polyflux/assets/shader/hueshift.glsl

uniform vec3 colorize_to;

// http://lolengine.net/blog/2013/07/27/rgb-to-hsv-in-glsl
vec3 rgb2hsv(vec3 c)
{
	vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
	vec4 p = c.g < c.b ? vec4(c.bg, K.wz) : vec4(c.gb, K.xy);
	vec4 q = c.r < p.x ? vec4(p.xyw, c.r) : vec4(c.r, p.yzx);

	float d = q.x - min(q.w, q.y);
	float e = 1.0e-10;
	return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x);
}

vec3 hsv2rgb(vec3 c)
{
	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);
}

vec4 effect(vec4 color, Image t, vec2 img_pos, vec2 screen_pos) {
	vec4 pixel = Texel(t, img_pos);
	vec3 hsv = rgb2hsv(pixel.xyz);
	vec3 shifted = vec3(mod(hsv.x + colorize_to.x, 1), hsv.yz * colorize_to.yz);
	return vec4(hsv2rgb(shifted).xyz, pixel.w) * color;
}