489776 - Clipping produces a bogus fringe effect

Status: NEW

(Core :: SVG, defect)

Description

[Jonathan Watt [:jwatt]]

Clipping produces a bogus fringe effect.

Comment 1

[Jonathan Watt [:jwatt]]

Attachment: testcase - you should not see a vague outline of a circle

Comment 2

[Jonathan Watt [:jwatt]]

Ignore the comment in the desc. I forgot to remove that.

Comment 3

[Robert Longson [:longsonr]]

Fixed by bug 489718 perhaps?

Comment 4

[Robert O'Callahan (:roc) (email my personal email if necessary)]

With bug 489718, authors could specify "crispEdges" to work around this bug, but it doesn't really fix this bug.

This is fundamental to the way cairo does clipping to non-pixel-aligned-rectangles --- setting up a mask surface with partial alpha values and applying that mask to each drawing operation individually. Not only does that lead to artifacts like in this bug, it's also incredibly slow.

As long as we're stuck with coverage-based AA, I think the best we can do is to use PushGroup to group together all components of a clipped element so that the clipping mask is applied just once. That would fix this bug and improve performance a lot when a complicated group is clipped.