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fonts: Unify letter-spacing/word-spacing implementations for harfbuzz and fast path (#38657)

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Servo two shapers: harfbuzz and a "fast path" for simple cases that
don't have kerning or substitution tables. Both currently implement
`letter-spacing` and `word-spacing` seperately. Theres also a slight
difference, harfbuzz will apply word spacing to preserved newlines and
the fast path won't.

This change unifies the two implementations.

Testing: Covered by existing tests

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
This commit is contained in:
Simon Wülker 2025-08-13 20:55:28 +02:00 • committed by GitHub
parent c741e45b21
commit dac34c0dfd
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2 changed files with 29 additions and 32 deletions
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35
components/fonts/font.rs
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@ -456,14 +456,11 @@ impl Font {
None => continue,
};
let mut advance = Au::from_f64_px(self.glyph_h_advance(glyph_id));
if character == ' ' {
// https://drafts.csswg.org/css-text-3/#word-spacing-property
advance += options.word_spacing;
}
if let Some(letter_spacing) = options.letter_spacing {
advance += letter_spacing;
}
let mut advance = advance_for_shaped_glyph(
Au::from_f64_px(self.glyph_h_advance(glyph_id)),
character,
options,
);
let offset = prev_glyph_id.map(|prev| {
let h_kerning = Au::from_f64_px(self.glyph_h_kerning(prev, glyph_id));
advance += h_kerning;
@ -935,3 +932,25 @@ pub(crate) fn map_platform_values_to_style_values(mapping: &[(f64, f64)], value:
mapping[mapping.len() - 1].1
}
/// Computes the total advance for a glyph, taking `letter-spacing` and `word-spacing` into account.
pub(super) fn advance_for_shaped_glyph(
mut advance: Au,
character: char,
options: &ShapingOptions,
) -> Au {
if let Some(letter_spacing) = options.letter_spacing {
advance += letter_spacing;
};
// CSS 2.1 § 16.4 states that "word spacing affects each space (U+0020) and non-breaking
// space (U+00A0) left in the text after the white space processing rules have been
// applied. The effect of the property on other word-separator characters is undefined."
// We elect to only space the two required code points.
if character == ' ' || character == '\u{a0}' {
// https://drafts.csswg.org/css-text-3/#word-spacing-property
advance += options.word_spacing;
}
advance
}