Subpixel glyph positioning

crates/epaint/src/text/font.rs

@@ -57,6 +57,67 @@ impl GlyphInfo {
     };
 }
 
+// Subpixel binning, taken from cosmic-text:
+// https://github.com/pop-os/cosmic-text/blob/974ddaed96b334f560b606ebe5d2ca2d2f9f23ef/src/glyph_cache.rs
+
+/// Bin for subpixel positioning of glyphs.
+///
+/// For accurate glyph positioning, we want to render each glyph at a subpixel coordinate. However, we also want to
+/// cache each glyph's bitmap. As a compromise, we bin each subpixel offset into one of four fractional values. This
+/// means one glyph can have up to four subpixel-positioned bitmaps in the cache.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
+pub(super) enum SubpixelBin {
+    #[default]
+    Zero,
+    One,
+    Two,
+    Three,
+}
+
+impl SubpixelBin {
+    /// Bin the given position and return the new integral coordinate.
+    fn new(pos: f32) -> (i32, Self) {
+        let trunc = pos as i32;
+        let fract = pos - trunc as f32;
+
+        #[expect(clippy::collapsible_else_if)]
+        if pos.is_sign_negative() {
+            if fract > -0.125 {
+                (trunc, Self::Zero)
+            } else if fract > -0.375 {
+                (trunc - 1, Self::Three)
+            } else if fract > -0.625 {
+                (trunc - 1, Self::Two)
+            } else if fract > -0.875 {
+                (trunc - 1, Self::One)
+            } else {
+                (trunc - 1, Self::Zero)
+            }
+        } else {
+            if fract < 0.125 {
+                (trunc, Self::Zero)
+            } else if fract < 0.375 {
+                (trunc, Self::One)
+            } else if fract < 0.625 {
+                (trunc, Self::Two)
+            } else if fract < 0.875 {
+                (trunc, Self::Three)
+            } else {
+                (trunc + 1, Self::Zero)
+            }
+        }
+    }
+
+    pub fn as_float(&self) -> f32 {
+        match self {
+            Self::Zero => 0.0,
+            Self::One => 0.25,
+            Self::Two => 0.5,
+            Self::Three => 0.75,
+        }
+    }
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Default)]
 pub struct GlyphAllocation {
     /// Used for pair-kerning.
@@ -65,7 +126,7 @@ pub struct GlyphAllocation {
     /// Use `ab_glyph::GlyphId(0)` if you just want to have an id, and don't care.
     pub(crate) id: ab_glyph::GlyphId,
 
-    /// Unit: points.
+    /// Unit: screen pixels.
     pub advance_width: f32,
 
     /// UV rectangle for drawing.
@@ -81,7 +142,7 @@ pub struct FontImpl {
     ab_glyph_font: ab_glyph::FontArc,
     tweak: FontTweak,
     glyph_info_cache: ahash::HashMap<char, GlyphInfo>,
-    glyph_alloc_cache: ahash::HashMap<(ab_glyph::GlyphId, u64), GlyphAllocation>,
+    glyph_alloc_cache: ahash::HashMap<(ab_glyph::GlyphId, SubpixelBin, u64), GlyphAllocation>,
 }
 
 trait FontExt {
@@ -203,14 +264,23 @@ impl FontImpl {
     }
 
     #[inline]
-    pub fn pair_kerning(
+    pub(super) fn pair_kerning_screen_space(
         &self,
         metrics: &ScaledMetrics,
         last_glyph_id: ab_glyph::GlyphId,
         glyph_id: ab_glyph::GlyphId,
     ) -> f32 {
         self.ab_glyph_font.kern_unscaled(last_glyph_id, glyph_id) * metrics.px_scale_factor
-            / metrics.pixels_per_point
+    }
+
+    #[inline]
+    pub fn pair_kerning(
+        &self,
+        metrics: &ScaledMetrics,
+        last_glyph_id: ab_glyph::GlyphId,
+        glyph_id: ab_glyph::GlyphId,
+    ) -> f32 {
+        self.pair_kerning_screen_space(metrics, last_glyph_id, glyph_id) / metrics.pixels_per_point
     }
 
