Originally we meant to match backtrack/lookahead across syllable
boundaries. But a bug in the code meant that this was NOT done for
backtrack. We "fixed" that in 2c7d0b6b80,
but that broke Myanmar shaping.
We now believe that for Indic-like shapers (which is where syllables are
used), all basic shaping forms should be fully contained within their
syllables, so now we limit backtrack/lookahead matching to the syllable
too. Unbreaks Myanmar.
Not for Arabic, but for Indic-like scripts. ZWJ/ZWNJ have special
meanings in those scripts, so let font lookups take full control.
This undoes the regression caused by automatic-joiners handling
introduced two commits ago.
We only disable automatic joiner handling for the "basic shaping
features" of Indic, Myanmar, and SEAsian shapers. The "presentation
forms" and other features are still applied with automatic-joiner
handling.
This change also changes the test suite failure statistics, such that
a few scripts show more "failures". The most affected is Kannada.
However, upon inspection, we believe that in most, if not all, of the
new failures, we are producing results superior to Uniscribe. Hard to
count those!
Here's an example of what is fixed by the recent joiner-handling
changes:
https://bugs.freedesktop.org/show_bug.cgi?id=58714
New numbers, for future reference:
BENGALI: 353892 out of 354188 tests passed. 296 failed (0.0835714%)
DEVANAGARI: 707336 out of 707394 tests passed. 58 failed (0.00819911%)
GUJARATI: 366262 out of 366457 tests passed. 195 failed (0.0532122%)
GURMUKHI: 60706 out of 60747 tests passed. 41 failed (0.067493%)
KANNADA: 950680 out of 951913 tests passed. 1233 failed (0.129529%)
KHMER: 299074 out of 299124 tests passed. 50 failed (0.0167155%)
LAO: 53611 out of 53644 tests passed. 33 failed (0.0615167%)
MALAYALAM: 1047983 out of 1048334 tests passed. 351 failed (0.0334817%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271539 out of 271847 tests passed. 308 failed (0.113299%)
TAMIL: 1091753 out of 1091754 tests passed. 1 failed (9.15957e-05%)
TELUGU: 970555 out of 970573 tests passed. 18 failed (0.00185457%)
TIBETAN: 208469 out of 208469 tests passed. 0 failed (0%)
When matching lookups, be smart about default-ignorable characters.
In particular:
Do nothing specific about ZWNJ, but for the other default-ignorables:
If the lookup in question uses the ignorable character in a sequence,
then match it as we used to do. However, if the sequence match will
fail because the default-ignorable blocked it, try skipping the
ignorable character and continue.
The most immediate thing it means is that if Lam-Alef forms a ligature,
then Lam-ZWJ-Alef will do to. Finally!
One exception: when matching for GPOS, or for backtrack/lookahead of
GSUB, we ignore ZWNJ too. That's the right thing to do.
It certainly is possible to build fonts that this feature will result
in undesirable glyphs, but it's hard to think of a real-world case
that that would happen.
This *does* break Indic shaping right now, since Indic Unicode has
specific rules for what ZWJ/ZWNJ mean, and skipping ZWJ is breaking
those rules. That will be fixed in upcoming commits.
Ouch, how did things ever work without this?! The added test that has a
dot-reph as well as a pre-base reordering Ra perfectly demonstrates the
bug (tested with Nirmala font from Win8 for example). Testing suggests
that Win8 shaper has the *exact* same bug / behavior that we used to
have. Odd.
This is a followup to 568000274c.
Looks like in the Latin shaper, Uniscribe zeroes all Unicode NSM
advances *after* GPOS, not before. Match that.
Can be tested using DejaVu Sans Mono, since that font has GPOS
rules to zero the mark advances on its own.
Before, we were zeroing advance width of attached marks for
non-Indic scripts, and not doing it for Indic.
We have now three different behaviors, which seem to better
reflect what Uniscribe is doing:
- For Indic, no explicit zeroing happens whatsoever, which
is the same as before,
- For Myanmar, zero advance width of glyphs marked as marks
*in GDEF*, and do that *before* applying GPOS. This seems
to be what the new Win8 Myanmar shaper does,
- For everything else, zero advance width of glyphs that are
from General_Category=Mn Unicode characters, and do so
before applying GPOS. This seems to be what Uniscribe does
for Latin at least.
With these changes, positioning of all tests matches for Myanmar,
except for the glitch in Uniscribe not applying 'mark'. See preivous
commit.
Implemented as a hack for now. Myanmar failures down from 23 to 15.
MYANMAR: 1123868 out of 1123883 tests passed. 15 failed (0.00133466%)
The remaining 15 cases are all where the syllable is wrong according to
the OpenType spec. We insert dottedcircle. Uniscribe fails to do that,
but it also fails to reorder the prebase-reordering medial-Ra. So it
gets it wrong.
Before, when matching ligatures, we never skipping over base / liga
glyphs even if that was what the LookupFlags asked for.
Fixed now. We carefully reviewed all instances of this, and tested with
Amiri as well as some Indic scripts, and are confident that this should
NOT break anyone's fonts. It's also how Uniscribe does it, from what
we can tell.
Before, for most scripts, we were not trying to recompose two characters
if the second one had ccc=0. That fails for Myanmar where U+1026
decomposes to U+1025,U+102E, both of which have ccc=0. However, we do
want to try to recompose those. We now check whether the second is a
mark, using general category instead.
At the same time, remove optimization that was conflicting with this.
[Let the Ngapi hackfest begin!]
Behdad Esfahbod