Bug 125963

Summary:

Subpixel layout: RenderLineBreak/RenderInline/RenderBlock::baselinePosition return value needs flooring (apple.com).

Product:

WebKit

Reporter:

alan <zalan>

Component:

Layout and Rendering

Assignee:

alan <zalan>

Status:

RESOLVED INVALID

Severity:

Normal

CC:

hyatt, jonlee, simon.fraser, webkit-bug-importer

Priority:

Keywords:

InRadar

Version:

528+ (Nightly build)

Hardware:

Unspecified

OS:

Unspecified

Bug Depends on:

Bug Blocks:

126283

Attachments:

Description	Flags
test case	none
screenshots	none
screenshot (apple.com)	none
test case (apple.com)	none

alan

Reported 2013-12-18 16:44:29 PST

Created attachment 219589 [details] test case In RenderBlock::baselinePosition the baseline value is calculated as follows: font baseline + (lineHeight - fontHeight)/2 return fontMetrics.ascent(baselineType) + (lineHeight(firstLine, direction, linePositionMode) - fontMetrics.height()) / 2; The font metric values are CSS pixel values, lineHeight() returns LayoutUnit. While calculating the baseline, the CSS pixel values (font metrics) get converted to layout units and at the end the result gets floored to CSS pixel value (return value). The floored return value can end up being different, if the /2 operation results in a negative value with fraction. subpixel on: ascent: 9px + offset: -0.5px = 8.5px -> 8px subpixel off: ascent: 9px + offset: -0.5px -> 0px = 9px observed on cnn.com

Attachments
test case (152 bytes, text/html) 2013-12-18 16:44 PST, alan	no flags	Details
screenshots (293.13 KB, image/gif) 2013-12-18 19:36 PST, alan	no flags	Details
screenshot (apple.com) (277.08 KB, image/gif) 2013-12-19 15:35 PST, alan	no flags	Details
test case (apple.com) (273 bytes, text/html) 2013-12-19 15:53 PST, alan	no flags	Details
View All Add attachment proposed patch, testcase, etc.

alan

Comment 1 2013-12-18 16:46:32 PST

<rdar://problem/15694755>

alan

Comment 2 2013-12-18 19:36:14 PST

Created attachment 219605 [details] screenshots adding anim gif to demonstrate the off-by-one rendering.

alan

Comment 3 2013-12-19 15:35:00 PST

Created attachment 219692 [details] screenshot (apple.com) RenderInline::baselinePosition has the exact same calculation as RenderBlock and the missing rounding shows up on apple.com (screenshot attached)

alan

Comment 4 2013-12-19 15:53:07 PST

Created attachment 219694 [details] test case (apple.com)

alan

Comment 5 2013-12-23 13:38:45 PST

And by flooring, I meant bias vertical centering similar to bug 101848

alan

Comment 6 2014-01-15 10:34:42 PST

The current subpixel rendering is closer to FF's rendering than it would be with the rounding (supbixel off in practice)

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