Vulkan: Write docs on format tables.

src/libANGLE/renderer/vulkan/README.md

@@ -204,3 +204,42 @@ code please see [TranslatorVulkan.cpp][TranslatorVulkan.cpp] under
 [Bresenham]: https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
 [VulkanVsGLLineRaster]: doc/img/LineRasterComparison.gif
 [DiamondRule]: doc/img/LineRasterPixelExample.png
+
+### Format Tables and Format Emulation
+
+#### Overrides and Fallbacks
+
+The [required Vulkan format support][VulkanRequiredSupport] tables do not implement the full set of
+formats needed for OpenGL conformance with extensions. ANGLE emulates missing formats using *format
+overrides* and *format fallbacks*.
+
+An *override* implements a missing GL format with a required format in all cases. For example, the
+luminance texture format `L8_UNORM` does not exist in Vulkan. We override `L8_UNORM` with the
+required image format `R8_UNORM`.
+
+A *fallback* is one or more non-required formats ANGLE checks for support at runtime. For example,
+`R8_UNORM` is not a required vertex buffer format. Some drivers do support `R8_UNORM` for vertex
+buffers. So at runtime we check for sampled image support and fall back to `R32_FLOAT` if `R8_UNORM`
+is not supported.
+
+#### The Vulkan Format Table
+
+Overrides and fallbacks are implemented in ANGLE's [Vulkan format
+table][vk_format_table_autogen.cpp]. The table is auto-generated by
+[`gen_vk_format_table.py`](gen_vk_format_table.py). We store the mapping from
+[`angle::Format::ID`](../FormatID_autogen.h) to [VkFormat][VkFormat] in
+[`vk_format_map.json`](vk_format_map.json). The format map also lists the overrides and fallbacks.
+To update the tables please modify the format map JSON and then run
+[`scripts/run_code_generation.py`][RunCodeGeneration].
+
+The [`vk::Format`](vk_format_utils.h) class describes the information ANGLE needs for a particular
+`VkFormat`. The 'ANGLE' format ID is a reference to the front-end format. The 'Image' or 'Buffer'
+format are the native Vulkan formats that implement a particular front-end format for `VkImages` and
+`VkBuffers`. For the above example of `R8_UNORM` overriding `L8_UNORM`, `L8_UNORM` is the ANGLE
+format and `R8_UNORM` is the Image format.
+
+For more information please see the source files.
+
+[VulkanRequiredSupport]: https://renderdoc.org/vkspec_chunked/chap37.html#features-required-format-support
+[VkFormat]: https://renderdoc.org/vkspec_chunked/chap37.html#VkFormat
+[RunCodeGeneration]: ../../../../scripts/run_code_generation.py
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