively disable the exclusive scissor test for
specific viewports by specifying a scissor rectangle with a width or height
of zero.

ifdef::VK_NV_shading_rate_image[]

When the use of a shading rate image results in a fragment covering multiple
pixels, the exclusive scissor test is performed independently for each pixel
in the fragment.
If a pixel covered by a fragment fails the exclusive scissor test, all
samples in the fragment associated with that pixel are treated as not
covered.
If the exclusive scissor test results in a fragment with no samples covered,
that fragment is discarded.

endif::VK_NV_shading_rate_image[]

endif::VK_NV_scissor_exclusive[]


[[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 <<pipelines-graphics>>.

pname:pSampleMask contains an array 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 [eq]#B# of mask word [eq]#M# corresponds to sample [eq]#32 {times} M
{plus} B#.
The array is sized to a length of [eq]#{lceil} pname: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.

[open,refpage='VkSampleMask',desc='Mask of sample coverage information',type='basetypes',xrefs='VkPipelineMultisampleStateCreateInfo']
--

The elements of the sample mask array are of type basetype:VkSampleMask,
each representing 32 bits of coverage information:

include::../api/basetypes/VkSampleMask.txt[]

--


[[fragops-early-mode]]
== Early Fragment Test Mode

The depth bounds test, stencil test, depth test,
ifdef::VK_NV_representative_fragment_test[]
representative fragment test,
endif::VK_NV_representative_fragment_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
<<shaders-fragment-earlytest,Early Fragment Tests>>).
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
[eq]#(x~f~,y~f~)# and depth [eq]#z#, as described in
<<primsrast,Rasterization>>.
The fragment is then modified by programmable fragment processing, which
adds associated data as described in <<shaders,Shaders>>.
The fragment is then further modified, and possibly discarded by the late
per-fragment operations described in this chapter.
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 <<fragops-dbt,Depth Bounds Test>>,
<<fragops-stencil,Stencil Test>>, <<fragops-depth,Depth Test>>,
<<fragops-samplecount,Sample Counting>> are instead performed prior to
fragment processing, as described in <<fragops-early-mode,Early Fragment
Test Mode>>, if requested by the fragment shader.


ifdef::VK_AMD_mixed_attachment_samples[]

[[fragops-mixed-attachment-samples]]
== Mixed attachment samples

Special rules apply to per-fragment operations when the number of samples of
the color attachments differs from the number of samples of the
depth/stencil attachment used in a subpass.

Let [eq]#C# be the number of color attachment samples and [eq]#D# be the
number of depth/stencil attachment samples used by a given subpass.

If [eq]#C < D# then only the first [eq]#C# number of samples are guaranteed
to have a corresponding fragment shader invocation and thus a corresponding
color output value, unless the fragment shaders produce inputs to the late
per-fragment tests (e.g. by outputting to a variable decorated with the
code:FragDepth built-in decoration).
Implementations are allowed to produce fragment shader invocations for
samples with indices greater than or equal to [eq]#C# but (other than
potential side effects) the color outputs of fragment shader invocations
corresponding to such samples are discarded.

endif::VK_AMD_mixed_attachment_samples[]


[[fragops-covg]]
== Multisample Coverage

ifndef::VK_NV_sample_mask_override_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.
endif::VK_NV_sample_mask_override_coverage[]
ifdef::VK_NV_sample_mask_override_coverage[]
If a fragment shader is active and its entry point's interface includes a
built-in output variable decorated with code:SampleMask and also decorated
with code:OverrideCoverageNV the fragment coverage is replaced with the
sample mask bits set in the shader.
Otherwise if the built-in output variable decorated with code:SampleMask is
not also decorated with code:OverrideCoverageNV then the fragment coverage
is code:ANDed with the bits of the sample mask to generate a new fragment
coverage value.
endif::VK_NV_sample_mask_override_coverage[]
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 the <<interfaces-fragmentoutput,Fragment Output Interface>>
section).
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 with
pname:rasterizationSamples bits 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 [eq]#[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 <<fragops-dbt,depth bounds tests>>,
<<fragops-stencil,stencil test>>, and <<fragops-depth,depth test>> is
specified through the members of the
sname:VkPipelineDepthStencilStateCreateInfo structure.

