| # 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 |