     #[inline(always)]
@@ -243,18 +313,21 @@ impl FontImpl {
         atlas: &mut TextureAtlas,
         metrics: &ScaledMetrics,
         glyph_info: GlyphInfo,
-    ) -> GlyphAllocation {
+        h_pos: f32,
+    ) -> (GlyphAllocation, i32) {
         let Some(glyph_id) = glyph_info.id else {
             // Invisible.
-            return GlyphAllocation::default();
+            return (GlyphAllocation::default(), h_pos as i32);
         };
 
+        let (h_pos_round, bin) = SubpixelBin::new(h_pos);
+
         let entry = match self
             .glyph_alloc_cache
-            .entry((glyph_id, metrics.glyph_cache_key()))
+            .entry((glyph_id, bin, metrics.glyph_cache_key()))
         {
             std::collections::hash_map::Entry::Occupied(glyph_alloc) => {
-                return *glyph_alloc.get();
+                return (*glyph_alloc.get(), h_pos_round);
             }
             std::collections::hash_map::Entry::Vacant(entry) => entry,
         };
@@ -268,7 +341,10 @@ impl FontImpl {
                 // (https://github.com/alexheretic/ab-glyph/issues/15), and this field is never accessed when
                 // rasterizing. We can just put anything here.
                 scale: PxScale::from(0.0),
-                position: ab_glyph::Point::default(),
+                position: ab_glyph::Point {
+                    x: bin.as_float(),
+                    y: 0.0,
+                },
             };
             let outlined = OutlinedGlyph::new(
                 glyph,
@@ -315,12 +391,11 @@ impl FontImpl {
 
         let allocation = GlyphAllocation {
             id: glyph_id,
-            advance_width: (glyph_info.advance_width_unscaled.0 * metrics.px_scale_factor)
-                / metrics.pixels_per_point,
+            advance_width: glyph_info.advance_width_unscaled.0 * metrics.px_scale_factor,
             uv_rect,
         };
         entry.insert(allocation);
-        allocation
+        (allocation, h_pos_round)
     }
 }
 

crates/epaint/src/text/text_layout.rs

@@ -45,7 +45,7 @@ impl PointScale {
 /// Temporary storage before line-wrapping.
 #[derive(Clone)]
 struct Paragraph {
-    /// Start of the next glyph to be added.
+    /// Start of the next glyph to be added. In screen-space / physical pixels.
     pub cursor_x: f32,
 
     /// This is included in case there are no glyphs
@@ -169,7 +169,7 @@ fn layout_section(
         paragraph.empty_paragraph_height = line_height; // TODO(emilk): replace this hack with actually including `\n` in the glyphs?
     }
 
-    paragraph.cursor_x += leading_space;
+    paragraph.cursor_x += leading_space * pixels_per_point;
 
     let mut last_glyph_id = None;
 
@@ -195,23 +195,32 @@ fn layout_section(
                     (font_impl, scaled_metrics.unwrap_or_default())
                 }
             };
-            let glyph_alloc = match font_impl.as_mut() {
-                Some(font_impl) => {
-                    font_impl.allocate_glyph(font.atlas, &font_impl_metrics, glyph_info)
-                }
+
+            if let (Some(font_impl), Some(last_glyph_id), Some(glyph_id)) =
+                (&font_impl, last_glyph_id, glyph_info.id)
+            {
+                paragraph.cursor_x += font_impl.pair_kerning_screen_space(
+                    &font_impl_metrics,
+                    last_glyph_id,
+                    glyph_id,
+                );
+                paragraph.cursor_x += extra_letter_spacing * pixels_per_point;
+            }
+
+            let (glyph_alloc, physical_x) = match font_impl.as_mut() {
+                Some(font_impl) => font_impl.allocate_glyph(
+                    font.atlas,
+                    &font_impl_metrics,
+                    glyph_info,
+                    paragraph.cursor_x,
+                ),
                 None => Default::default(),
             };
 