[open,refpage='VkPipelineDepthStencilStateCreateInfo',desc='Structure specifying parameters of a newly created pipeline depth stencil state',type='structs']
--

The sname:VkPipelineDepthStencilStateCreateInfo structure is defined as:

include::../api/structs/VkPipelineDepthStencilStateCreateInfo.txt[]

  * 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 <<fragops-depth,depth testing>>
    is enabled.
  * pname:depthWriteEnable controls whether <<fragops-depth-write,depth
    writes>> are enabled when pname:depthTestEnable is ename:VK_TRUE.
    Depth writes are always disabled when pname:depthTestEnable is
    ename:VK_FALSE.
  * pname:depthCompareOp is the comparison operator used in the
    <<fragops-depth,depth test>>.
  * pname:depthBoundsTestEnable controls whether <<fragops-dbt,depth bounds
    testing>> is enabled.
  * pname:stencilTestEnable controls whether <<fragops-stencil,stencil
    testing>> is enabled.
  * pname:front and pname:back control the parameters of the
    <<fragops-stencil,stencil test>>.
  * pname:minDepthBounds and pname:maxDepthBounds define the range of values
    used in the <<fragops-dbt,depth bounds test>>.

.Valid Usage
****
  * [[VUID-VkPipelineDepthStencilStateCreateInfo-depthBoundsTestEnable-00598]]
    If the <<features-features-depthBounds,depth bounds testing>> feature is
    not enabled, pname:depthBoundsTestEnable must: be ename:VK_FALSE
****

include::../validity/structs/VkPipelineDepthStencilStateCreateInfo.txt[]
--

[open,refpage='VkPipelineDepthStencilStateCreateFlags',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkPipelineDepthStencilStateCreateFlags.txt[]

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


[[fragops-dbt]]
== Depth Bounds Test

[open,refpage='vkCmdSetDepthBounds',desc='Set the depth bounds test values for a command buffer',type='protos']
--

The depth bounds test conditionally disables coverage of a sample based on
the outcome of a comparison between the value [eq]#z~a~# in the depth
attachment at location [eq]#(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::../api/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.

.Valid Usage
****
  * [[VUID-vkCmdSetDepthBounds-None-00599]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_DEPTH_BOUNDS dynamic state enabled
ifdef::VK_EXT_depth_range_unrestricted[]
  * [[VUID-vkCmdSetDepthBounds-minDepthBounds-00600]]
    Unless the `<<VK_EXT_depth_range_unrestricted>>` extension is enabled
    pname:minDepthBounds must: be between `0.0` and `1.0`, inclusive
endif::VK_EXT_depth_range_unrestricted[]
ifndef::VK_EXT_depth_range_unrestricted[]
  * [[VUID-vkCmdSetDepthBounds-minDepthBounds-00600]]
    pname:minDepthBounds must: be between `0.0` and `1.0`, inclusive
endif::VK_EXT_depth_range_unrestricted[]
ifdef::VK_EXT_depth_range_unrestricted[]
  * [[VUID-vkCmdSetDepthBounds-maxDepthBounds-00601]]
    Unless the `<<VK_EXT_depth_range_unrestricted>>` extension is enabled
    pname:maxDepthBounds must: be between `0.0` and `1.0`, inclusive
endif::VK_EXT_depth_range_unrestricted[]
ifndef::VK_EXT_depth_range_unrestricted[]
  * [[VUID-vkCmdSetDepthBounds-maxDepthBounds-00601]]
    pname:maxDepthBounds must: be between `0.0` and `1.0`, inclusive
endif::VK_EXT_depth_range_unrestricted[]
****

include::../validity/protos/vkCmdSetDepthBounds.txt[]
--

If [eq]#pname:minDepthBounds {leq} z~a~ {leq} pname: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 [eq]#(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.

[open,refpage='VkStencilOpState',desc='Structure specifying stencil operation state',type='structs']
--

The sname:VkStencilOpState structure is defined as:

include::../api/structs/VkStencilOpState.txt[]

  * pname:failOp is a elink:VkStencilOp value specifying the action
    performed on samples that fail the stencil test.
  * pname:passOp is a elink:VkStencilOp value specifying the action
    performed on samples that pass both the depth and stencil tests.
  * pname:depthFailOp is a elink:VkStencilOp value specifying the action
    performed on samples that pass the stencil test and fail the depth test.
  * pname:compareOp is a elink:VkCompareOp value specifying 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
front-facing fragments and use the back set of stencil state when processing
back-facing fragments.
Fragments rasterized from non-polygon primitives (points and lines) are
always considered front-facing.
Fragments rasterized from polygon primitives inherit their facingness from
the polygon, even if the polygon is 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 <<primsrast-polygons-basic,Basic Polygon
Rasterization>>).

