font(coretext): add config to adjust strength of font-thicken. (#4531)

This is achieved by rendering to an alpha-only context rather than a
normal single-channel context, and adjusting the brightness at which
CoreText thinks it's drawing the glyph, which affects how it applies
font smoothing (which is what `font-thicken` enables).

src/config/Config.zig

@@ -234,10 +234,20 @@ const c = @cImport({
 /// i.e. new windows, tabs, etc.
 @"font-codepoint-map": RepeatableCodepointMap = .{},
 
-/// Draw fonts with a thicker stroke, if supported. This is only supported
-/// currently on macOS.
+/// Draw fonts with a thicker stroke, if supported.
+/// This is currently only supported on macOS.
 @"font-thicken": bool = false,
 
+/// Strength of thickening when `font-thicken` is enabled.
+///
+/// Valid values are integers between `0` and `255`. `0` does not correspond to
+/// *no* thickening, rather it corresponds to the lightest available thickening.
+///
+/// Has no effect when `font-thicken` is set to `false`.
+///
+/// This is currently only supported on macOS.
+@"font-thicken-strength": u8 = 255,
+
 /// All of the configurations behavior adjust various metrics determined by the
 /// font. The values can be integers (1, -1, etc.) or a percentage (20%, -15%,
 /// etc.). In each case, the values represent the amount to change the original

src/font/face.zig

@@ -100,6 +100,15 @@ pub const RenderOptions = struct {
     ///
     /// This only works with CoreText currently.
     thicken: bool = false,
+
+    /// "Strength" of the thickening, between `0` and `255`.
+    /// Only has an effect when `thicken` is enabled.
+    ///
+    /// `0` does not correspond to *no* thickening,
+    /// just the *lightest* thickening available.
+    ///
+    /// CoreText only.
+    thicken_strength: u8 = 255,
 };
 
 test {

src/font/face/coretext.zig

@@ -354,7 +354,7 @@ pub const Face = struct {
             .depth = 1,
             .space = try macos.graphics.ColorSpace.createDeviceGray(),
             .context_opts = @intFromEnum(macos.graphics.BitmapInfo.alpha_mask) &
-                @intFromEnum(macos.graphics.ImageAlphaInfo.none),
+                @intFromEnum(macos.graphics.ImageAlphaInfo.only),
         } else .{
             .color = true,
             .depth = 4,
@@ -398,7 +398,7 @@ pub const Face = struct {
         if (color.color)
             context.setRGBFillColor(ctx, 1, 1, 1, 0)
         else
-            context.setGrayFillColor(ctx, 0, 0);
+            context.setGrayFillColor(ctx, 1, 0);
         context.fillRect(ctx, .{
             .origin = .{ .x = 0, .y = 0 },
             .size = .{
@@ -421,8 +421,9 @@ pub const Face = struct {
             context.setRGBFillColor(ctx, 1, 1, 1, 1);
             context.setRGBStrokeColor(ctx, 1, 1, 1, 1);
         } else {
-            context.setGrayFillColor(ctx, 1, 1);
-            context.setGrayStrokeColor(ctx, 1, 1);
+            const strength: f64 = @floatFromInt(opts.thicken_strength);
+            context.setGrayFillColor(ctx, strength / 255.0, 1);
+            context.setGrayStrokeColor(ctx, strength / 255.0, 1);
         }
 
         // If we are drawing with synthetic bold then use a fill stroke

src/renderer/Metal.zig

@@ -360,6 +360,7 @@ pub const DerivedConfig = struct {
     arena: ArenaAllocator,
 
     font_thicken: bool,
+    font_thicken_strength: u8,
     font_features: std.ArrayListUnmanaged([:0]const u8),
     font_styles: font.CodepointResolver.StyleStatus,
     cursor_color: ?terminal.color.RGB,
@@ -410,6 +411,7 @@ pub const DerivedConfig = struct {
         return .{
             .background_opacity = @max(0, @min(1, config.@"background-opacity")),
             .font_thicken = config.@"font-thicken",
+            .font_thicken_strength = config.@"font-thicken-strength",
             .font_features = font_features.list,
             .font_styles = font_styles,
 
@@ -2837,6 +2839,7 @@ fn addGlyph(
         .{
             .grid_metrics = self.grid_metrics,
             .thicken = self.config.font_thicken,
+            .thicken_strength = self.config.font_thicken_strength,
         },
     );
 

src/renderer/OpenGL.zig

@@ -272,6 +272,7 @@ pub const DerivedConfig = struct {
     arena: ArenaAllocator,
 
     font_thicken: bool,
+    font_thicken_strength: u8,
     font_features: std.ArrayListUnmanaged([:0]const u8),
     font_styles: font.CodepointResolver.StyleStatus,
     cursor_color: ?terminal.color.RGB,
@@ -321,6 +322,7 @@ pub const DerivedConfig = struct {
         return .{
             .background_opacity = @max(0, @min(1, config.@"background-opacity")),
             .font_thicken = config.@"font-thicken",
+            .font_thicken_strength = config.@"font-thicken-strength",
             .font_features = font_features.list,
             .font_styles = font_styles,
 
@@ -2093,6 +2095,7 @@ fn addGlyph(
         .{
             .grid_metrics = self.grid_metrics,
             .thicken = self.config.font_thicken,
+            .thicken_strength = self.config.font_thicken_strength,
         },
     );