Pixel-Composer/shaders/sh_channel_S/sh_channel_S.fsh

//
// Simple passthrough fragment shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;

vec3 rgb2hsv(vec3 c) {
	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);
    float e = 0.0000000001;
    return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x);
 }
 
void main() {
	vec4 col = texture2D( gm_BaseTexture, v_vTexcoord);
	vec3 hsv = rgb2hsv(col.rgb);
    
	gl_FragColor = vec4(hsv.g, hsv.g, hsv.g, col.a);
}