From RGB to HSV in OpenGL GLSL

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.

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.