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fix(metal): blend window-space layers instead of overwriting (#1788) #1789

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Merged
dfattal merged 2 commits into from
Oct 2, 2026
Merged

fix(metal): blend window-space layers instead of overwriting (#1788) #1789

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8977653
fix(metal): blend window-space layers instead of overwriting (#1788)
dfattal Oct 2, 2026
f3d2d37
tests: allow Metal's window-space SOURCE_ALPHA_BIT read (#1788)
dfattal Oct 2, 2026
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17 changes: 16 additions & 1 deletion src/xrt/compositor/metal/comp_metal_compositor.m
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Original file line number Diff line number Diff line change
Expand Up @@ -4018,6 +4018,21 @@ - (BOOL)wantsUpdateLayer
}
}

// #1788: window-space layers BLEND over the projection content,
// same rule as D3D11/D3D12, vk_native and GL (#1786/#1787):
// SOURCE_ALPHA_BIT clear means premultiplied bytes (colour src
// ONE), set means straight (colour src SOURCE_ALPHA); colour dst
// and the alpha "over" (ONE / ONE_MINUS_SOURCE_ALPHA) are shared,
// so atlas alpha survives for transparent sessions (#225/#1784).
// The blended projection pipelines carry exactly those factors.
// Deliberately NOT comp_layer_blend_mode() (#1599): see the
// comment in comp_d3d11_renderer.cpp's window-space draw. And
// never projection_pipeline — its REPLACE is for the base blit.
bool ws_premultiplied =
(layer->data.flags & XRT_LAYER_COMPOSITION_BLEND_TEXTURE_SOURCE_ALPHA_BIT) == 0;
id<MTLRenderPipelineState> ws_pipeline =
ws_premultiplied ? c->proj_premult_pipeline : c->proj_straight_pipeline;

// Source UV sub-rect (default to full texture if not specified)
struct xrt_normalized_rect nr = ws->sub.norm_rect;
if (nr.w <= 0.0f || nr.h <= 0.0f) {
Expand Down Expand Up @@ -4060,7 +4075,7 @@ - (BOOL)wantsUpdateLayer
}
[encoder setViewport:vp];

[encoder setRenderPipelineState:c->projection_pipeline];
[encoder setRenderPipelineState:ws_pipeline];
[encoder setDepthStencilState:c->depth_stencil_state];
[encoder setFragmentTexture:src_tex atIndex:0];
[encoder setFragmentSamplerState:c->sampler_linear atIndex:0];
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4 changes: 3 additions & 1 deletion tests/tests_comp_layer_view_camera.cpp
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Expand Up @@ -1138,7 +1138,9 @@ TEST_CASE("comp_layer_blend_mode: no backend reimplements the blend rule (#1621)
// channel D3D11's is (its own premul/straight decision, deliberately
// not comp_layer_blend_mode() -- see comp_d3d11_renderer.cpp).
{"gl/comp_gl_compositor.cpp", 1},
{"metal/comp_metal_compositor.m", 0},
// #1788: Metal's one allowance is the same Local2D / window-space
// channel as D3D11/D3D12/GL (#1581, #1786).
{"metal/comp_metal_compositor.m", 1},
// The Local2D / window-space channel, above.
{"d3d11/comp_d3d11_renderer.cpp", 1},
/*
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