font/coretext: support synthetic bold

src/config/Config.zig

@@ -62,7 +62,9 @@ const c = @cImport({
 /// Finally, some styles may be synthesized if they are not supported.
 /// For example, if a font does not have an italic style and no alternative
 /// italic font is specified, Ghostty will synthesize an italic style by
-/// applying a slant to the regular style.
+/// applying a slant to the regular style. If you want to disable these
+/// synthesized styles then you can use the `font-style` configurations
+/// as documented below.
 ///
 /// You can disable styles completely by using the `font-style` set of
 /// configurations. See the documentation for `font-style` for more information.

src/font/Collection.zig

@@ -273,9 +273,15 @@ pub fn completeStyles(self: *Collection, alloc: Allocator) CompleteError!void {
 
     // If we don't have bold, use the regular font.
     const bold_list = self.faces.getPtr(.bold);
-    if (bold_list.count() == 0) {
+    if (bold_list.count() == 0) bold: {
-        log.warn("bold style not available, using regular font", .{});
+        const synthetic = self.syntheticBold(regular_entry) catch |err| {
+            log.warn("failed to create synthetic bold, bold style will not be available err={}", .{err});
             try bold_list.append(alloc, .{ .alias = regular_entry });
+            break :bold;
+        };
+
+        log.info("synthetic bold face created", .{});
+        try bold_list.append(alloc, .{ .loaded = synthetic });
     }
 
     // If we don't have bold italic, use the regular italic font.
@@ -304,6 +310,26 @@ pub fn completeStyles(self: *Collection, alloc: Allocator) CompleteError!void {
     }
 }
 
+// Create an synthetic italic font face from the given entry and return it.
+fn syntheticBold(self: *Collection, entry: *Entry) !Face {
+    // Not all font backends support synthetic bold.
+    if (comptime !@hasDecl(Face, "syntheticBold")) return error.SyntheticBoldUnavailable;
+
+    // We require loading options to create a synthetic bold face.
+    const opts = self.load_options orelse return error.DeferredLoadingUnavailable;
+
+    // Try to bold it.
+    const regular = try self.getFaceFromEntry(entry);
+    const face = try regular.syntheticBold(opts.faceOptions());
+
+    var buf: [256]u8 = undefined;
+    if (face.name(&buf)) |name| {
+        log.info("font synthetic bold created family={s}", .{name});
+    } else |_| {}
+
+    return face;
+}
+
 // Create an synthetic italic font face from the given entry and return it.
 fn syntheticItalic(self: *Collection, entry: *Entry) !Face {
     // Not all font backends support synthetic italicization.

src/font/face/coretext.zig

@@ -24,6 +24,10 @@ pub const Face = struct {
     /// Set quirks.disableDefaultFontFeatures
     quirks_disable_default_font_features: bool = false,
 
+    /// True if this font face should be rasterized with a synthetic bold
+    /// effect. This is used for fonts that don't have a bold variant.
+    synthetic_bold: ?f64 = null,
+
     /// If the face can possibly be colored, then this is the state
     /// used to check for color information. This is null if the font
     /// can't possibly be colored (i.e. doesn't have SVG, sbix, etc
@@ -175,6 +179,25 @@ pub const Face = struct {
         return try initFont(ct_font, opts);
     }
 
+    /// Return a new face that is the same as this but applies a synthetic
+    /// bold effect to it. This is useful for fonts that don't have a bold
+    /// variant.
+    pub fn syntheticBold(self: *const Face, opts: font.face.Options) !Face {
+        const ct_font = try self.font.copyWithAttributes(0.0, null, null);
+        errdefer ct_font.release();
+        var face = try initFont(ct_font, opts);
+
+        // TO determine our synthetic bold line width we get a multiplier
+        // from the font size in points. This is a heuristic that is based
+        // on the fact that a line width of 1 looks good to me at 12 points
+        // and we want to scale that up roughly linearly with the font size.
+        const points_f64: f64 = @floatCast(opts.size.points);
+        const line_width = @max(points_f64 / 12.0, 1);
+        face.synthetic_bold = line_width;
+
+        return face;
+    }
+
     /// Returns the font name. If allocation is required, buf will be used,
     /// but sometimes allocation isn't required and a static string is
     /// returned.
@@ -300,11 +323,23 @@ pub const Face = struct {
             .advance_x = 0,
         };
 
+        // Additional padding we need to add to the bitmap context itself
+        // due to the glyph being larger than standard.
+        const padding_ctx: u32 = padding_ctx: {
             // If we're doing thicken, then getBoundsForGlyphs does not take
             // into account the anti-aliasing that will be added to the glyph.
             // We need to add some padding to allow that to happen. A padding of
             // 2 is usually enough for anti-aliasing.
-        const padding_ctx: u32 = if (opts.thicken) 2 else 0;
+            var result: u32 = if (opts.thicken) 2 else 0;
+
+            // If we have a synthetic bold, add padding for the stroke width
+            if (self.synthetic_bold) |line_width| {
+                // x2 for top and bottom padding
+                result += @intFromFloat(@ceil(line_width) * 2);
+            }
+
+            break :padding_ctx result;
+        };
         const padded_width: u32 = width + (padding_ctx * 2);
         const padded_height: u32 = height + (padding_ctx * 2);
 
@@ -390,6 +425,13 @@ pub const Face = struct {
             context.setGrayStrokeColor(ctx, 1, 1);
         }
 
+        // If we are drawing with synthetic bold then use a fill stroke
+        // which strokes the outlines of the glyph making a more bold look.
+        if (self.synthetic_bold) |line_width| {
+            context.setTextDrawingMode(ctx, .fill_stroke);
+            context.setLineWidth(ctx, line_width);
+        }
+
         // We want to render the glyphs at (0,0), but the glyphs themselves
         // are offset by bearings, so we have to undo those bearings in order
         // to get them to 0,0. We also add the padding so that they render