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Fix bitmap glyph positioning with scaled transforms #1306

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Fix bitmap glyph positioning with scaled transforms #1306

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17 changes: 11 additions & 6 deletions vello/src/scene.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -590,6 +590,10 @@ impl<'a> DrawGlyphs<'a> {
)
.to_vec();
let location = LocationRef::new(&coords);

let transform_scale = run_transform.determinant().abs().sqrt() as f32;
let scaled_font_size = self.run.font_size * transform_scale;

loop {
let ppem = self.run.font_size;
let outline_glyphs = (&mut glyphs).take_while(|glyph| {
Expand Down Expand Up @@ -711,15 +715,16 @@ impl<'a> DrawGlyphs<'a> {
}
};
let image = ImageBrush::new(image).multiply_alpha(self.brush_alpha);
// Split into multiple statements because rustfmt breaks
let transform =
run_transform.pre_translate(Vec2::new(glyph.x.into(), glyph.y.into()));

// Compute position in physical coordinates to handle scaled transforms correctly
let glyph_physical_pos =
run_transform * Point::new(glyph.x.into(), glyph.y.into());

// Logic copied from Skia without examination or careful understanding:
// https://github.com/google/skia/blob/61ac357e8e3338b90fb84983100d90768230797f/src/ports/SkTypeface_fontations.cpp#L664

let image_scale_factor = self.run.font_size / bitmap.ppem_y;
let font_units_to_size = self.run.font_size / upem;
let image_scale_factor = scaled_font_size / bitmap.ppem_y;
let font_units_to_size = scaled_font_size / upem;

// CoreText appears to special case Apple Color Emoji, adding
// a 100 font unit vertical offset. We do the same but only
Expand All @@ -734,7 +739,7 @@ impl<'a> DrawGlyphs<'a> {
bitmap.bearing_y
};

let transform = transform
let transform = Affine::translate(glyph_physical_pos.to_vec2())
.pre_translate(Vec2 {
x: (-bitmap.bearing_x * font_units_to_size).into(),
y: (bearing_y * font_units_to_size).into(),
Expand Down