224 lines
10 KiB
Plaintext
224 lines
10 KiB
Plaintext
include::meta/VK_NV_shader_image_footprint.txt[]
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*Last Modified Date*::
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2018-09-13
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*IP Status*::
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No known IP claims.
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*Contributors*::
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- Pat Brown, NVIDIA
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- Chris Lentini, NVIDIA
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- Daniel Koch, NVIDIA
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- Jeff Bolz, NVIDIA
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This extension adds Vulkan support for the `SPV_NV_shader_image_footprint`
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SPIR-V extension.
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That SPIR-V extension provides a new instruction
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code:OpImageSampleFootprintNV allowing shaders to determine the set of
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texels that would be accessed by an equivalent filtered texture lookup.
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Instead of returning a filtered texture value, the instruction returns a
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structure that can be interpreted by shader code to determine the footprint
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of a filtered texture lookup.
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This structure includes integer values that identify a small neighborhood of
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texels in the image being accessed and a bitfield that indicates which
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texels in that neighborhood would be used.
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The structure also includes a bitfield where each bit identifies whether any
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texel in a small aligned block of texels would be fetched by the texture
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lookup.
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The size of each block is specified by an access _granularity_ provided by
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the shader.
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The minimum granularity supported by this extension is 2x2 (for 2D textures)
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and 2x2x2 (for 3D textures); the maximum granularity is 256x256 (for 2D
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textures) or 64x32x32 (for 3D textures).
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Each footprint query returns the footprint from a single texture level.
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When using minification filters that combine accesses from multiple mipmap
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levels, shaders must perform separate queries for the two levels accessed
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("`fine`" and "`coarse`").
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The footprint query also returns a flag indicating if the texture lookup
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would access texels from only one mipmap level or from two neighboring
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levels.
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This extension should be useful for multi-pass rendering operations that do
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an initial expensive rendering pass to produce a first image that is then
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used as a texture for a second pass.
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If the second pass ends up accessing only portions of the first image (e.g.,
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due to visbility), the work spent rendering the non-accessed portion of the
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first image was wasted.
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With this feature, an application can limit this waste using an initial pass
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over the geometry in the second image that performs a footprint query for
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each visible pixel to determine the set of pixels that it needs from the
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first image.
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This pass would accumulate an aggregate footprint of all visible pixels into
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a separate "`footprint image`" using shader atomics.
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Then, when rendering the first image, the application can kill all shading
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work for pixels not in this aggregate footprint.
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This extension has a number of limitations.
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The code:OpImageSampleFootprintNV instruction only supports for two- and
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three-dimensional textures.
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Footprint evaluation only supports the CLAMP_TO_EDGE wrap mode; results are
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undefined for all other wrap modes.
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Only a limited set of granularity values and that set does not support
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separate coverage information for each texel in the original image.
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When using SPIR-V generated from the OpenGL Shading Language, the new
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instruction will be generated from code using the new
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code:textureFootprint*NV built-in functions from the
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`GL_NV_shader_texture_footprint` shading language extension.
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=== New Object Types
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None.
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=== New Enum Constants
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* Extending elink:VkStructureType:
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** ename:VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SHADER_IMAGE_FOOTPRINT_FEATURES_NV
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=== New Enums
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None.
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=== New Structures
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* slink:VkPhysicalDeviceShaderImageFootprintFeaturesNV
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=== New Functions
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None.
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=== New SPIR-V Capability
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* <<spirvenv-capabilities-table-imagefootprint,ImageFootprintNV>>
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=== Issues
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(1) The footprint returned by the SPIR-V instruction is a structure that
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includes an anchor, an offset, and a mask that represents a 8x8 or 4x4x4
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neighborhood of texel groups.
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But the bits of the mask are not stored in simple pitch order.
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Why is the footprint built this way?
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*RESOLVED*: We expect that applications using this feature will want to use
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a fixed granularity and accumulate coverage information from the returned
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footprints into an aggregate "`footprint image`" that tracks the portions of
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an image that would be needed by regular texture filtering.
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If an application is using a two-dimensional image with 4x4 pixel
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granularity, we expect that the footprint image will use 64-bit texels where
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each bit in an 8x8 array of bits corresponds to coverage for a 4x4 block in
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the original image.
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Texel (0,0) in the footprint image would correspond to texels (0,0) through
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(31,31) in the original image.
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In the usual case, the footprint for a single access will fully contained in
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a 32x32 aligned region of the original texture, which corresponds to a
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single 64-bit texel in the footprint image.
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In that case, the implementation will return an anchor coordinate pointing
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at the single footprint image texel, an offset vector of (0,0), and a mask
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whose bits are aligned with the bits in the footprint texel.
