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shaping: Don't assume there's a space glyph when rendering tabs (#32979)

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Previously if a font didn't have a space advance and it was needed to
make advances for tabs, Servo would try to read the advance from the
font. If the font didn't have a space glyph, Servo would panic. This
fixes that issue by making the space advance part of the `FontMetrics`
of a font (like Gecko) and falling back properly if that glyph doesn't
exist. The rendered glyph is still the "space" glyph, but we make
sure to select a font that supports that glyph explicitly.

This prevents a crash, but tabs still aren't handled properly. In
reality, tab stops should be calculated in layout and the size of
the space character of the current font shouldn't come into play.
The addition of the space advance metric will make this easier.

Fixes #32970.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
This commit is contained in:
Martin Robinson 2024-08-23 04:17:44 -07:00 • committed by GitHub
parent ebdae6094e
commit 78e2691d3f
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7 changed files with 75 additions and 40 deletions
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17
components/fonts/platform/windows/font.rs
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@ -19,6 +19,7 @@ use log::{debug, warn};
use style::computed_values::font_stretch::T as StyleFontStretch;
use style::computed_values::font_weight::T as StyleFontWeight;
use style::values::computed::font::FontStyle as StyleFontStyle;
use style::Zero;
use truetype::tables::WindowsMetrics;
use truetype::value::Read;
use webrender_api::FontInstanceFlags;
@ -242,6 +243,17 @@ impl PlatformFontMethods for PlatformFont {
.and_then(|idx| self.glyph_h_advance(idx))
.map(Au::from_f64_px);
// TODO: These should be retrieved from the OS/2 table if possible and then
// fall back to measuring 'x' if that is not available.
let average_advance = Au::zero();
let max_advance = Au::zero();
let space_advance = self
.glyph_index(' ')
.and_then(|index| self.glyph_h_advance(index))
.map(Au::from_f64_px)
.unwrap_or(average_advance);
let metrics = FontMetrics {
underline_size: au_from_du(dm.underlineThickness as i32),
underline_offset: au_from_du_s(dm.underlinePosition as i32),
@ -252,11 +264,12 @@ impl PlatformFontMethods for PlatformFont {
em_size: au_from_em(self.em_size as f64),
ascent: au_from_du_s(dm.ascent as i32),
descent: au_from_du_s(dm.descent as i32),
max_advance: au_from_pt(0.0), // FIXME
average_advance: au_from_pt(0.0), // FIXME
max_advance,
average_advance,
line_gap: au_from_du_s((dm.ascent + dm.descent + dm.lineGap as u16) as i32),
zero_horizontal_advance,
ic_horizontal_advance,
space_advance,
};
debug!("Font metrics (@{} pt): {:?}", self.em_size * 12., metrics);
metrics