Super compact HSV/RGB conversions for Arduino/C

hsv2rgb.ino

int redPin = 6;
int greenPin = 5;
int bluePin = 9;

float col[3];
float hue = 0.0;

void setup() {
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);
}

void loop() {
  setColor(hsv2rgb(hue, 1.0, 1.0, col));
  delay(50);
  hue += 0.01;
  if (hue >= 1.0) hue = 0.0;
}

void setColor(float *rgb) {
  analogWrite(redPin, (int)((1.0 - rgb[0]) * 255));
  analogWrite(greenPin, (int)((1.0 - rgb[1]) * 255));
  analogWrite(bluePin, (int)((1.0 - rgb[2]) * 255));  
}

// HSV->RGB conversion based on GLSL version
// expects hsv channels defined in 0.0 .. 1.0 interval
float fract(float x) { return x - int(x); }

float mix(float a, float b, float t) { return a + (b - a) * t; }

float step(float e, float x) { return x < e ? 0.0 : 1.0; }

float* hsv2rgb(float h, float s, float b, float* rgb) {
  rgb[0] = b * mix(1.0, constrain(abs(fract(h + 1.0) * 6.0 - 3.0) - 1.0, 0.0, 1.0), s);
  rgb[1] = b * mix(1.0, constrain(abs(fract(h + 0.6666666) * 6.0 - 3.0) - 1.0, 0.0, 1.0), s);
  rgb[2] = b * mix(1.0, constrain(abs(fract(h + 0.3333333) * 6.0 - 3.0) - 1.0, 0.0, 1.0), s);
  return rgb;
}

float* rgb2hsv(float r, float g, float b, float* hsv) {
  float s = step(b, g);
  float px = mix(b, g, s);
  float py = mix(g, b, s);
  float pz = mix(-1.0, 0.0, s);
  float pw = mix(0.6666666, -0.3333333, s);
  s = step(px, r);
  float qx = mix(px, r, s);
  float qz = mix(pw, pz, s);
  float qw = mix(r, px, s);
  float d = qx - min(qw, py);
  hsv[0] = abs(qz + (qw - py) / (6.0 * d + 1e-10));
  hsv[1] = d / (qx + 1e-10);
  hsv[2] = qx;
  return hsv;
}