font: further improve ul/st position calculations

src/font/face/coretext.zig

@@ -594,7 +594,10 @@ pub const Face = struct {
         // All of these metrics are based on our layout above.
         const cell_height = @ceil(layout_metrics.height);
         const cell_baseline = @ceil(layout_metrics.height - layout_metrics.ascent);
+
         const underline_thickness = @ceil(@as(f32, @floatCast(ct_font.getUnderlineThickness())));
+        const strikethrough_thickness = underline_thickness;
+
         const strikethrough_position = strikethrough_position: {
             // This is the height of lower case letters in our font.
             const ex_height = ct_font.getXHeight();
@@ -608,20 +611,21 @@ pub const Face = struct {
             // is the distance from the top down to the baseline, then
             // we subtract half of the ex height to go back up to the
             // correct height that should evenly split lowercase text.
-            const pos = layout_metrics.ascent - ex_height * 0.5 + 1;
+            const pos = layout_metrics.ascent -
+                ex_height * 0.5 +
+                strikethrough_thickness * 0.5 +
+                1;
 
             break :strikethrough_position @ceil(pos);
         };
-        const strikethrough_thickness = underline_thickness;
 
         // Underline position reported is usually something like "-1" to
         // represent the amount under the baseline. We add this to our real
         // baseline to get the actual value from the bottom (+y is up).
         // The final underline position is +y from the TOP (confusing)
         // so we have to subtract from the cell height.
-        const underline_position = cell_height -
+        const underline_position = @ceil(layout_metrics.ascent -
-            (cell_baseline + @ceil(@as(f32, @floatCast(ct_font.getUnderlinePosition())))) +
+            @as(f32, @floatCast(ct_font.getUnderlinePosition())) + 1);
-            1;
 
         // Note: is this useful?
         // const units_per_em = ct_font.getUnitsPerEm();

src/font/face/freetype.zig

@@ -660,52 +660,50 @@ pub const Face = struct {
         // is reversed.
         const cell_baseline = -1 * f26dot6ToFloat(size_metrics.descender);
 
+        const underline_thickness = @max(@as(f32, 1), fontUnitsToPxY(
+            face,
+            face.handle.*.underline_thickness,
+        ));
+
         // The underline position. This is a value from the top where the
         // underline should go.
         const underline_position: f32 = underline_pos: {
-            // The ascender is already scaled for scalable fonts, but the
+            const declared_px = @as(f32, @floatFromInt(freetype.mulFix(
-            // underline position is not.
-            const ascender_px = @as(i32, @intCast(size_metrics.ascender)) >> 6;
-            const declared_px = freetype.mulFix(
                 face.handle.*.underline_position,
                 @intCast(face.handle.*.size.*.metrics.y_scale),
-            ) >> 6;
+            ))) / 64;
 
             // We use the declared underline position if its available
-            const declared = ascender_px - declared_px;
+            const declared = cell_height - cell_baseline - declared_px;
             if (declared > 0)
-                break :underline_pos @floatFromInt(declared);
+                break :underline_pos declared;
 
             // If we have no declared underline position, we go slightly under the
             // cell height (mainly: non-scalable fonts, i.e. emoji)
             break :underline_pos cell_height - 1;
         };
-        const underline_thickness = @max(@as(f32, 1), fontUnitsToPxY(
-            face,
-            face.handle.*.underline_thickness,
-        ));
 
         // The strikethrough position. We use the position provided by the
         // font if it exists otherwise we calculate a best guess.
         const strikethrough: struct {
             pos: f32,
             thickness: f32,
-        } = if (face.getSfntTable(.os2)) |os2| .{
+        } = if (face.getSfntTable(.os2)) |os2| st: {
-            .pos = pos: {
+            const thickness = @max(@as(f32, 1), fontUnitsToPxY(face, os2.yStrikeoutSize));
-                // Ascender is scaled, strikeout pos is not
+
-                const ascender_px = @as(i32, @intCast(size_metrics.ascender)) >> 6;
+            const pos = @as(f32, @floatFromInt(freetype.mulFix(
-                const declared_px = freetype.mulFix(
                 os2.yStrikeoutPosition,
                 @as(i32, @intCast(face.handle.*.size.*.metrics.y_scale)),
-                ) >> 6;
+            ))) / 64;
 
-                break :pos @floatFromInt(ascender_px - declared_px);
+            break :st .{
-            },
+                .pos = @ceil(cell_height - cell_baseline - pos + thickness + 1),
-            .thickness = @max(@as(f32, 1), fontUnitsToPxY(face, os2.yStrikeoutSize)),
+                .thickness = thickness,
+            };
         } else .{
             // Exactly 50% of the ex height so that our strikethrough is
             // centered through lowercase text. This is a common choice.
-            .pos = cell_baseline - ex_height * 0.5 + 1,
+            .pos = @ceil(cell_height - cell_baseline - ex_height * 0.5 + underline_thickness),
             .thickness = underline_thickness,
         };