From RGB to HSV in OpenGL GLSL - opengl

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.
Edit: code is licensed under the WTFPL.

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.