feat(shaders): grayscale shader

shaders/grayscale.glsl.mustache

@@ -0,0 +1,54 @@
+/*
+ * Grayscale
+ */
+
+precision highp float;
+varying vec2 v_texcoord;
+uniform sampler2D tex;
+
+// Enum for type of grayscale conversion
+const int LUMINOSITY = 0;
+const int LIGHTNESS = 1;
+const int AVERAGE = 2;
+
+/**
+ * Type of grayscale conversion.
+ */
+const int Type = {{#nc}}{{type}} ? LUMINOSITY{{/nc}};
+
+// Enum for luma coefficients
+const int PAL = 0;
+const int HDTV = 1;
+const int HDR = 2;
+
+/**
+ * Standard for calculating luma component.
+ */
+const int LuminosityType = {{#nc}}{{luminosity_type}} ? HDR{{/nc}};
+
+void main() {
+    vec4 pixColor = texture2D(tex, v_texcoord);
+
+    float gray;
+    if (Type == LUMINOSITY) {
+        // https://en.wikipedia.org/wiki/Grayscale#Luma_coding_in_video_systems
+        if (LuminosityType == 0) {
+            gray = dot(pixColor.rgb, vec3(0.299, 0.587, 0.114));
+        } else if (LuminosityType == 1) {
+            gray = dot(pixColor.rgb, vec3(0.2126, 0.7152, 0.0722));
+        } else if (LuminosityType == 2) {
+            gray = dot(pixColor.rgb, vec3(0.2627, 0.6780, 0.0593));
+        }
+    } else if (Type == LIGHTNESS) {
+        float maxPixColor = max(pixColor.r, max(pixColor.g, pixColor.b));
+        float minPixColor = min(pixColor.r, min(pixColor.g, pixColor.b));
+        gray = (maxPixColor + minPixColor) / 2.0;
+    } else if (Type == AVERAGE) {
+        gray = (pixColor.r + pixColor.g + pixColor.b) / 3.0;
+    }
+    vec3 grayscale = vec3(gray);
+
+    gl_FragColor = vec4(grayscale, pixColor.a);
+}
+
+// vim: ft=glsl