Merge pull request #5628 from dotCooCoo/fix/rtl-kerning-whole-point-widths

Type: right-to-left manual kerning keeps whole-point spacing on both sides

crates/text/src/layout/vertical.rs

@@ -1,14 +1,24 @@
 use super::flow::flow;
 use super::region::Region;
-use super::{Affine, Ctx, Punct, Range, Rect, SGlyph, Tcy, TextLayout, TextObject, VerticalAlign, is_cjk, punct};
+use super::{Affine, Ctx, Punct, Range, Rect, SGlyph, Tcy, TextLayout, TextObject, VerticalAlign, continues_cluster, is_cjk, punct};
 
 /// System Layout on or Fractional Widths off: each character that takes space advances by a whole
 /// number of points, at least one, so small type does not collapse onto itself. A zero advance (a
-/// tate-chu-yoko continuation, a zero-width mark) stays zero.
+/// tate-chu-yoko continuation, a zero-width mark) stays zero. Right to left, a cluster with manual
+/// kerning moves by the difference the kerning makes to its rounded advance, so the rounding
+/// changes the space before the next character in reading order and not the space after the one
+/// before it.
 pub(super) fn whole_character_widths(glyphs: &mut [SGlyph]) {
-    for g in glyphs {
-        if g.adv.is_finite() && g.adv > 0.0 {
-            g.adv = g.adv.round().max(1.0);
+    let whole = |adv: f64| if adv.is_finite() && adv > 0.0 { adv.round().max(1.0) } else { adv };
+    for cluster in glyphs.chunk_by_mut(|a, b| continues_cluster(Some(a), Some(b))) {
+        // The cluster's last glyph carries the manual kerning in its advance.
+        let (adv, shift) = cluster.last().map_or((0.0, 0.0), |g| (g.adv, g.kern_shift));
+        let rounded_shift = Some(whole(adv) - whole(adv - shift)).filter(|s| s.is_finite()).unwrap_or(shift);
+        for g in cluster {
+            g.adv = whole(g.adv);
+            if shift != 0.0 {
+                g.kern_shift = rounded_shift;
+            }
         }
     }
 }

crates/text/src/shape.rs

@@ -34,7 +34,8 @@ pub(crate) struct SGlyph {
     pub dx: f64,
     pub dy: f64,
     /// Right to left: the run's manual kerning, which moves the glyph right along the baseline,
-    /// outside its Character Rotation. 0 otherwise.
+    /// outside its Character Rotation; with whole-point advances, the difference the kerning makes
+    /// to the cluster's rounded advance. 0 otherwise.
     pub kern_shift: f64,
     /// Outline scale (font units -> points).
     pub sx: f64,

crates/text/src/tests/typo.rs

@@ -98,6 +98,36 @@ fn rtl_manual_kerning_spaces_the_character_from_the_next_in_reading_order() {
     }
 }
 
+#[test]
+fn rtl_manual_kerning_with_whole_point_widths_keeps_the_previous_character_in_place() {
+    // Reading order: alef (bytes 0..2), bet (2..4), gimel (4..6); bet carries the kerning, which
+    // is a fraction of a point.
+    for (system_layout, fractional_widths) in [(true, true), (false, false)] {
+        for value in [3.0, 4.0, 6.0, 7.0] {
+            let kerned = |value: f64| {
+                let mut t = TextObject::point(Point::ZERO, "אבג", CharStyle { size: 100.0, ..CharStyle::default() });
+                t.system_layout = system_layout;
+                t.fractional_widths = fractional_widths;
+                edit::style_range(&mut t.runs, 2, 4, |s| s.kerning = Some(value));
+                layout(db(), &t)
+            };
+            let ink = |lay: &TextLayout, byte: usize| {
+                let g = lay.glyphs.iter().find(|g| g.byte == byte).unwrap();
+                assert!(g.rtl && g.gid != 0, "byte {byte} is set right to left with a glyph of its own");
+                (g.xf * Point::ZERO).x
+            };
+            let (plain, spaced) = (kerned(0.0), kerned(value));
+            let gap = |lay: &TextLayout, right: usize, left: usize| ink(lay, right) - ink(lay, left);
+            assert!(
+                (gap(&spaced, 0, 2) - gap(&plain, 0, 2)).abs() < 1e-9,
+                "alef, before bet in reading order, keeps its place (kerning {value}, System Layout {system_layout})"
+            );
+            let wider = gap(&spaced, 2, 4) - gap(&plain, 2, 4);
+            assert!((wider - wider.round()).abs() < 1e-9, "bet moves from gimel by whole points: {wider} (kerning {value})");
+        }
+    }
+}
+
 #[test]
 fn rtl_manual_kerning_on_a_rotated_character_moves_it_along_the_baseline() {
     // Reading order: alef (bytes 0..2), bet (2..4), gimel (4..6); bet is turned 90 degrees and