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.

[open,refpage='vkCmdSetStencilCompareMask',desc='Set the stencil compare mask dynamic state',type='protos']
--

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::../api/protos/vkCmdSetStencilCompareMask.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 compare mask.
  * pname:compareMask is the new value to use as the stencil compare mask.

.Valid Usage
****
  * [[VUID-vkCmdSetStencilCompareMask-None-00602]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_STENCIL_COMPARE_MASK dynamic state enabled
****

include::../validity/protos/vkCmdSetStencilCompareMask.txt[]
--

[open,refpage='VkStencilFaceFlagBits',desc='Bitmask specifying sets of stencil state for which to update the compare mask',type='enums']
--

Bits which can: be set in the
flink:vkCmdSetStencilCompareMask::pname:faceMask parameter, and similar
parameters of other commands specifying which stencil state to update
stencil masks for, are:

include::../api/enums/VkStencilFaceFlagBits.txt[]

  * ename:VK_STENCIL_FACE_FRONT_BIT specifies that only the front set of
    stencil state is updated.
  * ename:VK_STENCIL_FACE_BACK_BIT specifies 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
    specifies that both sets of stencil state are updated.

--

[open,refpage='VkStencilFaceFlags',desc='Bitmask of VkStencilFaceFlagBits',type='enums']
--
include::../api/flags/VkStencilFaceFlags.txt[]

sname:VkStencilFaceFlags is a bitmask type for setting a mask of zero or
more slink:VkStencilFaceFlagBits.
--

[open,refpage='vkCmdSetStencilWriteMask',desc='Set the stencil write mask dynamic state',type='protos']
--

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::../api/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.

.Valid Usage
****
  * [[VUID-vkCmdSetStencilWriteMask-None-00603]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_STENCIL_WRITE_MASK dynamic state enabled
****

include::../validity/protos/vkCmdSetStencilWriteMask.txt[]
--

[open,refpage='vkCmdSetStencilReference',desc='Set the stencil reference dynamic state',type='protos']
--

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::../api/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.

.Valid Usage
****
  * [[VUID-vkCmdSetStencilReference-None-00604]]
    The bound graphics pipeline must: have been created with the
    ename:VK_DYNAMIC_STATE_STENCIL_REFERENCE dynamic state enabled
****

include::../validity/protos/vkCmdSetStencilReference.txt[]
--

pname:reference is an integer reference value that is used in the unsigned
stencil comparison.
The reference value used by stencil comparison must be within the range
[eq]#[0,2^s^-1]# , where [eq]#s# is the number of bits in the stencil
framebuffer attachment, otherwise the reference value is considered
undefined.
The [eq]#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 [eq]#R# be the masked reference value and [eq]#S# be the masked stored
stencil value.

[open,refpage='VkCompareOp',desc='Stencil comparison function',type='enums']
--

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

include::../api/enums/VkCompareOp.txt[]

  * ename:VK_COMPARE_OP_NEVER specifies that the test never passes.
  * ename:VK_COMPARE_OP_LESS specifies that the test passes when [eq]#R <
    S#.
  * ename:VK_COMPARE_OP_EQUAL specifies that the test passes when [eq]#R =
    S#.
  * ename:VK_COMPARE_OP_LESS_OR_EQUAL specifies that the test passes when
    [eq]#R {leq} S#.
  * ename:VK_COMPARE_OP_GREATER specifies that the test passes when [eq]#R >
    S#.
  * ename:VK_COMPARE_OP_NOT_EQUAL specifies that the test passes when [eq]#R
    {neq} S#.
  * ename:VK_COMPARE_OP_GREATER_OR_EQUAL specifies that the test passes when
    [eq]#R {geq} S#.
  * ename:VK_COMPARE_OP_ALWAYS specifies that the test always passes.