-            if let (Some(font_impl), Some(last_glyph_id)) = (&font_impl, last_glyph_id) {
-                paragraph.cursor_x +=
-                    font_impl.pair_kerning(&font_impl_metrics, last_glyph_id, glyph_alloc.id);
-                paragraph.cursor_x += extra_letter_spacing;
-            }
-
             paragraph.glyphs.push(Glyph {
                 chr,
-                pos: pos2(paragraph.cursor_x, f32::NAN),
-                advance_width: glyph_alloc.advance_width,
+                pos: pos2(physical_x as f32 / pixels_per_point, f32::NAN),
+                advance_width: glyph_alloc.advance_width / pixels_per_point,
                 line_height,
                 font_impl_height: font_impl_metrics.row_height,
                 font_impl_ascent: font_impl_metrics.ascent,
@@ -222,7 +231,6 @@ fn layout_section(
             });
 
             paragraph.cursor_x += glyph_alloc.advance_width;
-            paragraph.cursor_x = paragraph.cursor_x.round_to_pixels(pixels_per_point);
             last_glyph_id = Some(glyph_alloc.id);
         }
     }
@@ -454,14 +462,6 @@ fn replace_last_glyph_with_overflow_character(
 
         let (font_id, glyph_info) = font.glyph_info(overflow_character);
         let mut font_impl = font.fonts_by_id.get_mut(&font_id);
-        let font_impl_metrics = font_impl
-            .as_ref()
-            .map(|f| f.scaled_metrics(pixels_per_point, font_size))
-            .unwrap_or_default();
-        let replacement_glyph_alloc = font_impl
-            .as_mut()
-            .map(|f| f.allocate_glyph(font.atlas, &font_impl_metrics, glyph_info))
-            .unwrap_or_default();
         let font_impl_metrics = font_impl
             .as_mut()
             .map(|f| f.scaled_metrics(pixels_per_point, font_size))
@@ -488,12 +488,30 @@ fn replace_last_glyph_with_overflow_character(
             0.0 // TODO(emilk): heed paragraph leading_space 😬
         };
 
+        let replacement_glyph_width = font_impl
+            .as_mut()
+            .and_then(|f| f.glyph_info(overflow_character))
+            .map(|i| i.advance_width_unscaled.0 * font_impl_metrics.px_scale_factor)
+            .unwrap_or_default();
+
         // Check if we're within width budget:
-        if overflow_glyph_x + replacement_glyph_alloc.advance_width <= job.effective_wrap_width()
+        if overflow_glyph_x + replacement_glyph_width <= job.effective_wrap_width()
             || row.glyphs.is_empty()
         {
             // we are done
 
+            let (replacement_glyph_alloc, physical_x) = font_impl
+                .as_mut()
+                .map(|f| {
+                    f.allocate_glyph(
+                        font.atlas,
+                        &font_impl_metrics,
+                        glyph_info,
+                        overflow_glyph_x * pixels_per_point,
+                    )
+                })
+                .unwrap_or_default();
+
             let font_metrics = font.scaled_metrics(pixels_per_point, font_size);
             let line_height = section
                 .format
@@ -502,7 +520,7 @@ fn replace_last_glyph_with_overflow_character(
 
             row.glyphs.push(Glyph {
                 chr: overflow_character,
-                pos: pos2(overflow_glyph_x, f32::NAN),
+                pos: pos2(physical_x as f32 / pixels_per_point, f32::NAN),
                 advance_width: replacement_glyph_alloc.advance_width,
                 line_height,
                 font_impl_height: font_impl_metrics.row_height,