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