On 9/6/14 22:15, Behdad Esfahbod wrote:
On 14-06-09 10:12 AM, Jonathan Kew wrote:
Hi Behdad,
Our current mark-zeroing code, in zero_mark_widths_by_gdef() and
zero_mark_widths_by_unicode(), modifies only the advance of the glyphs, so
that they no longer take up any space on the line.
Right.
I'm wondering whether we should also adjust the offset, by subtracting the
advance from it before we zero the advance. (Though perhaps only if there's no
GPOS positioning?)
We definitely need to test what Uniscribe does, eg. for Latin. Note that any
offsetting should only happen in LTR, not RTL.
Good point - doing it in RTL has the opposite effect, and makes things
worse. (Indeed, that showed up as a test failure when I ran an
experimental patch.)
In particular, this is required for correct rendering of old fonts such as MS
Sans Serif codepage 874 (Thai), which has spacing glyphs for the Thai
vowel/tone marks.
Note that Thai and Hangul might be more affected as they have
fallback_position turned off. The default shaper has fallback_position turned
on which takes care of this.
Ah, right - with fallback_position it shouldn't be an issue. So this
really is primarily about Thai.
AFAICT, what Uniscribe does with it is to zero their advance
width *and* offset them to the left by their original advance, so that they
overstrike the preceding glyph, even though the images in the font appear to
the right of the origin.
WDYT? Can you think of fonts where this would *not* be appropriate?
The reason we turned fallback positioning off for Thai had to do with vertical
positioning IIRC. Maybe we should turn it on for horizontal but not vertical?
I don't think so. The Thai glyphs are designed to be placed relative to
the right-hand edge of the base, not centered. (And the results I'm
getting with this exactly match what Uniscribe does.)
I don't remember why we did that for Hangul, perhaps because of the special
logic we have for tone marks.
Sounds right. Hangul doesn't have a whole lot of marks to worry about,
but the ones it does have are pretty special, and fallback positioning
wouldn't do the right thing with them.
Suggested patch in https://bugzilla.mozilla.org/show_bug.cgi?id=985220
(for the version of harfbuzz we currently have in our tree, though I
don't think this code has changed substantially?). This fixes the Thai
version of MS Sans Serif, and shouldn't disrupt anything else AFAICS.
Please take a look and see if it seems reasonable to you - thanks.
JK
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