Measure ascii height and use to upper bound ic_width

src/font/Metrics.zig

@@ -117,6 +117,16 @@ pub const FaceMetrics = struct {
     /// lowercase x glyph.
     ex_height: ?f64 = null,
 
+    /// The measured height of the bounding box containing all printable
+    /// ASCII characters. This can be different from ascent - descent for
+    /// two reasons: non-letter symbols like @ and $ often exceed the
+    /// the ascender and descender lines; and fonts often bake the line
+    /// gap into the ascent and descent metrics (as per, e.g., the Google
+    /// Fonts guidelines: https://simoncozens.github.io/gf-docs/metrics.html).
+    ///
+    /// Positive value in px
+    ascii_height: ?f64 = null,
+
     /// The width of the character "水" (CJK water ideograph, U+6C34),
     /// if present. This is used for font size adjustment, to normalize
     /// the width of CJK fonts mixed with latin fonts.
@@ -144,11 +154,20 @@ pub const FaceMetrics = struct {
         return 0.75 * self.capHeight();
     }
 
+    /// Convenience function for getting the ASCII height. If we
+    /// couldn't measure this, we use 1.5 * cap_height as our
+    /// estimator, based on measurements across programming fonts.
+    pub inline fn asciiHeight(self: FaceMetrics) f64 {
+        if (self.ascii_height) |value| if (value > 0) return value;
+        return 1.5 * self.capHeight();
+    }
+
     /// Convenience function for getting the ideograph width. If this is
-    /// not defined in the font, we estimate it as two cell widths.
+    /// not defined in the font, we estimate it as the minimum of the
+    /// ascii height and two cell widths.
     pub inline fn icWidth(self: FaceMetrics) f64 {
         if (self.ic_width) |value| if (value > 0) return value;
-        return 2 * self.cell_width;
+        return @min(self.asciiHeight(), 2 * self.cell_width);
     }
 
     /// Convenience function for getting the underline thickness. If

src/font/face/coretext.zig

@@ -775,7 +775,10 @@ pub const Face = struct {
         // Cell width is calculated by calculating the widest width of the
         // visible ASCII characters. Usually 'M' is widest but we just take
         // whatever is widest.
-        const cell_width: f64 = cell_width: {
+        //
+        // ASCII height is calculated as the height of the overall bounding
+        // box of the same characters.
+        const cell_width: f64, const ascii_height: f64 = measurements: {
             // Build a comptime array of all the ASCII chars
             const unichars = comptime unichars: {
                 const len = 127 - 32;
@@ -803,7 +806,10 @@ pub const Face = struct {
                 max = @max(advances[i].width, max);
             }
 
-            break :cell_width max;
+            // Get the overall bounding rect for the glyphs
+            const rect = ct_font.getBoundingRectsForGlyphs(.horizontal, &glyphs, null);
+
+            break :measurements .{ max, rect.size.height };
         };
 
         // Measure "水" (CJK water ideograph, U+6C34) for our ic width.
@@ -864,6 +870,7 @@ pub const Face = struct {
 
             .cap_height = cap_height,
             .ex_height = ex_height,
+            .ascii_height = ascii_height,
             .ic_width = ic_width,
         };
     }

src/font/face/freetype.zig

@@ -960,13 +960,19 @@ pub const Face = struct {
         // visible ASCII characters. Usually 'M' is widest but we just take
         // whatever is widest.
         //
+        // ASCII height is calculated as the height of the overall bounding
+        // box of the same characters.
+        //
         // If we fail to load any visible ASCII we just use max_advance from
-        // the metrics provided by FreeType.
+        // the metrics provided by FreeType, and set ascii_height to null as
-        const cell_width: f64 = cell_width: {
+        // it's optional.
+        const cell_width: f64, const ascii_height: ?f64 = measurements: {
             self.ft_mutex.lock();
             defer self.ft_mutex.unlock();
 
             var max: f64 = 0.0;
+            var top: f64 = 0.0;
+            var bottom: f64 = 0.0;
             var c: u8 = ' ';
             while (c < 127) : (c += 1) {
                 if (face.getCharIndex(c)) |glyph_index| {
@@ -974,20 +980,37 @@ pub const Face = struct {
                         .render = false,
                         .no_svg = true,
                     })) {
+                        const glyph = face.handle.*.glyph;
                         max = @max(
-                            f26dot6ToF64(face.handle.*.glyph.*.advance.x),
+                            f26dot6ToF64(glyph.*.advance.x),
                             max,
                         );
+                        top = @max(
+                            f26dot6ToF64(glyph.*.metrics.horiBearingY),
+                            top,
+                        );
+                        bottom = @min(
+                            f26dot6ToF64(glyph.*.metrics.horiBearingY - glyph.*.metrics.height),
+                            bottom,
+                        );
                     } else |_| {}
                 }
             }
 
-            // If we couldn't get any widths, just use FreeType's max_advance.
+            // If we couldn't get valid measurements, just use
+            // FreeType's max_advance and null, respectively.
             if (max == 0.0) {
-                break :cell_width f26dot6ToF64(size_metrics.max_advance);
+                max = f26dot6ToF64(size_metrics.max_advance);
             }
+            const rect_height: ?f64 = rect_height: {
+                const estimate = top - bottom;
+                if (estimate <= 0.0) {
+                    break :rect_height null;
+                }
+                break :rect_height estimate;
+            };
 
-            break :cell_width max;
+            break :measurements .{ max, rect_height };
         };
 
         // We use the cap and ex heights specified by the font if they're
@@ -1089,6 +1112,7 @@ pub const Face = struct {
 
             .cap_height = cap_height,
             .ex_height = ex_height,
+            .ascii_height = ascii_height,
             .ic_width = ic_width,
         };
     }