feat: ensure min contrast ratio as in xterm.js

kitty/cell_defines.glsl

@@ -7,6 +7,8 @@
 #define HAS_TRANSPARENCY {TRANSPARENT}
 #define FG_OVERRIDE {FG_OVERRIDE}
 #define FG_OVERRIDE_THRESHOLD {FG_OVERRIDE_THRESHOLD}
+#define APPLY_MIN_CONTRAST_RATIO {APPLY_MIN_CONTRAST_RATIO}
+#define MIN_CONTRAST_RATIO {MIN_CONTRAST_RATIO}
 #define TEXT_NEW_GAMMA {TEXT_NEW_GAMMA}
 
 #define DECORATION_SHIFT {DECORATION_SHIFT}

kitty/cell_fragment.glsl

@@ -87,6 +87,53 @@ vec3 fg_override(float under_luminance, float over_lumininace, vec3 over) {
 	return original_level * over + override_level * vec3(step(under_luminance, 0.5f));
 }
 #endif
+#if (APPLY_MIN_CONTRAST_RATIO == 1)
+float contrast_ratio(float under_luminance, float over_luminance) {
+    return (max(under_luminance, over_luminance) + 0.05f) / (min(under_luminance, over_luminance) + 0.05f);
+}
+vec3 increase_luminance(vec3 over, float ratio, float target_ratio, float under_luminance) {
+    vec3 one = vec3(1.f);
+    while (ratio < target_ratio && any(lessThan(over, one))) {
+        over = clamp(over + (one - over) * 0.1f + 0.02f, 0.f, 1.f); // add 0.02 to converge faster towards 1
+        ratio = contrast_ratio(under_luminance, dot(over, Y));
+    }
+    return over;
+}
+vec3 decrease_luminance(vec3 over, float ratio, float target_ratio, float under_luminance) {
+    vec3 zero = vec3(0.f);
+    while (ratio < target_ratio && any(greaterThan(over, zero))) {
+        over = clamp(over * 0.9f - 0.02f, 0.f, 1.f); // subtract 0.02 to converge faster towards 0
+        ratio = contrast_ratio(under_luminance, dot(over, Y));
+    }
+    return over;
+}
+vec3 ensure_min_contrast_ratio(float under_luminance, float over_luminance, vec3 under, vec3 over) {
+    // as in https://github.com/xtermjs/xterm.js/blob/2042bb85023714e55c0c2e986b5000e33b17c414/src/common/Color.ts#L288
+    float ratio = contrast_ratio(under_luminance, over_luminance);
+    if (ratio < MIN_CONTRAST_RATIO) {
+        if (over_luminance < under_luminance) {
+            vec3 result_a = decrease_luminance(over, ratio, MIN_CONTRAST_RATIO, under_luminance);
+            float result_a_ratio = contrast_ratio(under_luminance, dot(result_a, Y));
+            if (result_a_ratio < ratio) {
+                vec3 result_b = increase_luminance(over, ratio, MIN_CONTRAST_RATIO, under_luminance);
+                float result_b_ratio = contrast_ratio(under_luminance, dot(result_b, Y));
+                return result_a_ratio > result_b_ratio ? result_a : result_b;
+            }
+            return result_a;
+        } else {
+            vec3 result_a = increase_luminance(over, ratio, MIN_CONTRAST_RATIO, under_luminance);
+            float result_a_ratio = contrast_ratio(under_luminance, dot(result_a, Y));
+            if (result_a_ratio < ratio) {
+                vec3 result_b = decrease_luminance(over, ratio, MIN_CONTRAST_RATIO, under_luminance);
+                float result_b_ratio = contrast_ratio(under_luminance, dot(result_b, Y));
+                return result_a_ratio > result_b_ratio ? result_a : result_b;
+            }
+            return result_a;
+        }
+    }
+    return over;
+}
+#endif
 
 vec4 foreground_contrast(vec4 over, vec3 under) {
     float under_luminance = dot(under, Y);
@@ -96,6 +143,10 @@ vec4 foreground_contrast(vec4 over, vec3 under) {
     over.rgb = fg_override(under_luminance, over_lumininace, over.rgb);
     over_lumininace = dot(over.rgb, Y);
 #endif
+#if (APPLY_MIN_CONTRAST_RATIO == 1)
+    over.rgb = ensure_min_contrast_ratio(under_luminance, over_lumininace, under, over.rgb);
+    over_lumininace = dot(over.rgb, Y);
+#endif
 
     // Apply additional gamma-adjustment scaled by the luminance difference, the darker the foreground the more adjustment we apply.
     // A multiplicative contrast is also available to increase saturation.
@@ -111,6 +162,10 @@ vec4 foreground_contrast_incorrect(vec4 over, vec3 under) {
 #if (FG_OVERRIDE == 1)
     over.rgb = fg_override(under_luminance, over_lumininace, over.rgb);
     over_lumininace = dot(over.rgb, Y);
+#endif
+#if (APPLY_MIN_CONTRAST_RATIO == 1)
+    over.rgb = ensure_min_contrast_ratio(under_luminance, over_lumininace, under, over.rgb);
+    over_lumininace = dot(over.rgb, Y);
 #endif
     // This is the original gamma-incorrect rendering, it is the solution of the following equation:
     //

kitty/shaders.py

@@ -150,7 +150,10 @@ class LoadShaderPrograms:
         self.semi_transparent = semi_transparent
         opts = get_options()
         self.text_old_gamma = opts.text_composition_strategy == 'legacy'
-        self.text_fg_override_threshold = max(0, min(opts.text_fg_override_threshold, 100)) * 0.01
+        if opts.text_fg_override_threshold < 0:
+            self.text_fg_override_threshold = opts.text_fg_override_threshold
+        else:
+            self.text_fg_override_threshold = max(0, min(opts.text_fg_override_threshold, 100)) * 0.01
         cell = program_for('cell')
         if self.cell_program_replacer is null_replacer:
             self.cell_program_replacer = MultiReplacer(
@@ -168,7 +171,9 @@ class LoadShaderPrograms:
             r['WHICH_PHASE'] = f'PHASE_{which}'
             r['TRANSPARENT'] = '1' if semi_transparent else '0'
             r['FG_OVERRIDE_THRESHOLD'] = str(self.text_fg_override_threshold)
-            r['FG_OVERRIDE'] = '1' if self.text_fg_override_threshold != 0. else '0'
+            r['FG_OVERRIDE'] = '1' if self.text_fg_override_threshold > 0 else '0'
+            r['MIN_CONTRAST_RATIO'] = str(-self.text_fg_override_threshold)
+            r['APPLY_MIN_CONTRAST_RATIO'] = '1' if self.text_fg_override_threshold < 0 else '0'
             r['TEXT_NEW_GAMMA'] = '0' if self.text_old_gamma else '1'
             return self.cell_program_replacer(src)