Fix fg threshold not applying when using legacy text composition

Also note that it does not work with multi-pass rendering.

kitty/cell_fragment.glsl

@@ -129,37 +129,43 @@ float clamp_to_unit_float(float x) {
     return clamp(x, 0.0f, 1.0f);
 }
 
-vec4 foreground_contrast(vec4 over, vec3 under) {
-    float underL = dot(under, Y);
-    float overL = dot(over.rgb, Y);
-
-#if defined(FG_OVERRIDE)
+#ifdef FG_OVERRIDE
+vec3 fg_override(float under_luminance, float over_lumininace, vec3 over) {
     // If the difference in luminance is too small,
     // force the foreground color to be black or white.
-    float diffL = abs(underL - overL);
-	float overrideLvl = (1.f - colored_sprite) * step(diffL, text_fg_override_threshold);
-	float originalLvl = 1.f - overrideLvl;
-	over.rgb = (
-        originalLvl * over.rgb +
-        overrideLvl * vec3(step(underL, 0.5f))
-    );
+    float diff_luminance = abs(under_luminance - over_lumininace);
+	float override_level = (1.f - colored_sprite) * step(diff_luminance, text_fg_override_threshold);
+	float original_level = 1.f - override_level;
+	return original_level * over + override_level * vec3(step(under_luminance, 0.5f));
+}
+#endif
+
+vec4 foreground_contrast(vec4 over, vec3 under) {
+    float under_luminance = dot(under, Y);
+    float over_lumininace = dot(over.rgb, Y);
+
+#ifdef FG_OVERRIDE
+    over.rgb = fg_override(under_luminance, over_lumininace, over.rgb);
 #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.
-    over.a = clamp_to_unit_float(mix(over.a, pow(over.a, text_gamma_adjustment), (1 - overL + underL) * text_gamma_scaling) * text_contrast);
+    over.a = clamp_to_unit_float(mix(over.a, pow(over.a, text_gamma_adjustment), (1 - over_lumininace + under_luminance) * text_gamma_scaling) * text_contrast);
     return over;
 }
 
 vec4 foreground_contrast_incorrect(vec4 over, vec3 under) {
     // Simulation of gamma-incorrect blending
-    float underL = dot(under, Y);
-    float overL = dot(over.rgb, Y);
+    float under_luminance = dot(under, Y);
+    float over_lumininace = dot(over.rgb, Y);
+#ifdef FG_OVERRIDE
+    over.rgb = fg_override(under_luminance, over_lumininace, over.rgb);
+#endif
     // This is the original gamma-incorrect rendering, it is the solution of the following equation:
     //
     // linear2srgb(over * overA2 + under * (1 - overA2)) = linear2srgb(over) * over.a + linear2srgb(under) * (1 - over.a)
     // ^ gamma correct blending with new alpha             ^ gamma incorrect blending with old alpha
-    over.a = clamp_to_unit_float((srgb2linear(linear2srgb(overL) * over.a + linear2srgb(underL) * (1.0f - over.a)) - underL) / (overL - underL));
+    over.a = clamp_to_unit_float((srgb2linear(linear2srgb(over_lumininace) * over.a + linear2srgb(under_luminance) * (1.0f - over.a)) - under_luminance) / (over_lumininace - under_luminance));
     return over;
 }
 
@@ -184,6 +190,7 @@ vec4 calculate_foreground() {
     vec4 text_fg = foreground_color();
     return foreground_with_decorations(text_fg);
 }
+
 vec4 calculate_foreground(vec3 bg) {
     // When rendering on a background we can adjust the alpha channel contrast
     // to improve legibility based on the source and destination colors

kitty/options/definition.py

@@ -276,7 +276,9 @@
 foreground and background is below this threshold, the foreground color will be set
 to white if the background is dark or black if the background is light. The default
 value is :code:`0`, which means no overriding is performed. Useful when working with applications
-that use colors that do not contrast well with your preferred color scheme.
+that use colors that do not contrast well with your preferred color scheme. Note that this
+will not work in situations where kitty has to render in multiple passes, for example,
+when rendering with images under text.
 ''')
 
 egr()  # }}}