On Thu, 3 Apr 2025 11:23:42 GMT, Daniel Gredler <dgred...@openjdk.org> wrote:
> It looks like this regression actually fits into a longer series of fixes / > regressions in this area: > > - [JDK-4517298](https://bugs.openjdk.org/browse/JDK-4517298) fixed metrics > for zero-width characters, but broke some ligatures / glyph substitutions > - [JDK-7017058](https://bugs.openjdk.org/browse/JDK-7017058) fixed the > ligatures / glyph substitutions, but broke some zero-width metrics > - [JDK-8208377](https://bugs.openjdk.org/browse/JDK-8208377) fixed some > metrics and rendering for zero-width characters, but broke some ligatures / > glyph substitutions > - Now, with this PR, we aim to fix the ligatures without re-breaking > zero-width metrics and display > > We have two different types of use cases pulling `CharToGlyphMapper` in two > different directions: the users who need raw, untransformed glyph info, and > the users who need normalized / transformed glyph info. > > It looks to me like, in the current code base, the only `CharToGlyphMapper` > user which requires raw font data is HarfBuzz (explicitly confirmed with the > HarfBuzz team here: https://github.com/harfbuzz/harfbuzz/discussions/5234). > > The regression mechanism at play here is that the HarfBuzz font callbacks are > currently providing HarfBuzz with transformed glyph info (e.g. ZWJ -> > INVISIBLE_GLYPH_ID), which prevents HarfBuzz from recognizing and applying > the correct font GSUB substitutions (which involve ZWJ). > > In order to fix this without (yet again) breaking metrics and display > behavior elsewhere, I've added two methods to `CharToGlyphMapper` which > provide access to raw glyph info, to be used by the HarfBuzz font callbacks: > `charToGlyphRaw(int)` and `charToVariationGlyphRaw(int)`. > > Note two intricacies related to `CompositeGlyphMapper`: > 1. We need to be careful to only cache raw (untransformed) values, to avoid > conflicts between requests for a raw version of a glyph and a transformed > version of the same glyph. Another option would have been two separate > caches, but I don't think that's necessary. > 2. Consumers who are using `CompositeGlyphMapper.SLOTMASK` to check glyph > slots (e.g. `FontRunIterator` and `CTextPipe`) will "see" invisible glyphs as > having come from slot 0. This isn't new, and I think it's OK, but something > to be aware of. > > The glyph cache handling in `CCharToGlyphMapper` (for macOS) also requires > care to avoid mixing value types. > > Please also note that I'm not sure if the tweak to `sunFont.c` is being > tested, since FFM is being used by default for HarfBuzz integration. (Is > there a plan to remove... We had similar emoji-related regressions at JetBrains. Although our font-related code diverged from OpenJDK a bit, porting this patch seems to resolve them too. I am not an OpenJDK reviewer, but LGTM nevertheless. ------------- PR Comment: https://git.openjdk.org/jdk/pull/24412#issuecomment-2806961451
