393128 - Drag image code does not deal with pre-multiplied alpha

Status: RESOLVED FIXED

(Core :: Widget: Cocoa, defect)

Description

[Colin Barrett [:cbarrett]]

STR:
1. Start up Minefield, have it go to the default Minefield homepage
2. Go to www.google.com
3. Drag the Google image

Expected results:
Image appears with 65% opacity

Actual results:
Background is 100% opaque white, colors show through. Definitely not normal SourceOver compositing.

I have two theories.

1) The dragging code in nsDragService works with pixels directly. Cocoa uses premultiplied alpha, so this type of direct manipulation might not be ideal. Instead, we should consider using disolveToPoint:fraction:.

2) Our use of CGContextSetCompositeOperation (http://mxr.mozilla.org/seamonkey/search?string=CGContextSetCompositeOperatio) might be screwing with the compositing operation used. We might want to do a gsave, set it to SourceOver when the drag begins, and when the drag ends do a grestore.

I'll do some investigation as time permits. I've been meaning to file this for a while now.

Comment 1

[Colin Barrett [:cbarrett]]

Attachment: fix v1.0

Here's my proposed fix.

Turns out that gfxImageSurface returns pre-multiplied alpha, and NSBitmapImageRep also expects pre-multiplied alpha. The previous code was not pre-multiplying things, thus the odd appearance.

I took the liberty of changing return nsnull to return nil in ConstructDragImage -- it returns an ObjC type, it should use the ObjC version of null.

Comment 2

[Colin Barrett [:cbarrett]]

Checking in widget/src/cocoa/nsDragService.mm;
/cvsroot/mozilla/widget/src/cocoa/nsDragService.mm,v  <--  nsDragService.mm
new revision: 1.23; previous revision: 1.22
done
(typo'd bug number in commit, oops)