--

[open,refpage='VkStencilOp',desc='Stencil comparison function',type='enums']
--

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

include::../api/enums/VkStencilOp.txt[]

  * 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 [eq]#s# bits of pname:writeMask, where [eq]#s# is the
number of bits in the stencil framebuffer attachment, specify an integer
mask.
Where a [eq]#1# appears in this mask, the corresponding bit in the stencil
value in the depth/stencil attachment is written; where a [eq]#0# appears,
the bit is not written.
The pname:writeMask value uses either the front-facing or back-facing state
based on the facingness 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 [eq]#(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 [eq]#pname:z~f~# be the incoming fragment's depth value for a sample,
and let [eq]#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 [eq]#z~f~ < z~a~#.
  * ename:VK_COMPARE_OP_EQUAL: the test passes when [eq]#z~f~ = z~a~#.
  * ename:VK_COMPARE_OP_LESS_OR_EQUAL: the test passes when [eq]#z~f~ {leq}
    z~a~#.
  * ename:VK_COMPARE_OP_GREATER: the test passes when [eq]#z~f~ > z~a~#.
  * ename:VK_COMPARE_OP_NOT_EQUAL: the test passes when [eq]#z~f~ {neq}
    z~a~#.
  * ename:VK_COMPARE_OP_GREATER_OR_EQUAL: the test passes when [eq]#z~f~
    {geq} z~a~#.
  * ename:VK_COMPARE_OP_ALWAYS: the test always passes.

If slink:VkPipelineRasterizationStateCreateInfo::pname:depthClampEnable is
enabled, before the incoming fragment's [eq]#pname:z~f~# is compared to
[eq]#pname:z~a~#, [eq]#pname:z~f~# is clamped to [eq]#[min(n,f),max(n,f)]#,
where [eq]#n# and [eq]#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) [eq]#pname: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.
ifdef::VK_EXT_depth_range_unrestricted[]
If the depth framebuffer attachment is a fixed-point format and the depth
value is outside of the `0.0` to `1.0` range the depth value is clamped
between `0.0` and `1.0` inclusive before writing.
endif::VK_EXT_depth_range_unrestricted[]
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.


ifdef::VK_NV_representative_fragment_test[]
[[fragops-rep-frag-test]]
== Representative Fragment Test

The representative fragment test allows implementations to reduce the amount
of rasterization and fragment processing work performed for each point,
line, or triangle primitive.
For any primitive that produces one or more fragments that pass all prior
early fragment tests, the implementation may: choose one or more
"representative" fragments for processing and discard all other fragments.
For draw calls rendering multiple points, lines, or triangles arranged in
lists, strips, or fans, the representative fragment test is performed
independently for each of those primitives.
The set of fragments discarded by the representative fragment test is
implementation-dependent.
In some cases, the representative fragment test may not discard any
fragments for a given primitive.

[open,refpage='VkPipelineRepresentativeFragmentTestStateCreateInfoNV',desc='Structure specifying representative fragment test',type='structs']
--

If the pname:pNext chain of slink:VkGraphicsPipelineCreateInfo includes a
sname:VkPipelineRepresentativeFragmentTestStateCreateInfoNV structure, then
that structure includes parameters that control the representative fragment
test.

The sname:VkPipelineRepresentativeFragmentTestStateCreateInfoNV structure is
defined as:

include::../api/structs/VkPipelineRepresentativeFragmentTestStateCreateInfoNV.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:representativeFragmentTestEnable controls whether the
    representative fragment test is enabled.

If this structure is not present, pname:representativeFragmentTestEnable is
considered to be ename:VK_FALSE, and the representative fragment test is
disabled.

If <<fragops-early-mode, early fragment tests>> are not enabled in the
active fragment shader, the representative fragment shader test has no
effect, even if enabled.

include::../validity/structs/VkPipelineRepresentativeFragmentTestStateCreateInfoNV.txt[]
--

endif::VK_NV_representative_fragment_test[]


[[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
<<queries-occlusion,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,
ifdef::VK_NV_scissor_exclusive[]
exclusive scissor,
endif::VK_NV_scissor_exclusive[]
sample mask, alpha to coverage, stencil, and depth tests.

ifdef::VK_NV_fragment_coverage_to_color[]

[[fragops-coverage-to-color]]
== Fragment Coverage To Color

[open,refpage='VkPipelineCoverageToColorStateCreateInfoNV',desc='Structure specifying whether fragment coverage replaces a color',type='structs']
--

If the pname:pNext chain of slink:VkPipelineMultisampleStateCreateInfo
includes a sname:VkPipelineCoverageToColorStateCreateInfoNV structure, then
that structure controls whether the fragment coverage is substituted for a
fragment color output and, if so, which output is replaced.

