# 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