From a212cf56bd7408f2e540b53c575658de4b4ed3a1 Mon Sep 17 00:00:00 2001
From: Pierre Le Fevre <hello@pierrelf.com>
Date: Sat, 18 Jul 2026 21:07:51 +0800
Subject: [PATCH] Scale fractional glyph atlas quads

References isu issue 281.
---
 crates/render/src/atlas.rs | 45 +++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/crates/render/src/atlas.rs b/crates/render/src/atlas.rs
index 60903ee..9ce5218 100644
--- a/crates/render/src/atlas.rs
+++ b/crates/render/src/atlas.rs
@@ -375,6 +375,12 @@ impl GlyphAtlas {
         // the device-pixel bitmap metrics back down by `scale`. Shaping uses
         // the logical size for advances/offsets.
         let render_px = size_px * scale;
+        let raster_px = GlyphCache::quantize_size(render_px) as f32;
+        let bitmap_to_logical = if raster_px > 0.0 {
+            (render_px / raster_px) / scale
+        } else {
+            1.0 / scale
+        };
         let shaped = font.shape_text(&line.text, size_px);
         let color = color_to_f32(&line.color);
         let synthesize_bold = should_synthesize_bold(line, font);
@@ -404,8 +410,8 @@ impl GlyphAtlas {
             let gx = line.x
                 + sg.x_offset
                 + line.letter_spacing * glyph_index as f32
-                + region.bearing_x as f32 / scale;
-            let gy = baseline - region.bearing_y as f32 / scale;
+                + region.bearing_x as f32 * bitmap_to_logical;
+            let gy = baseline - region.bearing_y as f32 * bitmap_to_logical;
 
             // UV coordinates in the atlas page.
             let u0 = region.x as f32 / page_w as f32;
@@ -416,8 +422,8 @@ impl GlyphAtlas {
             quads.push(TexturedQuad {
                 x: gx,
                 y: gy,
-                width: region.width as f32 / scale,
-                height: region.height as f32 / scale,
+                width: region.width as f32 * bitmap_to_logical,
+                height: region.height as f32 * bitmap_to_logical,
                 u0,
                 v0,
                 u1,
@@ -435,8 +441,8 @@ impl GlyphAtlas {
                 quads.push(TexturedQuad {
                     x: gx + offset,
                     y: gy,
-                    width: region.width as f32 / scale,
-                    height: region.height as f32 / scale,
+                    width: region.width as f32 * bitmap_to_logical,
+                    height: region.height as f32 * bitmap_to_logical,
                     u0,
                     v0,
                     u1,
@@ -749,6 +755,33 @@ mod tests {
         );
     }
 
+    #[test]
+    fn text_quads_scale_quantized_bitmaps_to_requested_fractional_size() {
+        let font = we_text::font::load_system_font().expect("system font");
+        let mut atlas = GlyphAtlas::with_page_size(256);
+
+        let mut whole = text_line("H", 0.0);
+        whole.font_size = 16.0;
+        whole.baseline_offset = 16.0;
+        let whole_quads = atlas.build_text_quads(&whole, &font, 1.0);
+
+        let mut fractional = text_line("H", 0.0);
+        fractional.font_size = 15.5;
+        fractional.baseline_offset = 15.5;
+        let fractional_quads = atlas.build_text_quads(&fractional, &font, 1.0);
+
+        if whole_quads.is_empty() || fractional_quads.is_empty() {
+            return;
+        }
+
+        let expected = fractional.font_size / whole.font_size;
+        let actual = fractional_quads[0].height / whole_quads[0].height;
+        assert!(
+            (actual - expected).abs() < 0.02,
+            "fractional glyph quad height should scale by {expected}, got {actual}"
+        );
+    }
+
     #[test]
     fn synthetic_bold_is_skipped_for_resolved_bold_face() {
         let Some((regular, bold)) = regular_and_bold_system_faces() else {
-- 
2.51.2