HSV shift

shader/hue.frag

@@ -1,12 +1,36 @@
 
 uniform sampler2D texture;
-uniform lowp mat3 rotationMat;
+uniform mediump float hueAdjust;
 
 varying vec2 v_texCoord;
 
+/* Source: gamedev.stackexchange.com/a/59808/24839 */
+vec3 rgb2hsv(vec3 c)
+{
+	const vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
+	vec4 p = mix(vec4(c.bg, K.wz), vec4(c.gb, K.xy), step(c.b, c.g));
+	vec4 q = mix(vec4(p.xyw, c.r), vec4(c.r, p.yzx), step(p.x, c.r));
+
+	float d = q.x - min(q.w, q.y);
+	/* Avoid divide-by-zero situations by adding a very tiny delta */
+	const float eps = 1.0e-10;
+	return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + eps)), d / (q.x + eps), q.x);
+}
+
+vec3 hsv2rgb(vec3 c)
+{
+	const vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
+	vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
+	return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
+}
+
 void main ()
 {
 	vec4 color = texture2D (texture, v_texCoord.xy);
+	vec3 hsv = rgb2hsv(color.rgb);
 
-	gl_FragColor = vec4(rotationMat * color.rgb, color.a);
+	hsv.x += hueAdjust;
+	color.rgb = hsv2rgb(hsv);
+
+	gl_FragColor = color;
 }