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
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
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
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
VkBuffers. For the above example of
L8_UNORM is the intended format and
R8_UNORM is the actual Image format.
For more information please see the source files.