The sname:VkPipelineCoverageToColorStateCreateInfoNV structure is defined
as:

include::../api/structs/VkPipelineCoverageToColorStateCreateInfoNV.txt[]

  * 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:coverageToColorEnable controls whether the fragment coverage value
    replaces a fragment color output.
  * pname:coverageToColorLocation controls which fragment shader color
    output value is replaced.

If pname:coverageToColorEnable is ename:VK_TRUE, the fragment coverage
information is treated as a bitmask with one bit for each sample (as in the
<<fragops-samplemask,Sample Mask>> section), and this bitmask replaces the
first component of the color value corresponding to the fragment shader
output location with code:Location equal to pname:coverageToColorLocation
and code:Index equal to zero.
If the color attachment format has fewer bits than the sample coverage, the
low bits of the sample coverage bitmask are taken without any clamping.
If the color attachment format has more bits than the sample coverage, the
high bits of the sample coverage bitmask are filled with zeros.

If <<primsrast-sampleshading,Sample Shading>> is in use, the coverage
bitmask only has bits set for samples that correspond to the fragment shader
invocation that shades those samples.

This pipeline stage occurs after sample counting and before blending, and is
always performed after fragment shading regardless of the setting of
code:EarlyFragmentTests.

If pname:coverageToColorEnable is ename:VK_FALSE, these operations are
skipped.
If this structure is not present, it is as if pname:coverageToColorEnable is
ename:VK_FALSE.

.Valid Usage
****
  * [[VUID-VkPipelineCoverageToColorStateCreateInfoNV-coverageToColorEnable-01404]]
    If pname:coverageToColorEnable is ename:VK_TRUE, then the render pass
    subpass indicated by
    slink:VkGraphicsPipelineCreateInfo::pname:renderPass and
    slink:VkGraphicsPipelineCreateInfo::pname:subpass must: have a color
    attachment at the location selected by pname:coverageToColorLocation,
    with a elink:VkFormat of ename:VK_FORMAT_R8_UINT,
    ename:VK_FORMAT_R8_SINT, ename:VK_FORMAT_R16_UINT,
    ename:VK_FORMAT_R16_SINT, ename:VK_FORMAT_R32_UINT, or
    ename:VK_FORMAT_R32_SINT
****

include::../validity/structs/VkPipelineCoverageToColorStateCreateInfoNV.txt[]
--

[open,refpage='VkPipelineCoverageToColorStateCreateFlagsNV',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkPipelineCoverageToColorStateCreateFlagsNV.txt[]

sname:VkPipelineCoverageToColorStateCreateFlagsNV is a bitmask type for
setting a mask, but is currently reserved for future use.
--

endif::VK_NV_fragment_coverage_to_color[]


[[fragops-coverage-reduction]]
== 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.

ifndef::VK_NV_framebuffer_mixed_samples[]

Each color sample is associated with a unique rasterization sample, and the
value of the coverage mask is assigned to the color sample mask.

endif::VK_NV_framebuffer_mixed_samples[]

ifdef::VK_NV_framebuffer_mixed_samples[]

If the pipeline's
slink:VkPipelineMultisampleStateCreateInfo::pname:rasterizationSamples is
greater than one and the slink:VkAttachmentDescription::pname:samples of the
color attachments is one, then the fragment's coverage is reduced from
pname:rasterizationSamples bits to a single bit, where the color sample mask
is 1 if any bit in the fragment's coverage is on, and 0 otherwise.

If the pipeline's
slink:VkPipelineMultisampleStateCreateInfo::pname:rasterizationSamples is
greater than the slink:VkAttachmentDescription::pname:samples of the color
attachments in the subpass, then the fragment's coverage is reduced from
pname:rasterizationSamples bits to a color sample mask with
slink:VkAttachmentDescription::pname:samples bits.
There is an implementation-dependent association of raster samples to color
samples.
The reduced color sample mask is computed such that the bit for each color
sample is 1 if any of the associated bits in the fragment's coverage is on,
and 0 otherwise.


