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.
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.