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font: render colour emoji from bitmap strikes, scaled to the cell

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Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: If30e5f13ee24e691b417ad35c588a6226a6a6964
This commit is contained in:
raf 2026-06-24 14:01:25 +03:00
commit 5682027a94
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GPG key ID: 29D95B64378DB4BF
2 changed files with 62 additions and 29 deletions
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39
src/font.rs
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@ -213,9 +213,7 @@ impl Fonts {
}
let index = matched.face_index().unwrap_or(0) as isize;
let face = self.library.new_face(&path, index)?;
// Bitmap-strike-only fonts reject an arbitrary pixel size; skip them
// rather than guess a strike.
if face.set_pixel_sizes(0, self.size_px).is_err() {
if size_face(&face, self.size_px).is_err() {
return Ok(None);
}
self.faces.push(face);
@ -240,10 +238,43 @@ fn resolve_face(
.find(family, Some(style.fontconfig_style()))
.map_err(|_| FontError::NoFamily(family.to_owned()))?;
let face = library.new_face(&font.path, font.index.unwrap_or(0) as isize)?;
face.set_pixel_sizes(0, size_px)?;
size_face(&face, size_px)?;
Ok(face)
}
/// Set a face to `size_px`. Scalable faces size directly; bitmap-strike faces
/// (e.g. colour-emoji fonts) cannot, so select the nearest available strike and
/// let the renderer scale its glyphs into the cell.
fn size_face(face: &Face, size_px: u32) -> Result<(), FontError> {
match face.set_pixel_sizes(0, size_px) {
Ok(()) => Ok(()),
Err(_) if face.has_fixed_sizes() => {
face.select_size(nearest_strike(face, size_px))?;
Ok(())
}
Err(err) => Err(err.into()),
}
}
/// Index of the fixed strike whose pixel height is closest to `target`.
fn nearest_strike(face: &Face, target: u32) -> i32 {
let rec = face.raw();
let target = i32::try_from(target).unwrap_or(i32::MAX);
let mut best = 0;
let mut best_delta = i32::MAX;
for i in 0..rec.num_fixed_sizes {
// SAFETY: `available_sizes` points to `num_fixed_sizes` valid
// `FT_Bitmap_Size` entries for the face's lifetime; `i` is in range.
let height = i32::from(unsafe { (*rec.available_sizes.offset(i as isize)).height });
let delta = (height - target).abs();
if delta < best_delta {
best = i;
best_delta = delta;
}
}
best
}
fn cell_metrics(face: &Face, family: &str) -> Result<CellMetrics, FontError> {
let metrics = face
.size_metrics()