1# Format Tables and Format Emulation 2 3## Overrides and Fallbacks 4 5The [required Vulkan format support][VulkanRequiredSupport] tables do not implement the full set of 6formats needed for OpenGL conformance with extensions. ANGLE emulates missing formats using *format 7overrides* and *format fallbacks*. 8 9An *override* implements a missing GL format with a required format in all cases. For example, the 10luminance texture format `L8_UNORM` does not exist in Vulkan. We override `L8_UNORM` with the 11required image format `R8_UNORM`. 12 13A *fallback* is one or more non-required formats ANGLE checks for support at runtime. For example, 14`R8_UNORM` is not a required vertex buffer format. Some drivers do support `R8_UNORM` for vertex 15buffers. So at runtime we check for sampled image support and fall back to `R32_FLOAT` if `R8_UNORM` 16is not supported. 17 18## The Vulkan Format Table 19 20Overrides and fallbacks are implemented in ANGLE's [Vulkan format table][../vk_format_table_autogen.cpp]. 21The table is auto-generated by [`gen_vk_format_table.py`](../gen_vk_format_table.py). We store the mapping from 22[`angle::Format::ID`](../../FormatID_autogen.h) to [VkFormat][VkFormat] in 23[`vk_format_map.json`](../vk_format_map.json). The format map also lists the overrides and fallbacks. 24To update the tables please modify the format map JSON and then run 25[`scripts/run_code_generation.py`][RunCodeGeneration]. 26 27The [`vk::Format`](../vk_format_utils.h) class describes the information ANGLE needs for a particular 28`VkFormat`. The 'intended' format ID is a reference to the front-end format. The 'actual Image' and 29'actual Buffer' formats are the native Vulkan formats that implement a particular front-end format 30for `VkImages` and `VkBuffers`. For the above example of `R8_UNORM` overriding `L8_UNORM`, 31`L8_UNORM` is the intended format and `R8_UNORM` is the actual Image format. 32 33For more information please see the source files. 34 35[VulkanRequiredSupport]: https://renderdoc.org/vkspec_chunked/chap37.html#features-required-format-support 36[VkFormat]: https://renderdoc.org/vkspec_chunked/chap37.html#VkFormat 37[RunCodeGeneration]: ../../../../scripts/run_code_generation.py 38