[Webkit-unassigned] [Bug 66574] New: Canvas 2D - Source rectangles that overlap the source image boundary, not supported by drawImage

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=66574

           Summary: Canvas 2D - Source rectangles that overlap the source
                    image boundary, not supported by drawImage
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: ASSIGNED
          Severity: Normal
          Priority: P2
         Component: Canvas
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: junov at chromium.org
                CC: ian at hixie.ch, senorblanco at chromium.org,
                    jmuizelaar at mozilla.com, mdelaney at apple.com


This bug is a spin-off of https://bugs.webkit.org/show_bug.cgi?id=65709

Removing the DOM exception that was thrown by drawImage when the source rectangle is out of bounds exposed the question of how to handle source rectangles that are only partially out of bounds

In the discussion for bug 65709, we agreed that the out-of bound region should be treated as transparent black. There was still an open question about whether this result should be achieved by 
a) clipping the the source rectangle to the bounds of the source image, and clipping the destination rectangle in such a way that preserves the src->dst coordinate mapping;
or
b) padding the source rectangle with transparent black pixels. This would cause the encroached source image edges to be subject to filtering (i.e. not treated as border pixels)

Currently, the HTML spec does not provide a clear answer as to what the correct approach should be.

The resolution of this bug is currently blocked on resolving the ambiguity in the HTML5 canvas element specification
The question was raised on whatwg : http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2011-August/032849.html
But this thread did not yield a satisfying answer (yet).

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