123 lines
4.7 KiB
Plaintext
123 lines
4.7 KiB
Plaintext
// Copyright (c) 2018 Khronos Group. This work is licensed under a
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// Creative Commons Attribution 4.0 International License; see
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// http://creativecommons.org/licenses/by/4.0/
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[[fragmentdensitymapops]]
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= Fragment Density Map Operations
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== Fragment Density Map Operations Overview
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When a fragment is generated in a render pass that has a fragment density
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map attachment, its area is determined by the properties of the local
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framebuffer region that the fragment occupies.
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The framebuffer is divided into a uniform grid of these local regions, and
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their fragment area property is derived from the density map with the
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following operations:
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* <<fragmentdensitymap-fetch-density-value,Fetch density value>>
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** <<fragmentdensitymap-component-swizzle,Component swizzle>>
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** <<fragmentdensitymap-component-mapping,Component mapping>>
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* <<fragmentdensitymap-conversion-to-fragment-area,Fragment area
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conversion>>
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** <<fragmentdensitymap-fragment-area-filter,Fragment area filter>>
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** <<fragmentdensitymap-fragment-area-clamp,Fragment area clamp>>
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[[fragmentdensitymap-fetch-density-value]]
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== Fetch Density Value
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Each local framebuffer region at center coordinate [eq]#(x,y)# fetches a
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texel from the fragment density map at integer coordinates:
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:: latexmath:[i =
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\lfloor{\frac{x}{fragmentDensityTexelSize_{width}}}\rfloor]
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:: latexmath:[j =
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\lfloor{\frac{y}{fragmentDensityTexelSize_{height}}}\rfloor]
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Where the size of each region in the framebuffer is:
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:: latexmath:[fragmentDensityTexelSize'_{width} =
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{2^{\lceil{\log_2(\frac{framebuffer_{width}}{fragmentDensityMap_{width}})}\rceil}}]
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:: latexmath:[fragmentDensityTexelSize'_{height} =
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{2^{\lceil{\log_2(\frac{framebuffer_{height}}{fragmentDensityMap_{height}})}\rceil}}]
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This region is subject to the limits in
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sname:VkPhysicalDeviceFragmentDensityMapPropertiesEXT and therefore the
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final region size is clamped:
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:: latexmath:[fragmentDensityTexelSize_{width} =
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\mathbin{clamp}(fragmentDensityTexelSize'_{width},minFragmentDensityTexelSize_{width},maxFragmentDensityTexelSize_{height})]
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:: latexmath:[fragmentDensityTexelSize_{height} =
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\mathbin{clamp}(fragmentDensityTexelSize'_{height},minFragmentDensityTexelSize_{height},maxFragmentDensityTexelSize_{height})]
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When multiview is enabled for the render pass and the fragment density map
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attachment view was created with pname:layerCount greater than `1`, the
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density map layer that the texel is fetched from is:
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:: latexmath:[layer = baseArrayLayer + ViewIndex]
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Otherwise:
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:: latexmath:[layer = baseArrayLayer]
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The texel fetched from the density map at [eq]#(i,j,layer)# is next
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converted to density with the following operations.
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[[fragmentdensitymap-component-swizzle]]
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=== Component Swizzle
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The pname:components member of slink:VkImageViewCreateInfo is applied to the
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fetched texel as defined in <<textures-component-swizzle,Image component
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swizzle>>.
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[[fragmentdensitymap-component-mapping]]
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=== Component Mapping
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The swizzled texel's components are mapped to a density value:
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:: latexmath:[densityValue_{xy} = (C'_{r},C'_{g})]
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[[fragmentdensitymap-conversion-to-fragment-area]]
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== Fragment Area Conversion
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Fragment area for the framebuffer region is undefined if the density fetched
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is not a normalized floating-point value greater than `0.0`.
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Otherwise, the fetched fragment area for that region is derived as:
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:: latexmath:[fragmentArea_{wh} = \frac{1.0}{densityValue_{xy}}]
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[[fragmentdensitymap-fragment-area-filter]]
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=== Fragment Area Filter
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Optionally, the implementation may: fetch additional density map texels in
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an implementation defined window around [eq]#(i,j)#.
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The texels follow the standard conversion steps up to and including
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<<fragmentdensitymap-conversion-to-fragment-area,fragment area conversion>>.
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A single fetched fragment area for the framebuffer region is chosen by the
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implementation and must: have an area between the _min_ and _max_ areas of
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the fetched set.
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[[fragmentdensitymap-fragment-area-clamp]]
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=== Fragment Area Clamp
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The implementation may: clamp the fetched fragment area to one that it
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supports.
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The clamped fragment area must: have a size less than or equal to the
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original fetched value.
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Implementations may: vary the supported set of fragment areas per
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framebuffer region.
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Fragment area [eq]#(1,1)# must: always be in the supported set.
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[NOTE]
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.Note
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====
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For example, if the fetched fragment area is [eq]#(1,4)# but the
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implementation only supports areas of [eq]#{(1,1),(2,2)}#, it could choose
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to clamp the area to [eq]#(2,2)# since it has the same size as [eq]#(1,4)#.
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While this would produce fragments that have lower quality strictly in the
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x-axis, the overall density is maintained.
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====
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The clamped fragment area is assigned to the corresponding framebuffer
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region.
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