color.c

#include "color.h"
#include "utilities.h"

#include <math.h>

/*
 * Algorithm adapted from https://gist.github.com/hdznrrd/656996. Uses a little libmath.
 * */
void color_HSV2RGB(struct color_ColorHSV const *hsv, struct color_ColorRGB *rgb) {
   int i;
   float f,p,q,t;
   float h, s, v;

   //expand the u8 hue in range 0->255 to 0->359* (there are problems at exactly 360)
   h = 359.0 * ((float)hsv->h / 255.0);

   h = MAX(0.0, MIN(360.0, h));
   s = MAX(0.0, MIN(100.0, hsv->s));
   v = MAX(0.0, MIN(100.0, hsv->v));

   s /= 100;
   v /= 100;

   if(s == 0) {
      // Achromatic (grey)
      rgb->r = rgb->g = rgb->b = round(v*255);
      return;
   }

   h /= 60; // sector 0 to 5
   i = floor(h);
   f = h - i; // factorial part of h
   p = v * (1 - s);
   q = v * (1 - s * f);
   t = v * (1 - s * (1 - f));
   switch(i) {
      case 0:
         rgb->r = round(255*v);
         rgb->g = round(255*t);
         rgb->b = round(255*p);
         break;
      case 1:
         rgb->r = round(255*q);
         rgb->g = round(255*v);
         rgb->b = round(255*p);
         break;
      case 2:
         rgb->r = round(255*p);
         rgb->g = round(255*v);
         rgb->b = round(255*t);
         break;
      case 3:
         rgb->r = round(255*p);
         rgb->g = round(255*q);
         rgb->b = round(255*v);
         break;
      case 4:
         rgb->r = round(255*t);
         rgb->g = round(255*p);
         rgb->b = round(255*v);
         break;
      default: // case 5:
         rgb->r = round(255*v);
         rgb->g = round(255*p);
         rgb->b = round(255*q);
   }
}

color.h

#ifndef COLOR_H__
#define COLOR_H__

/*
 * Generic colors and utilities. Derived from
 * https://github.com/borgel/sympetrum-v2
 */

#include <stdint.h>

struct color_ColorRGB {
   uint8_t     b;
   uint8_t     g;
   uint8_t     r;
};

#define HSV_CHANNEL_MAX          255
#define HSV_CHANNEL_MIN          0

struct color_ColorHSV {
   uint8_t     h;
   uint8_t     s;
   uint8_t     v;
};

void color_HSV2RGB(struct color_ColorHSV const *hsv, struct color_ColorRGB *rgb);

#endif//COLOR_H__

utilities.h

#ifndef UTILITIES_H__
#define UTILITIES_H__

#define MAX(a,b) \
   ({ __typeof__ (a) _a = (a); \
    __typeof__ (b) _b = (b); \
    _a > _b ? _a : _b; })

#define MIN(a,b) \
   ({ __typeof__ (a) _a = (a); \
    __typeof__ (b) _b = (b); \
    _a > _b ? _b : _a; })

#endif//UTILITIES_H__