[[fragops-coverage-modulation]]
=== Coverage Modulation

[open,refpage='VkPipelineCoverageModulationStateCreateInfoNV',desc='Structure specifying parameters controlling coverage modulation',type='structs']
--

As part of coverage reduction, fragment color values can: also be modulated
(multiplied) by a value that is a function of fraction of covered
rasterization samples associated with that color sample.

Pipeline state controlling coverage reduction is specified through the
members of the sname:VkPipelineCoverageModulationStateCreateInfoNV
structure.

The sname:VkPipelineCoverageModulationStateCreateInfoNV structure is defined
as:

include::../api/structs/VkPipelineCoverageModulationStateCreateInfoNV.txt[]

  * 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:coverageModulationMode controls which color components are
    modulated and is of type elink:VkCoverageModulationModeNV.
  * pname:coverageModulationTableEnable controls whether the modulation
    factor is looked up from a table in pname:pCoverageModulationTable.
  * pname:coverageModulationTableCount is the number of elements in
    pname:pCoverageModulationTable.
  * pname:pCoverageModulationTable is a table of modulation factors
    containing a value for each number of covered samples.

If pname:coverageModulationTableEnable is ename:VK_FALSE, then for each
color sample the associated bits of the fragment's coverage are counted and
divided by the number of associated bits to produce a modulation factor
[eq]#R# in the range [eq]#(0,1]# (a value of zero would have been killed due
to a color coverage of 0).
Specifically:

  * [eq]#N# = value of pname:rasterizationSamples
  * [eq]#M# = value of slink:VkAttachmentDescription::pname:samples for any
    color attachments
  * [eq]#R = popcount(associated coverage bits) / (N / M)#

If pname:coverageModulationTableEnable is ename:VK_TRUE, the value [eq]#R#
is computed using a programmable lookup table.
The lookup table has [eq]#N / M# elements, and the element of the table is
selected by:

  * [eq]#R = pname:pCoverageModulationTable[popcount(associated coverage
    bits)-1]#

Note that the table does not have an entry for [eq]#popcount(associated
coverage bits) = 0#, because such samples would have been killed.

The values of pname:pCoverageModulationTable may: be rounded to an
implementation-dependent precision, which is at least as fine as [eq]#1 /
N#, and clamped to [eq]#[0,1]#.

For each color attachment with a floating point or normalized color format,
each fragment output color value is replicated to [eq]#M# values which can:
each be modulated (multiplied) by that color sample's associated value of
[eq]#R#.
Which components are modulated is controlled by
pname:coverageModulationMode.

If this structure is not present, it is as if coverageModulationMode is
ename:VK_COVERAGE_MODULATION_MODE_NONE_NV.

.Valid Usage
****
  * [[VUID-VkPipelineCoverageModulationStateCreateInfoNV-coverageModulationTableEnable-01405]]
    If pname:coverageModulationTableEnable is ename:VK_TRUE,
    pname:coverageModulationTableCount must: be equal to the number of
    rasterization samples divided by the number of color samples in the
    subpass.
****

include::../validity/structs/VkPipelineCoverageModulationStateCreateInfoNV.txt[]
--

[open,refpage='VkPipelineCoverageModulationStateCreateFlagsNV',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkPipelineCoverageModulationStateCreateFlagsNV.txt[]

sname:VkPipelineCoverageModulationStateCreateFlagsNV is a bitmask type for
setting a mask, but is currently reserved for future use.
--

[open,refpage='VkCoverageModulationModeNV',desc='Specify the discard rectangle mode',type='enums']
--
Possible values of
slink:VkPipelineCoverageModulationStateCreateInfoNV::pname:coverageModulationMode,
specifying which color components are modulated, are:

include::../api/enums/VkCoverageModulationModeNV.txt[]

  * ename:VK_COVERAGE_MODULATION_MODE_NONE_NV specifies that no components
    are multiplied by the modulation factor.
  * ename:VK_COVERAGE_MODULATION_MODE_RGB_NV specifies that the red, green,
    and blue components are multiplied by the modulation factor.
  * ename:VK_COVERAGE_MODULATION_MODE_ALPHA_NV specifies that the alpha
    component is multiplied by the modulation factor.
  * ename:VK_COVERAGE_MODULATION_MODE_RGBA_NV specifies that all components
    are multiplied by the modulation factor.
--

endif::VK_NV_framebuffer_mixed_samples[]