GLSL hsv2rgb macro for constexpr initialization

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GLSL hsv2rgb macro for constexpr initialization

One piece of GLSL code I am constantly coming back to is Sam Hocevar's hsv2rbg and rgb2hsv functions posted on stackoverflow: https://stackoverflow.com/a/17897228/418488

The code

// Sam Hovevar has licensed the code under WTFPL.

// 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);
}

My small contribution

One thing I find myself wishing for is to initialize global const values from hsv2rbg but it doesn't work above because the const global values must be initialize from constexpr expressions

You can't declare a function constexpr in GLSL (yet) so I wrote this simple macro which is just hsv2rgb made into a macro

// License WTFPL
const vec4 hsv2rgb_K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
#define HSV2RGB(c)  (c.z * mix(hsv2rgb_K.xxx, clamp(abs(fract(c.xxx + hsv2rgb_K.xyz) * 6.0 - hsv2rgb_K.www) - hsv2rgb_K.xxx, 0.0, 1.0), c.y))

Example Usage

const vec3 grid_color   = HSV2RGB(vec3(0.6, 0.6, 1.0)); 
const vec3 plate_color  = HSV2RGB(vec3(0.0, 0.0, 0.125)); 
const vec3 plane_color  = HSV2RGB(vec3(0.7, 0.125, 1.0/32.0)); 

That's it.

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