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adventures in shaping

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This commit is a result of debugging a problem with a particular font
where a VS2 variation of a glyph produced incorrect shaping.

Further investigation showed that this was specific to using freetype
font functions and that switching to opentype functions produces the
correct shaping information in that case.

Further-further investigation showed that this difference in behavior
was really due to the font file being out of spec and freetype returning
unexpected data as a result.

This commit allows switching to using harfbuzz's own opentype font+face
(rather than freetype) and/or switching the freetype-based font to using
opentype font functions.  These two options don't yield equivalent
results in the wezterm integration: a couple of our shaping tests
fail due to the x-advances not being the same. Those can be drastically
different in some cases (eg: I seem to get 0 for certain bitmap strikes,
and for Roboto the value is off by a factor of about 1.5).

This is incomplete and not something I want to turn on by default at
the moment, but I don't want to lose this work, hence this commit.

refs: https://github.com/wez/wezterm/issues/2475
refs: https://github.com/harfbuzz/harfbuzz/issues/3806
This commit is contained in:
Wez Furlong 2022-09-03 08:34:19 -07:00
parent 2af5a05d19
commit c4d19afa75
2 changed files with 46 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
test-data/vs1-vs2.txt Normal file
View File

@ -0,0 +1,3 @@
1d49c 𝒜 𝒜︀ 𝒜︁
212c ℬ ℬ︀ ℬ︁
1d49e 𝒞 𝒞︀ 𝒞︁

59
wezterm-font/src/shaper/harfbuzz.rs
View File

@ -14,6 +14,16 @@ use thiserror::Error;
use unicode_segmentation::UnicodeSegmentation;
use wezterm_bidi::Direction;
// Changing these will switch to using harfbuzz's opentype functions.
// There's something awry with our integration in that mode: the advances
// aren't equivalent to its freetype functions and this manifests most
// prominently in proportional fonts as well as with fonts with bitmap
// strikes.
// Until we get to the bottom of this, these are compile-time rather
// than runtime configs.
const USE_OT_FUNCS: bool = false;
const USE_OT_FACE: bool = false;
#[derive(Clone, Debug)]
struct Info {
cluster: usize,
@ -138,13 +148,19 @@ impl HarfbuzzShaper {
let handle = &self.handles[font_idx];
log::trace!("shaper wants {} {:?}", font_idx, handle);
let face = self.lib.face_from_locator(&handle.handle)?;
let mut font = harfbuzz::Font::new(face.face);
let (load_flags, _) = ftwrap::compute_load_flags_from_config(
handle.freetype_load_flags,
handle.freetype_load_target,
handle.freetype_render_target,
);
font.set_load_flags(load_flags);
let font = if USE_OT_FACE {
harfbuzz::Font::from_locator(&handle.handle)?
} else {
let (load_flags, _) = ftwrap::compute_load_flags_from_config(
handle.freetype_load_flags,
handle.freetype_load_target,
handle.freetype_render_target,
);
let mut font = harfbuzz::Font::new(face.face);
font.set_load_flags(load_flags);
font
};
let features = match &handle.harfbuzz_features {
Some(features) => features
@ -220,22 +236,33 @@ impl HarfbuzzShaper {
}
}
}
pair.face.set_font_size(
font_size * self.handles[font_idx].scale.unwrap_or(1.),
dpi,
)?;
let point_size = font_size * self.handles[font_idx].scale.unwrap_or(1.);
pair.face.set_font_size(point_size, dpi)?;
// Tell harfbuzz to recompute important font metrics!
let mut font = pair.font.borrow_mut();
if USE_OT_FACE {
font.set_ppem(0, 0);
font.set_ptem(0.);
let scale = (point_size * 2f64.powf(6.)) as i32;
font.set_font_scale(scale, scale);
}
font.font_changed();
if USE_OT_FUNCS {
font.set_ot_funcs();
}
shaped_any = pair.shaped_any;
font.shape(&mut buf, pair.features.as_slice());
/*
log::info!(
"shaped font_idx={} as: {}",
log::trace!(
"shaped font_idx={} {:?} as: {}",
font_idx,
buf.serialize(Some(&pair.font))
&s[range.start..range.end],
buf.serialize(Some(&*font))
);
*/
break;
}
None => {