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For this case, the shader can simply atomically OR the mask bits into the
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contents of the footprint texel to accumulate footprint coverage.
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In the worst case, the footprint for a single access spans multiple 32x32
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aligned regions and may require updates to four separate footprint image
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texels.
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In this case, the implementation will return an anchor coordinate pointing
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at the lower right footprint image texel and an offset will identify how
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many "`columns`" and "`rows`" of the returned 8x8 mask correspond to
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footprint texels to the left and above the anchor texel.
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If the anchor is (2,3), the 64 bits of the returned mask are arranged
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spatially as follows, where each 4x4 block is assigned a bit number that
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matches its bit number in the footprint image texels:
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----
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+-------------------------+-------------------------+
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- 46 47 | 40 41 42 43 44 45 -- -- |
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| -- -- -- -- -- -- 54 55 | 48 49 50 51 52 53 -- -- |
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| -- -- -- -- -- -- 62 63 | 56 57 58 59 60 61 -- -- |
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+-------------------------+-------------------------+
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| -- -- -- -- -- -- 06 07 | 00 01 02 03 04 05 -- -- |
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| -- -- -- -- -- -- 14 15 | 08 09 10 11 12 13 -- -- |
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| -- -- -- -- -- -- 22 23 | 16 17 18 19 20 21 -- -- |
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| -- -- -- -- -- -- 30 31 | 24 25 26 27 28 29 -- -- |
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| -- -- -- -- -- -- 38 39 | 32 33 34 35 36 37 -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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| -- -- -- -- -- -- -- -- | -- -- -- -- -- -- -- -- |
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+-------------------------+-------------------------+
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----
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To accumulate coverage for each of the four footprint image texels, a shader
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can AND the returned mask with simple masks derived from the x and y offset
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values and then atomically OR the updated mask bits into the contents of the
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corresponding footprint texel.
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[source,c++]
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----
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uint64_t returnedMask = (uint64_t(footprint.mask.x) | (uint64_t(footprint.mask.y) << 32));
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uint64_t rightMask = ((0xFF >> footprint.offset.x) * 0x0101010101010101UL);
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uint64_t bottomMask = 0xFFFFFFFFFFFFFFFFUL >> (8 * footprint.offset.y);
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uint64_t bottomRight = returnedMask & bottomMask & rightMask;
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uint64_t bottomLeft = returnedMask & bottomMask & (~rightMask);
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uint64_t topRight = returnedMask & (~bottomMask) & rightMask;
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uint64_t topLeft = returnedMask & (~bottomMask) & (~rightMask);
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----
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(2) What should an application do to ensure maximum performance when
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accumulating footprints into an aggregate footprint image?
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*RESOLVED*: We expect that the most common usage of this feature will be to
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accumulate aggregate footprint coverage, as described in the previous issue.
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Even if you ignore the anisotropic filtering case where the implementation
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may return a granularity larger than that requested by the caller, each
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shader invocation will need to use atomic functions to update up to four
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footprint image texels for each level of detail accessed.
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Having each active shader invocation perform multiple atomic operations can
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be expensive, particularly when neighboring invocations will want to update
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the same footprint image texels.
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Techniques can be used to reduce the number of atomic operations performed
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when accumulating coverage include:
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* Have logic that detects returned footprints where all components of the
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returned offset vector are zero.
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In that case, the mask returned by the footprint function is guaranteed
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to be aligned with the footprint image texels and affects only a single
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footprint image texel.
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* Have fragment shaders communicate using built-in functions from the
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`VK_NV_shader_subgroup_partitioned` extension or other shader subgroup
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extensions.
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If you have multiple invocations in a subgroup that need to update the
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same texel (x,y) in the footprint image, compute an aggregate footprint
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mask across all invocations in the subgroup updating that texel and have
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a single invocation perform an atomic operation using that aggregate
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mask.
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* When the returned footprint spans multiple texels in the footprint
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image, each invocation need to perform four atomic operations.
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In the previous issue, we had an example that computed separate masks
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for "`topLeft`", "`topRight`", "`bottomLeft`", and "`bottomRight`".
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When the invocations in a subgroup have good locality, it might be the
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case the "`top left`" for some invocations might refer to footprint
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image texel (10,10), while neighbors might have their "`top left`"
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texels at (11,10), (10,11), and (11,11).
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If you compute separate masks for even/odd x and y values instead of
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left/right or top/bottom, the "`odd/odd`" mask for all invocations in
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the subgroup hold coverage for footprint image texel (11,11), which can
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be updated by a single atomic operation for the entire subgroup.
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=== Examples
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TBD
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=== Version History
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* Revision 2, 2018-09-13 (Pat Brown)
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- Add issue (2) with performance tips.
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* Revision 1, 2018-08-12 (Pat Brown)
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- Initial draft
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