readbackFromWebGPUCanvas:transferToImageBitmap_unconfigured_nonzero_size expects opaque black image (alpha == 255) #4259

jamienicol · 2025-03-06T16:50:33Z

I'm looking at making the above test pass in Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=1948580)

I can see the test expects the returned bitmaps pixels to have an alpha value of 255. But looking at the spec (steps 1 to 3) it seems to me the returned image should be transparent black, ie alpha == 0, as the canvas is unconfigured.

Does the CTS need updated, or am I misreading the spec?

…red webgpu canvas. r?#webgpu-reviewers The spec[1] (steps 1-3) mandates we return a transparent black image of the same size as the canvas when attempting to snapshot an unconfigured webgpu canvas context. Currently, CanvasContext::GetSurfaceSnapshot() returns null in such cases. nsLayoutUtils::SurfaceFromOffscreenCanvas() then handles this by returning a transparent black surface. This should be sufficient. However, the relevant CTS tests[2] currently expect an *opaque* black image to be returned. An issue[3] has been filed upstream. This patch makes us match the CTS' expected behaviour, by specifically handling the unconfigured case in CanvasContext::GetSurfaceSnapshot(), and returning an opaque black surface of the required size. We clear the pixels using libyuv::ARGBRect() for performance reasons, like we do in BufferTexture.cpp when initializing BufferTextureData buffers. If and when the CTS is updated to expect a transparent black surface, we can either revert this patch in its entirety, or simply pass the correct value for aIsOpaque to MakeFallbackImage(). [1] https://www.w3.org/TR/webgpu/#abstract-opdef-get-a-copy-of-the-image-contents-of-a-context [2] webgpu:web_platform,canvas,readbackFromWebGPUCanvas:transferToImageBitmap_unconfigured_nonzero_size:* [3] gpuweb/cts#4259

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readbackFromWebGPUCanvas:transferToImageBitmap_unconfigured_nonzero_size expects opaque black image (alpha == 255) #4259

readbackFromWebGPUCanvas:transferToImageBitmap_unconfigured_nonzero_size expects opaque black image (alpha == 255) #4259

jamienicol commented Mar 6, 2025 •

edited

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readbackFromWebGPUCanvas:transferToImageBitmap_unconfigured_nonzero_size expects opaque black image (alpha == 255) #4259

readbackFromWebGPUCanvas:transferToImageBitmap_unconfigured_nonzero_size expects opaque black image (alpha == 255) #4259

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jamienicol commented Mar 6, 2025 • edited Loading

jamienicol commented Mar 6, 2025 •

edited

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