renderer/metal: minimum contrast ratio is configurable

src/config/Config.zig

@@ -195,6 +195,12 @@ foreground: Color = .{ .r = 0xFF, .g = 0xFF, .b = 0xFF },
 /// the selection color will vary across the selection.
 @"selection-invert-fg-bg": bool = false,
 
+/// The minimum contrast ratio between the foreground and background
+/// colors. The contrast ratio is a value between 1 and 21. A value of
+/// 1 allows for no contrast (i.e. black on black). This value is
+/// the contrast ratio as defined by the WCAG 2.0 specification.
+@"minimum-contrast": f64 = 1,
+
 /// Color palette for the 256 color form that many terminal applications
 /// use. The syntax of this configuration is "N=HEXCODE" where "n"
 /// is 0 to 255 (for the 256 colors) and HEXCODE is a typical RGB
@@ -1569,6 +1575,9 @@ pub fn finalize(self: *Config) !void {
     // Clamp our split opacity
     self.@"unfocused-split-opacity" = @min(1.0, @max(0.15, self.@"unfocused-split-opacity"));
 
+    // Clamp our contrast
+    self.@"minimum-contrast" = @min(21, @max(1, self.@"minimum-contrast"));
+
     // Minimmum window size
     if (self.@"window-width" > 0) self.@"window-width" = @max(10, self.@"window-width");
     if (self.@"window-height" > 0) self.@"window-height" = @max(4, self.@"window-height");

src/renderer/Metal.zig

@@ -152,6 +152,7 @@ pub const DerivedConfig = struct {
     selection_background: ?terminal.color.RGB,
     selection_foreground: ?terminal.color.RGB,
     invert_selection_fg_bg: bool,
+    min_contrast: f32,
     custom_shaders: std.ArrayListUnmanaged([]const u8),
     custom_shader_animation: bool,
     links: link.Set,
@@ -203,6 +204,7 @@ pub const DerivedConfig = struct {
             .background = config.background.toTerminalRGB(),
             .foreground = config.foreground.toTerminalRGB(),
             .invert_selection_fg_bg = config.@"selection-invert-fg-bg",
+            .min_contrast = @floatCast(config.@"minimum-contrast"),
 
             .selection_background = if (config.@"selection-background") |bg|
                 bg.toTerminalRGB()
@@ -374,6 +376,7 @@ pub fn init(alloc: Allocator, options: renderer.Options) !Metal {
             .cell_size = undefined,
             .strikethrough_position = @floatFromInt(metrics.strikethrough_position),
             .strikethrough_thickness = @floatFromInt(metrics.strikethrough_thickness),
+            .min_contrast = options.config.min_contrast,
         },
 
         // Fonts
@@ -531,6 +534,7 @@ pub fn setFontSize(self: *Metal, size: font.face.DesiredSize) !void {
         },
         .strikethrough_position = @floatFromInt(metrics.strikethrough_position),
         .strikethrough_thickness = @floatFromInt(metrics.strikethrough_thickness),
+        .min_contrast = self.uniforms.min_contrast,
     };
 
     // Recalculate our cell size. If it is the same as before, then we do
@@ -1257,6 +1261,9 @@ pub fn changeConfig(self: *Metal, config: *DerivedConfig) !void {
         self.font_shaper = font_shaper;
     }
 
+    // Set our new minimum contrast
+    self.uniforms.min_contrast = config.min_contrast;
+
     self.config.deinit();
     self.config = config.*;
 }
@@ -1305,6 +1312,7 @@ pub fn setScreenSize(
         },
         .strikethrough_position = old.strikethrough_position,
         .strikethrough_thickness = old.strikethrough_thickness,
+        .min_contrast = old.min_contrast,
     };
 
     // Reset our buffer sizes so that we free memory when the screen shrinks.
@@ -1705,7 +1713,12 @@ pub fn updateCell(
         });
 
         break :bg .{ rgb.r, rgb.g, rgb.b, bg_alpha };
-    } else .{ 0, 0, 0, 0 };
+    } else .{
+        self.current_background_color.r,
+        self.current_background_color.g,
+        self.current_background_color.b,
+        @intFromFloat(@max(0, @min(255, @round(self.config.background_opacity * 255)))),
+    };
 
     // If the cell has a character, draw it
     if (cell.char > 0) {

src/renderer/metal/shaders.zig

@@ -126,6 +126,10 @@ pub const Uniforms = extern struct {
     /// Metrics for underline/strikethrough
     strikethrough_position: f32,
     strikethrough_thickness: f32,
+
+    /// The minimum contrast ratio for text. The contrast ratio is calculated
+    /// according to the WCAG 2.0 spec.
+    min_contrast: f32,
 };
 
 /// The uniforms used for custom postprocess shaders.

src/renderer/shaders/cell.metal

@@ -13,6 +13,7 @@ struct Uniforms {
   float2 cell_size;
   float strikethrough_position;
   float strikethrough_thickness;
+  float min_contrast;
 };
 
 struct VertexIn {
@@ -29,7 +30,11 @@ struct VertexIn {
   // the text color. For styles, this is the color of the style.
   uchar4 color [[ attribute(5) ]];
 
-  // The fields below are present only when rendering text.
+  // The fields below are present only when rendering text (fg mode)
+
+  // The background color of the cell. This is used to determine if
+  // we need to render the text with a different color to ensure
+  // contrast.
   uchar4 bg_color [[ attribute(7) ]];
 
   // The position of the glyph in the texture (x,y)
@@ -79,13 +84,17 @@ float contrast_ratio(float3 color1, float3 color2) {
   return (max(l1, l2) + 0.05f) / (min(l1, l2) + 0.05f);
 }
 
-float4 contrasted_color(float4 fg, float4 bg) {
+// Return the fg if the contrast ratio is greater than min, otherwise
+// return a color that satisfies the contrast ratio. Currently, the color
+// is always white or black, whichever has the highest contrast ratio.
+float4 contrasted_color(float min, float4 fg, float4 bg) {
     float3 fg_premult = fg.rgb * fg.a;
     float3 bg_premult = bg.rgb * bg.a;
     float ratio = contrast_ratio(fg_premult, bg_premult);
-    if (ratio <= 3.0f) {
-        float ratio = contrast_ratio(float3(1.0f), bg_premult);
-        if (ratio > 3.0f) {
+    if (ratio < min) {
+        float white_ratio = contrast_ratio(float3(1.0f), bg_premult);
+        float black_ratio = contrast_ratio(float3(0.0f), bg_premult);
+        if (white_ratio > black_ratio) {
             return float4(1.0f);
         } else {
             return float4(0.0f, 0.0f, 0.0f, 1.0f);
@@ -159,7 +168,7 @@ vertex VertexOut uber_vertex(
     // (between 0.0 and 1.0) and must be done in the fragment shader.
     out.tex_coord = float2(input.glyph_pos) + float2(input.glyph_size) * position;
 
-    out.color = contrasted_color(out.color, float4(input.bg_color) / 255.0f);
+    out.color = contrasted_color(uniforms.min_contrast, out.color, float4(input.bg_color) / 255.0f);
     break;
   }