Bug 76137 – Borders and box masks behave incorrectly with overlapping offsets

• Component: WebCore Misc.
• Status: RESOLVED FIXED

[Ian Henderson]

Created attachment 122147 [details]
test case

See attached test case.  An 8x8 image is used as a box mask and a border image.  The test case includes three offsets to divide the image into nine parts -- 3, 4, and 5.  The rendering is incorrect in the 4 and 5 cases (though it may not appear so in the box mask case -- the untouched areas are left unclipped unless we fall into the transparency layer path by e.g. pinch zooming).

It seems like we should do what it says at <http://www.w3.org/TR/css3-background/#border-image-width>:
If two opposite ‘border-image-width’ offsets are large enough that they overlap, then the used values of all ‘border-image-width’ offsets are proportionally reduced until they no longer overlap. In mathematical notation: Given Lwidth as the width of the border image area, Lheight as its height, and Wside as the border image width offset for the side side, let f = min(Lwidth/(Wleft+Wright), Lheight/(Wtop+Wbottom)). If f < 1, then all W are reduced by multiplying them by f.

[Ian Henderson]

(In reply to comment #0)
> It seems like we should do what it says at <http://www.w3.org/TR/css3-background/#border-image-width>:
> If two opposite ‘border-image-width’ offsets are large enough that they overlap, then the used values of all ‘border-image-width’ offsets are proportionally reduced until they no longer overlap. In mathematical notation: Given Lwidth as the width of the border image area, Lheight as its height, and Wside as the border image width offset for the side side, let f = min(Lwidth/(Wleft+Wright), Lheight/(Wtop+Wbottom)). If f < 1, then all W are reduced by multiplying them by f.

Looking at the code, we already make this correction, but it's not enough to ensure there's at least one pixel in the middle to stretch.

[Simon Fraser (smfr)]

Debug builds assert when zooming with the testcase:

ASSERTION FAILED: CGImageGetHeight(image.get()) == currHeight - CGRectIntegral(srcRect).origin.y
/Volumes/SSData/Development/OSX/webkit/OpenSource/Source/WebCore/platform/graphics/cg/GraphicsContextCG.cpp(197) : void WebCore::GraphicsContext::drawNativeImage(NativeImagePtr, const WebCore::FloatSize &, WebCore::ColorSpace, const WebCore::FloatRect &, const WebCore::FloatRect &, WebCore::CompositeOperator)
1   0x10422a238 WebCore::GraphicsContext::drawNativeImage(CGImage*, WebCore::FloatSize const&, WebCore::ColorSpace, WebCore::FloatRect const&, WebCore::FloatRect const&, WebCore::CompositeOperator)

[Simon Fraser (smfr)]

Isn't the relevant spec text actually:
"The regions given by the ‘border-image-slice’ values may overlap. However if the sum of the right and left widths is equal to or greater than the width of the image, the images for the top and bottom edge and the middle part are empty, which has the same effect as if a nonempty transparent image had been specified for those parts. Analogously for the top and bottom values."

[Simon Fraser (smfr)]

Created attachment 122288 [details]
Patch

[Simon Fraser (smfr)]

Will investigate issues with border-image-width separately.

[WebKit Review Bot]

Comment on attachment 122288 [details]
Patch

Attachment 122288 [details] did not pass chromium-ews (chromium-xvfb):
Output: http://queues.webkit.org/results/11227168

New failing tests:
animations/cross-fade-webkit-mask-box-image.html
fast/borders/border-image-slice-constrained.html

[Dave Hyatt]

Comment on attachment 122288 [details]
Patch

r=me

[Simon Fraser (smfr)]

Comment on attachment 122288 [details]
Patch

http://trac.webkit.org/changeset/104851
Keeping bug open to look at border-image-width behavior.

[Simon Fraser (smfr)]

Created attachment 122313 [details]
Patch to fix mask-box-image behavior with large slices

[WebKit Review Bot]

Comment on attachment 122313 [details]
Patch to fix mask-box-image behavior with large slices

Attachment 122313 [details] did not pass chromium-ews (chromium-xvfb):
Output: http://queues.webkit.org/results/11115617

New failing tests:
fast/backgrounds/mask-box-image.html

[Simon Fraser (smfr)]

http://trac.webkit.org/changeset/105091