FormatTablesAndEmulation.md

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Hash : c79b0ea5
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Date : 2020-02-25T08:26:23

Vulkan: Document command access APIs.

Includes a short description of the ContextVk command APIs.

Also a few small cleanups to other Vulkan back-end documentation.

Bug: angleproject:4029
Change-Id: I3bd04861f27b718e6a94fdae00644c124452bc62
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2070597
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>

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Format Tables and Format Emulation

Overrides and Fallbacks

The required Vulkan format support 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. We store the mapping from angle::Format::ID to VkFormat in 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.

The vk::Format class describes the information ANGLE needs for a particular VkFormat. The ‘intended’ format ID is a reference to the front-end format. The ‘actual Image’ and ‘actual Buffer’ formats 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 intended format and R8_UNORM is the actual Image format.

For more information please see the source files.

Source

src/libANGLE/renderer/vulkan/doc/FormatTablesAndEmulation.md

# 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 'intended' format ID is a reference to the front-end format. The 'actual Image' and
'actual Buffer' formats 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 intended format and `R8_UNORM` is the actual 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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