Add color blind accessibility filter

shaders.lua

@@ -57,4 +57,75 @@ glow = love.graphics.newShader[[
         }
         return pixel_me;
       }
-    ]]
+    ]]
+
+-- Accessibility post-process.  The puzzle's RGB channels are normally also
+-- its visual language, so this runs only after the entire scene has been
+-- composed.  It leaves neutral UI/text alone and remaps saturated colours to
+-- a palette whose red, green, and yellow accents remain distinct for people
+-- with deuteranopia.
+accessibilityColorShader = love.graphics.newShader[[
+  extern number accessibilityTime;
+
+  vec3 rgbToHsv(vec3 c) {
+    float maximum = max(c.r, max(c.g, c.b));
+    float minimum = min(c.r, min(c.g, c.b));
+    float delta = maximum - minimum;
+    float hue = 0.0;
+
+    if (delta > 0.0001) {
+      if (maximum == c.r) {
+        hue = mod((c.g - c.b) / delta, 6.0);
+      } else if (maximum == c.g) {
+        hue = ((c.b - c.r) / delta) + 2.0;
+      } else {
+        hue = ((c.r - c.g) / delta) + 4.0;
+      }
+      hue = hue / 6.0;
+      if (hue < 0.0) hue += 1.0;
+    }
+
+    float saturation = maximum <= 0.0 ? 0.0 : delta / maximum;
+    return vec3(hue, saturation, maximum);
+  }
+
+  vec4 effect(vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords) {
+    vec4 pixel = Texel(texture, texture_coords) * color;
+    vec3 hsv = rgbToHsv(pixel.rgb);
+
+    // Preserve greys, white (including additive RGB overlap), and near-black
+    // background detail.  Only colourful puzzle elements are recoloured.
+    if (hsv.y < 0.22 || hsv.z < 0.06) return pixel;
+
+    // Keep every additive pair in its own bucket.  In particular, magenta
+    // (red + blue) must not become the same colour as red, and cyan
+    // (green + blue) must not become the same colour as green.
+    vec3 replacement;
+    float pattern = 0.0;
+    // Advance in whole pixels so the texture stays crisp instead of swimming.
+    float ditherStep = floor(accessibilityTime * 1.5);
+    if (hsv.x < 0.055 || hsv.x >= 0.94) {
+      replacement = vec3(0.93, 0.18, 0.50); // red: pink
+    } else if (hsv.x < 0.19) {
+      replacement = vec3(1.00, 0.78, 0.08); // red + green: gold
+      vec2 cell = mod(floor(screen_coords + vec2(ditherStep, 0.0)), vec2(9.0));
+      pattern = length(cell - vec2(4.0)) < 1.6 ? 1.0 : 0.0; // dithered dots
+    } else if (hsv.x < 0.43) {
+      replacement = vec3(0.00, 0.72, 0.62); // green: turquoise
+    } else if (hsv.x < 0.58) {
+      replacement = vec3(0.12, 0.55, 0.96); // green + blue: sky blue
+      pattern = mod(floor(screen_coords.x + screen_coords.y + ditherStep), 10.0) < 2.0 ? 1.0 : 0.0; // dithered slashes
+    } else if (hsv.x < 0.77) {
+      replacement = vec3(0.38, 0.32, 0.96); // blue: indigo
+    } else {
+      replacement = vec3(0.72, 0.25, 0.92); // red + blue: purple
+      pattern = mod(floor((screen_coords.x + ditherStep) / 4.0) + floor(screen_coords.y / 4.0), 2.0); // dithered checker
+    }
+
+    // Secondary colours get a gently animated dither cue, so they are
+    // identifiable even when their hue is hard to perceive.  Lighting and
+    // fades remain.
+    replacement = mix(replacement, vec3(1.0), pattern * 0.55);
+    return vec4(replacement * hsv.z, pixel.a);
+  }
+]]