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kainino0xkainino0x
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Allow reading back from canvases after present (#2905)
* Allow reading back from canvases after present This allows drawImage/toDataURL/etc. to see the canvas contents presented in the previous frame, as long as getCurrentTexture (or configure/unconfigure) hasn't been called yet this frame. alphaMode (née compositingAlphaMode) now affects using the canvas as an image source (drawImage/etc.) as well as compositing, so that the observed contents don't change on a frame boundary. As a weird aside (necessary to fully define the image source behavior), defines super-luminant values as being in the extended color space (i.e. once un-premultiplied). This definition emerges naturally, but it's also weird. Fixes #2743 Fixes #1847 Fixes a leftover bit from #2373 (placeholder canvases) * nit * Remove the "cancel present" behavior of destroy() This fixes several problems: - Unnecessary complexity in how currentTexture logic works. - Errors in the previous commit, where this was just not fully handled. - Using the "destroyed" state on the content process (minor issue). * nits * nit
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‎explainer/index.bs‎

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@@ -1040,15 +1040,17 @@ even though context state does not persist across loss/restoration.)
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In order to access a canvas, an app gets a `GPUTexture` from the `GPUCanvasContext`
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and then writes to it, as it would with a normal `GPUTexture`.
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### Swap Chains ### {#canvas-output-swap-chains}
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### Canvas Configuration ### {#canvas-output-swap-chains}
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Canvas `GPUTexture`s are vended in a very structured way:
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- `canvas.getContext('gpupresent')` provides a `GPUCanvasContext`.
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- `GPUCanvasContext.configureSwapChain({ device, format, usage })` provides a `GPUSwapChain`,
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invalidating any previous swapchains, attaching the canvas to the provided device, and
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setting the `GPUTextureFormat` and `GPUTextureUsage` for vended textures.
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- `GPUSwapChain.getCurrentTexture()` provides a `GPUTexture`.
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- `canvas.getContext('webgpu')` provides a `GPUCanvasContext`.
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- `GPUCanvasContext.configure({ device, format, usage })` modifies the current configuration
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invalidating any previous texture object, attaching the canvas to the provided device,
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and setting options for vended textures and canvas behavior.
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- Resizing the canvas also invalidates previous texture objects.
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- `GPUCanvasContext.getCurrentTexture()` provides a `GPUTexture`.
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- `GPUCanvasContext.unconfigure()` returns the context to its initial, unconfigured state.
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This structure provides maximal compatibility with optimized paths in native graphics APIs.
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In these, typically, a platform-specific "surface" object can produce an API object called a
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### Current Texture ### {#canvas-output-current-texture}
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A `GPUSwapChain` provides a "current texture" via `getCurrentTexture()`.
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A `GPUCanvasContext` provides a "current texture" via `getCurrentTexture()`.
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For <{canvas}> elements, this returns a texture for the *current frame*:
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- On `getCurrentTexture()`, `[[currentTexture]]` is created if it doesn't exist, then returned.
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- During the "[=Update the rendering=]" step, the browser compositor takes ownership of the
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`[[currentTexture]]` for display, and that internal slot is cleared for the next frame.
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- On `getCurrentTexture()`, a new `[[drawingBuffer]]` is created if one doesn't exist for the
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current frame, wrapped in a `GPUTexture`, and returned.
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- During the "[=Update the rendering=]" step, the `[[drawingBuffer]]` becomes readonly. Then, it is
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shared by the browser compositor (for display) and the page's canvas (readable using
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drawImage/toDataURL/etc.)
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### `getSwapChainPreferredFormat()` ### {#canvas-output-preferred-format}
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### `getPreferredCanvasFormat()` ### {#canvas-output-preferred-format}
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Due to framebuffer hardware differences, different devices have different preferred byte layouts
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for display surfaces.
@@ -1089,8 +1093,8 @@ As today with WebGL, user agents can make their own decisions about how to expos
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capabilities, e.g. choosing the capabilities of the initial, primary, or most-capable display.
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In the future, an event might be provided that allows applications to detect when a canvas moves
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to a display with different properties so they can call `getSwapChainPreferredFormat()` and
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`configureSwapChain()` again.
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to a display with different properties so they can call `getPreferredCanvasFormat()` and
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`configure()` again.
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#### Multiple Adapters #### {#canvas-output-multiple-adapters}
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