kc3-lang/angle

2020-09-23T21:31:05

Metal: Implement transform feedback - XFB is currently emulated by writing to storage buffers. - Metal doesn't allow vertex shader to both write to storage buffers and to stage output (i.e clip position). So if GL_RASTERIZER_DISCARD is NOT enabled, the draw with XFB enabled will have 2 passes: + First pass: vertex shader writes to XFB buffers + not write to stage output + disable rasterizer. + Second pass: vertex shader writes to stage output (i.e. [[position]]) + enable rasterizer. If GL_RASTERIZER_DISCARD is enabled, the second pass is omitted. + This effectively executes the same vertex shader twice. TODO: possible improvement is writing vertex outputs to buffer in first pass then re-use that buffer as input for second pass which has a passthrough vertex shader. - If GL_RASTERIZER_DISCARD is enabled, and XFB is enabled: + Only first pass above will be executed, and the render pass will use an empty 1x1 texture attachment since rasterization is not needed. - If GL_RASTERIZER_DISCARD is enabled, but XFB is NOT enabled: + we still enable Metal rasterizer. + but vertex shader must emulate the discard by writing gl_Position = (-3, -3, -3, 1). This effectively moves the vertex out of clip space's visible area. + This is because GLSL still allows vertex shader to write to stage output when rasterizer is disabled. However, Metal doesn't allow that. In Metal, if rasterizer is disabled, then vertex shader must not write to stage output. - See src/libANGLE/renderer/metal/doc/TransformFeedback.md for more details. Bug: angleproject:2634 Change-Id: I6c700e031052560326b7f660ee7597202d38e6aa Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2408594 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>