[Webkit-unassigned] [Bug 177132] New: WebGL canvas passed to drawImage is very slow

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=177132

            Bug ID: 177132
           Summary: WebGL canvas passed to drawImage is very slow
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: macOS 10.13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Canvas
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ashley at scirra.com
                CC: dino at apple.com

Steps to reproduce:

1. Visit: http://www.scirra.com/labs/bugs/webgl-canvas2d-perf.html
2. Observe the FPS reading


Actual results:

The demo simply creates a window-sized WebGL canvas which changes every frame, and copies it to a window-sized 2D canvas with drawImage(). On a mid-2015 Macbook Pro, this runs at about ~25 FPS.


Expected results:

It should run at 60 FPS, as it does in Chrome.

Drawing a WebGL canvas to a 2D canvas is a simple workaround for rendering multiple views of one WebGL context. We use this in Construct 3 at https://editor.construct.net, and so this overload of drawImage() is critical to the performance of our web app and this issue causes it to be unnecessarily slow. The poor performance suggests a CPU readback is happening per frame, when it should be possible to fully GPU accelerate this.

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