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Make subpixel positioning match that of Gecko. #1715

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Merged
merged 1 commit into from
Sep 19, 2017
Merged

Make subpixel positioning match that of Gecko. #1715

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8 changes: 6 additions & 2 deletions webrender/res/prim_shared.glsl
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Original file line number Diff line number Diff line change
Expand Up @@ -289,10 +289,14 @@ Glyph fetch_glyph(int specific_prim_address,
case SUBPX_DIR_NONE:
break;
case SUBPX_DIR_HORIZONTAL:
glyph.x = trunc(glyph.x);
// Glyphs positioned [-0.125, 0.125] get a
// subpx position of zero. So include that
// offset in the glyph position to ensure
// we truncate to the correct whole position.
glyph.x = trunc(glyph.x + 0.125);
break;
case SUBPX_DIR_VERTICAL:
glyph.y = trunc(glyph.y);
glyph.y = trunc(glyph.y + 0.125);
break;
}

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89 changes: 66 additions & 23 deletions webrender_api/src/font.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -101,33 +101,26 @@ pub enum SubpixelDirection {
Vertical,
}

const FIXED16_SHIFT: i32 = 16;

// This matches the behaviour of SkScalarToFixed
fn f32_truncate_to_fixed16(x: f32) -> i32 {
let fixed1 = (1 << FIXED16_SHIFT) as f32;
(x * fixed1) as i32
}

impl FontRenderMode {
// Skia quantizes subpixel offets into 1/4 increments.
// Given the absolute position, return the quantized increment
fn subpixel_quantize_offset(&self, pos: f32) -> SubpixelOffset {
const SUBPIXEL_BITS: i32 = 2;
const SUBPIXEL_FIXED16_MASK: i32 =
((1 << SUBPIXEL_BITS) - 1) << (FIXED16_SHIFT - SUBPIXEL_BITS);

const SUBPIXEL_ROUNDING: f32 = 0.5 / (1 << SUBPIXEL_BITS) as f32;
let pos = pos + SUBPIXEL_ROUNDING;
let fraction = (f32_truncate_to_fixed16(pos) & SUBPIXEL_FIXED16_MASK) >>
(FIXED16_SHIFT - SUBPIXEL_BITS);

match fraction {
0 => SubpixelOffset::Zero,
1 => SubpixelOffset::Quarter,
2 => SubpixelOffset::Half,
3 => SubpixelOffset::ThreeQuarters,
_ => panic!("Should only be given the fractional part"),
// Following the conventions of Gecko and Skia, we want
// to quantize the subpixel position, such that abs(pos) gives:
// [0.0, 0.125) -> Zero
// [0.125, 0.375) -> Quarter
// [0.375, 0.625) -> Half
// [0.625, 0.875) -> ThreeQuarters,
// [0.875, 1.0) -> Zero
// The unit tests below check for this.
let apos = (pos.fract() * 8.0).abs() as i32;

match apos {
0 | 7 => SubpixelOffset::Zero,
1...2 => SubpixelOffset::Quarter,
3...4 => SubpixelOffset::Half,
5...6 => SubpixelOffset::ThreeQuarters,
_ => unreachable!("bug: unexpected quantized result"),
}
}

Expand Down Expand Up @@ -274,3 +267,53 @@ pub struct GlyphInstance {
pub index: GlyphIndex,
pub point: LayoutPoint,
}

#[cfg(test)]
mod test {
use super::{FontRenderMode, SubpixelOffset};

#[test]
fn test_subpx_quantize() {
let rm = FontRenderMode::Subpixel;

assert_eq!(rm.subpixel_quantize_offset(0.0), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(-0.0), SubpixelOffset::Zero);

assert_eq!(rm.subpixel_quantize_offset(0.1), SubpixelOffset::Zero);
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nit: could be nicer if those subpixel_quantize_offset calls are batched per expected SubpixelOffset

assert_eq!(rm.subpixel_quantize_offset(0.01), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.05), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.12), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.124), SubpixelOffset::Zero);

assert_eq!(rm.subpixel_quantize_offset(0.125), SubpixelOffset::Quarter);
assert_eq!(rm.subpixel_quantize_offset(0.2), SubpixelOffset::Quarter);
assert_eq!(rm.subpixel_quantize_offset(0.25), SubpixelOffset::Quarter);
assert_eq!(rm.subpixel_quantize_offset(0.33), SubpixelOffset::Quarter);
assert_eq!(rm.subpixel_quantize_offset(0.374), SubpixelOffset::Quarter);

assert_eq!(rm.subpixel_quantize_offset(0.375), SubpixelOffset::Half);
assert_eq!(rm.subpixel_quantize_offset(0.4), SubpixelOffset::Half);
assert_eq!(rm.subpixel_quantize_offset(0.5), SubpixelOffset::Half);
assert_eq!(rm.subpixel_quantize_offset(0.58), SubpixelOffset::Half);
assert_eq!(rm.subpixel_quantize_offset(0.624), SubpixelOffset::Half);

assert_eq!(rm.subpixel_quantize_offset(0.625), SubpixelOffset::ThreeQuarters);
assert_eq!(rm.subpixel_quantize_offset(0.67), SubpixelOffset::ThreeQuarters);
assert_eq!(rm.subpixel_quantize_offset(0.7), SubpixelOffset::ThreeQuarters);
assert_eq!(rm.subpixel_quantize_offset(0.78), SubpixelOffset::ThreeQuarters);
assert_eq!(rm.subpixel_quantize_offset(0.874), SubpixelOffset::ThreeQuarters);

assert_eq!(rm.subpixel_quantize_offset(0.875), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.89), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.91), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.967), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(0.999), SubpixelOffset::Zero);

assert_eq!(rm.subpixel_quantize_offset(-1.0), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(1.0), SubpixelOffset::Zero);
assert_eq!(rm.subpixel_quantize_offset(1.5), SubpixelOffset::Half);
assert_eq!(rm.subpixel_quantize_offset(-1.625), SubpixelOffset::ThreeQuarters);
assert_eq!(rm.subpixel_quantize_offset(-4.33), SubpixelOffset::Quarter);

}
}