[Webkit-unassigned] [Bug 240100] New: [GPU Process] [iOS] REGRESSION(r293570): Snapshot rendering is not scaled with the device scale factor

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=240100

            Bug ID: 240100
           Summary: [GPU Process] [iOS] REGRESSION(r293570): Snapshot
                    rendering is not scaled with the device scale factor
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: sabouhallawa at apple.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

In r293570, the GPU Process was enabled for snapshot images. Before r293570, a local ImageBuffer was created in snapshotFrameRectWithClip(). Then in WebFrame::createSelectionSnapshot(), the pixels of the ImageBuffer were copied to a WebImage, which was backed by a ShareableBitmap.

r293570 made snapshotFrameRectWithClip() create a remote ImageBuffer which is moved to a WebImage in WebFrame::createSelectionSnapshot(). This remote ImageBuffer has to have ImageBufferShareableBitmapBackend so it can provide ShareableBitmap without allocating any extra memory.

The problem is in initializing the GraphicsContext of ImageBufferShareableBitmapBackend. The scaling factor is not set to the CTM of the ImageBufferShareableBitmapBackend context. See the comment in ImageBufferShareableBitmapBackend constructor and notice that this function does not call applyBaseTransformToContext().

A simple approach to fix this bug is to make snapshotFrameRectWithClip() handle the scaling outside the ImageBuffer creation. This is similar to what we do in GraphicsContext::createAlignedImageBuffer() where we scale the size and create the ImageBuffer with scaleFactor = 1.

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