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Conversion RGB-HSV

Memo for myself: how to convert from/to RGB and HSV. It is explained on this Wikipedia page, and can be implemented in C as follow (given equalf, modulof are respectively the equality and modulo operators for floating point values):

// Convert a color from RGB to HSV.
// Input:
//   rgb: RGB values in [0.0, 1.0]
// Output:
//   hsv: result HSV values in [0.0, 1.0]
void rgb2hsv(float const* const rgb, float* const hsv) {

  float max = rgb[2];
  loop (i, 2) if (rgb[i] > max) max = rgb[i];
  float min = rgb[2];
  loop (i, 2) if (rgb[i] < min) min = rgb[i];
  float range = max - min;
  if (equalf(range, 0.0) == true) 
    hsv[0] = 0.0;
  else if (equalf(max, rgb[0]))
    hsv[0] = modulof(((rgb[1] - rgb[2]) / range), 6.0);
  else if (equalf(max, rgb[1]))
    hsv[0] = (rgb[2] - rgb[0]) / range + 2.0;
  else if (equalf(max, rgb[2]))
    hsv[0] = (rgb[0] - rgb[1]) / range + 4.0;
  hsv[0] /= 6.0;
  if (hsv[0] < 0.0)
    hsv[0] = 1.0 + hsv[0];
  hsv[2] = max;
  if (equalf(hsv[2], 0.0) == true) hsv[1] = 0.0;
  else hsv[1] = range / hsv[2];

}

and:

// Convert a color from HSV to RGB
// Input:
//   hsv: HSV values in [0.0, 1.0]
// Output:
//   rgb: result RGB values in [0.0, 1.0]
void rgb2hsv(float const* const hsv, float* const rgb) {

  float c = hsv[1] * hsv[2];
  float x = c * (1.0 - fabsf(modulof(hsv[0] * 6.0, 2.0) - 1.0));
  float m = hsv[2] - c;
  if (hsv[0] < 1.0 / 6.0) {
    rgb[0] = c; rgb[1] = x; rgb[2] = 0;
  } else if (1.0 / 6.0 <= hsv[0] && hsv[0] < 2.0 / 6.0) {
    rgb[0] = x; rgb[1] = c; rgb[2] = 0;
  } else if (2.0 / 6.0 <= hsv[0] && hsv[0] < 3.0 / 6.0) {
    rgb[0] = 0; rgb[1] = c; rgb[2] = x;
  } else if (3.0 / 6.0 <= hsv[0] && hsv[0] < 4.0 / 6.0) {
    rgb[0] = 0; rgb[1] = x; rgb[2] = c;
  } else if (4.0 / 6.0 <= hsv[0] && hsv[0] < 5.0 / 6.0) {
    rgb[0] = x; rgb[1] = 0; rgb[2] = c;
  } else if (5.0 / 6.0 <= hsv[0]) {
    rgb[0] = c; rgb[1] = 0; rgb[2] = x;
  }
  loop (i, 3) rgb[i] += m;
 
}

If you want to know more about other color spaces, read this article.

2021-10-10
in All, C programming, Computer graphics,
68 views
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