Disable subpixel binning for CJK glyphs

crates/epaint/src/text/font.rs

@@ -342,6 +342,7 @@ impl FontImpl {
         atlas: &mut TextureAtlas,
         metrics: &ScaledMetrics,
         glyph_info: GlyphInfo,
+        chr: char,
         h_pos: f32,
     ) -> (GlyphAllocation, i32) {
         let Some(glyph_id) = glyph_info.id else {
@@ -349,7 +350,13 @@ impl FontImpl {
             return (GlyphAllocation::default(), h_pos as i32);
         };
 
-        let (h_pos_round, bin) = SubpixelBin::new(h_pos);
+        // CJK scripts contain a lot of characters and could hog the glyph atlas if we stored 4 subpixel offsets per
+        // glyph.
+        let (h_pos_round, bin) = if is_cjk(chr) {
+            (h_pos.round() as i32, SubpixelBin::Zero)
+        } else {
+            SubpixelBin::new(h_pos)
+        };
 
         let entry = match self
             .glyph_alloc_cache
@@ -581,3 +588,28 @@ fn invisible_char(c: char) -> bool {
             | '\u{FEFF}' // ZERO WIDTH NO-BREAK SPACE
     )
 }
+
+#[inline]
+pub(super) fn is_cjk_ideograph(c: char) -> bool {
+    ('\u{4E00}' <= c && c <= '\u{9FFF}')
+        || ('\u{3400}' <= c && c <= '\u{4DBF}')
+        || ('\u{2B740}' <= c && c <= '\u{2B81F}')
+}
+
+#[inline]
+pub(super) fn is_kana(c: char) -> bool {
+    ('\u{3040}' <= c && c <= '\u{309F}') // Hiragana block
+        || ('\u{30A0}' <= c && c <= '\u{30FF}') // Katakana block
+}
+
+#[inline]
+pub(super) fn is_cjk(c: char) -> bool {
+    // TODO(bigfarts): Add support for Korean Hangul.
+    is_cjk_ideograph(c) || is_kana(c)
+}
+
+#[inline]
+pub(super) fn is_cjk_break_allowed(c: char) -> bool {
+    // See: https://en.wikipedia.org/wiki/Line_breaking_rules_in_East_Asian_languages#Characters_not_permitted_on_the_start_of_a_line.
+    !")]｝〕〉》」』】〙〗〟'\"｠»ヽヾーァィゥェォッャュョヮヵヶぁぃぅぇぉっゃゅょゎゕゖㇰㇱㇲㇳㇴㇵㇶㇷㇸㇹㇺㇻㇼㇽㇾㇿ々〻‐゠–〜?!‼⁇⁈⁉・、:;,。.".contains(c)
+}

crates/epaint/src/text/text_layout.rs

@@ -5,7 +5,10 @@ use emath::{Align, GuiRounding as _, NumExt as _, Pos2, Rect, Vec2, pos2, vec2};
 use crate::{
     Color32, Mesh, Stroke, Vertex,
     stroke::PathStroke,
-    text::{font::ScaledMetrics, fonts::FontFaceKey},
+    text::{
+        font::{ScaledMetrics, is_cjk, is_cjk_break_allowed},
+        fonts::FontFaceKey,
+    },
 };
 
 use super::{FontsImpl, Galley, Glyph, LayoutJob, LayoutSection, PlacedRow, Row, RowVisuals};
@@ -214,6 +217,7 @@ fn layout_section(
                     font.atlas,
                     &font_impl_metrics,
                     glyph_info,
+                    chr,
                     paragraph.cursor_x,
                 ),
                 None => Default::default(),
@@ -509,6 +513,7 @@ fn replace_last_glyph_with_overflow_character(
                         font.atlas,
                         &font_impl_metrics,
                         glyph_info,
+                        overflow_character,
                         overflow_glyph_x * pixels_per_point,
                     )
                 })
@@ -1043,31 +1048,6 @@ impl RowBreakCandidates {
     }
 }
 
-#[inline]
-fn is_cjk_ideograph(c: char) -> bool {
-    ('\u{4E00}' <= c && c <= '\u{9FFF}')
-        || ('\u{3400}' <= c && c <= '\u{4DBF}')
-        || ('\u{2B740}' <= c && c <= '\u{2B81F}')
-}
-
-#[inline]
-fn is_kana(c: char) -> bool {
-    ('\u{3040}' <= c && c <= '\u{309F}') // Hiragana block
-        || ('\u{30A0}' <= c && c <= '\u{30FF}') // Katakana block
-}
-
-#[inline]
-fn is_cjk(c: char) -> bool {
-    // TODO(bigfarts): Add support for Korean Hangul.
-    is_cjk_ideograph(c) || is_kana(c)
-}
-
-#[inline]
-fn is_cjk_break_allowed(c: char) -> bool {
-    // See: https://en.wikipedia.org/wiki/Line_breaking_rules_in_East_Asian_languages#Characters_not_permitted_on_the_start_of_a_line.
-    !")]｝〕〉》」』】〙〗〟'\"｠»ヽヾーァィゥェォッャュョヮヵヶぁぃぅぇぉっゃゅょゎゕゖㇰㇱㇲㇳㇴㇵㇶㇷㇸㇹㇺㇻㇼㇽㇾㇿ々〻‐゠–〜?!‼⁇⁈⁉・、:;,。.".contains(c)
-}
-
 // ----------------------------------------------------------------------------
 
 #[cfg(test)]