Bug 135686

Summary: Subpixel rendering: Border thickness and length flooring can result empty borders due to losing precision during multiple float <-> LayoutUnit conversions.
Product: WebKit Reporter: zalan <zalan>
Component: Layout and RenderingAssignee: zalan <zalan>
Status: RESOLVED FIXED    
Severity: Normal CC: commit-queue, ddkilzer, esprehn+autocc, glenn, kondapallykalyan, simon.fraser
Priority: P2    
Version: 528+ (Nightly build)   
Hardware: Unspecified   
OS: Unspecified   
Bug Depends on:    
Bug Blocks: 135898    
Attachments:
Description Flags
Patch
none
Patch none

zalan
Reported 2014-08-06 20:18:48 PDT
in RenderObject::drawLineForBoxSide() we check whether the border thickness and length are > 0. Flooring is used to avoid painting empty lines. In certain cases, when we lose precession while converting float <-> LayoutUnit back and forth, flooring a border width value of 0.5px (before conversions and 0.49999 after conversions) becomes 0 and we early return from painting.
Attachments
Patch (10.07 KB, patch)
2014-08-06 20:33 PDT, zalan 		no flags
DetailsFormatted DiffDiff
Patch (10.06 KB, patch)
2014-08-07 20:13 PDT, zalan 		no flags
DetailsFormatted DiffDiff
Show Obsolete (1) View All Add attachment proposed patch, testcase, etc.
zalan
Comment 1 2014-08-06 20:33:47 PDT
Created attachment 236169 [details] Patch
Simon Fraser (smfr)
Comment 2 2014-08-07 13:52:12 PDT
Comment on attachment 236169 [details] Patch View in context: https://bugs.webkit.org/attachment.cgi?id=236169&action=review > Source/WebCore/ChangeLog:10 > + empty border rect drawing can lead to false positives. false positives of what?
zalan
Comment 3 2014-08-07 20:13:01 PDT
Created attachment 236256 [details] Patch
WebKit Commit Bot
Comment 4 2014-08-07 20:52:00 PDT
Comment on attachment 236256 [details] Patch Clearing flags on attachment: 236256 Committed r172326: <http://trac.webkit.org/changeset/172326>
WebKit Commit Bot
Comment 5 2014-08-07 20:52:03 PDT
All reviewed patches have been landed. Closing bug.
David Kilzer (:ddkilzer)
Comment 6 2015-01-05 20:41:57 PST
(In reply to comment #4) > Comment on attachment 236256 [details] > Patch > > Clearing flags on attachment: 236256 > > Committed r172326: <http://trac.webkit.org/changeset/172326> Note that this change caused: Bug 135898: Dashed/dotted borders do not paint. <https://bugs.webkit.org/show_bug.cgi?id=135898>
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