615121 - vertical glyph offsets are inverted

Status: RESOLVED FIXED

(Core :: Layout: Text and Fonts, defect)

Description

[Karl Tomlinson (:karlt)]

An operator precedence error from bug 605043 means that the y-axis glyph
offsets are not correctly inverted, so GPOS offsets get applied in the wrong
direction when the HarfBuzz shaper is enabled on Linux.

Comment 1

[Karl Tomlinson (:karlt)]

Attachment: patch

I don't understand why the bug is not showing up in the same way on other
platforms.  With GDI and DWrite, fixing the error changes the positions in 
layout/reftests/text/475092-pos.html by only one pixel, which makes it pass.
(The reftest still doesn't pass on Linux or Mac - see bug 482596 - but is much
closer on Linux at least.)

Comment 2

[Jonathan Kew [:jfkthame]]

(In reply to comment #1)
> Created attachment 493586 [details] [diff] [review]
> patch
> 
> I don't understand why the bug is not showing up in the same way on other
> platforms.

It does... I just noticed it a couple of days ago on OS X, when testing a complex Arabic-script font that used to work; was intending to file and investigate, but you got there first - thanks!

Comment 4

[Jonathan Kew [:jfkthame]]

We should add a testcase to catch this, especially as y-axis sign errors are so easy to make, and not always obvious at first because vertical deltas are rarely used. I think we can do something using @font-face with a font that's known to support vertically-stacked marks...

Comment 5

[Jonathan Kew [:jfkthame]]

Attachment: reftests for direction of vertical glyph offsets

Here are a couple of reftests using the Scheherazade font that we have in the reftest fonts directory; they will fail if we make a sign error handling the glyph y-offsets.

Comment 6

[Karl Tomlinson (:karlt)]

Comment on attachment 493667 [details] [diff] [review]
reftests for direction of vertical glyph offsets

Nice, thank you!

Comment 7

[Jonathan Kew [:jfkthame]]

Pushed this to trunk:
http://hg.mozilla.org/mozilla-central/rev/3e3726fc8083 (patch)
http://hg.mozilla.org/mozilla-central/rev/a5aafda069f7 (tests)