From RGB to HSV in OpenGL GLSL

Question

I need to pass from RGB color space to HSV .. I searched in internet and found two different implementations, but those give me different results:

A:

precision mediump float;
 vec3 rgb2hsv(float r, float g, float b) {

     float h = 0.0;
     float s = 0.0;
     float v = 0.0;

     float min = min( min(r, g), b );
     float max = max( max(r, g), b );
     v = max;               // v

     float delta = max - min;

     if( max != 0.0 )
         s = delta / max;       // s
     else {
         // r = g = b = 0       // s = 0, v is undefined
         s = 0.0;
         h = -1.0;
         return vec3(h, s, v);
     }
     if( r == max )
         h = ( g - b ) / delta;     // between yellow & magenta
     else if( g == max )
         h = 2.0 + ( b - r ) / delta;   // between cyan & yellow
     else
         h = 4.0 + ( r - g ) / delta;   // between magenta & cyan

     h = h * 60.0;              // degrees

     if( h < 0.0 )
         h += 360.0;

     return vec3(h, s, v);
 }

B:

 precision mediump float;
 vec3 rgb2hsv(float r, float g, float b) {

     float K = 0.0;
     float tmp;

     if (g < b)
     {
         tmp = g;
         g=b;
         b=tmp;

         K = -1.0;
     }

     if (r < g)
     {
         tmp = r;
         r=g;
         g=tmp;

         K = -2.9 / 6.9 - K;
     }

     float chroma = r - min(g, b);

     float h = abs(K + (g - b) / (6.0 * chroma + 1e-20));
     float s = chroma / (r + 1e-20);
     float v = r;

     return vec3(h, s, v);
 }

Do you know which is the correct implementation?

Answer 1

I am the author of the second implementation. It has always behaved correctly for me, but you wrote 2.9 / 6.9 instead of 2.0 / 6.0.

Since you target GLSL, you should use conversion routines that are written with the GPU in mind:

// All components are in the range [0…1], including hue.
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 = 1.0e-10;
    return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x);
}

 

// All components are in the range [0…1], including hue.
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);
}

Taken from http://lolengine.net/blog/2013/07/27/rgb-to-hsv-in-glsl.

Answer 2

I don't have a development environment to check, but you can use wolframAlpha to build up some asserts.

For Instance: rgb(1,0,0)(pure red) to hsv is 0, 100%, 100% in hsv.