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Metal Backend Design Notes

Minimum Requirements

rafx-api requires metal 2.0. iOS GPU family 3 or higher is recommended. (A9 or higher on mobile)

For more info, see the Metal Feature Set Tables provided by Apple.

Status

Metal support is fairly complete but could probably be optimized. See Future Work.

Platform Support

Metal is supported on macOS and iOS only. A9 or higher is recommended on mobile.

Implementation Notes

Coordinate System

rafx-api uses the default metal coordinate system with no modifications.

Validation

Set the environment variable METAL_DEVICE_WRAPPER_TYPE=1

Debugging

Xcode can be used to debug/trace a frame. It can be used by rust by making an empty xcode project and changing the debug settings to launch an external program.

Shader Translation

rafx-api accepts metal source code or compiled shader libraries. You can compile this yourself, or use rafx-shader-processor. The shader processor currently uses vulkan GLSL as input. spirv-cross is used to translate the shader to metal shader language. There are a few things about this process to be aware of:

  • We translate vulkan descriptor sets into metal argument buffers
  • The argument buffer definition must be identical in the vertex/fragment shaders.
  • The bindings in metal must be sequential, 0..n. The shader processor remaps the bindings in the GLSL to match. If you use rafx-framework or other higher-level rafx crates to load the shader, the setup and API will handle this for you automatically.
  • Immutable samplers in MSL are called const samplers. These are specified in the generated MSL.

Subpasses/Tile Shading

Tile shading is not supported in rafx-api. It's unclear how metal's abstraction can be mapped to other backends. rafx-api makes the internally created metal objects available to you, so you can always natively implement a performance-critical part of your pipeline if needed.

Future Work

  • Descriptor set "default" values (i.e. bind a 1x1 texture)
  • Inline constant data in argument buffers
  • Use metal heaps for better performance
  • A general optimization pass over the backend. (For some scenes with few draw calls, the vulkan backend is slightly faster, which is not expected!)