RGB to HSL and HSL to RGB conversion functions

gistfile1.c

#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))

typedef struct rgb {
  float r, g, b;
} RGB;

typedef struct hsl {
  float h, s, l;
} HSL;

/*
 * Converts an RGB color value to HSL. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns HSL in the set [0, 1].
 */
HSL rgb2hsl(float r, float g, float b) {
  
  HSL result;
  
  r /= 255;
  g /= 255;
  b /= 255;
  
  float max = MAX(MAX(r,g),b);
  float min = MIN(MIN(r,g),b);
  
  result.h = result.s = result.l = (max + min) / 2;

  if (max == min) {
    result.h = result.s = 0; // achromatic
  }
  else {
    float d = max - min;
    result.s = (result.l > 0.5) ? d / (2 - max - min) : d / (max + min);
    
    if (max == r) {
      result.h = (g - b) / d + (g < b ? 6 : 0);
    }
    else if (max == g) {
      result.h = (b - r) / d + 2;
    }
    else if (max == b) {
      result.h = (r - g) / d + 4;
    }
    
    result.h /= 6;
  }

  return result;
  
}

////////////////////////////////////////////////////////////////////////

/*
 * Converts an HUE to r, g or b.
 * returns float in the set [0, 1].
 */
float hue2rgb(float p, float q, float t) {

  if (t < 0) 
    t += 1;
  if (t > 1) 
    t -= 1;
  if (t < 1./6) 
    return p + (q - p) * 6 * t;
  if (t < 1./2) 
    return q;
  if (t < 2./3)   
    return p + (q - p) * (2./3 - t) * 6;
    
  return p;
  
}

////////////////////////////////////////////////////////////////////////

/*
 * Converts an HSL color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes h, s, and l are contained in the set [0, 1] and
 * returns RGB in the set [0, 255].
 */
RGB hsl2rgb(float h, float s, float l) {

  RGB result;
  
  if(0 == s) {
    result.r = result.g = result.b = l; // achromatic
  }
  else {
    float q = l < 0.5 ? l * (1 + s) : l + s - l * s;
    float p = 2 * l - q;
    result.r = hue2rgb(p, q, h + 1./3) * 255;
    result.g = hue2rgb(p, q, h) * 255;
    result.b = hue2rgb(p, q, h - 1./3) * 255;
  }

  return result;

}

Hi @ciembor, nice job, whats the license for this? Thanks!

It's the do what you want license :). I don't even know when I wrote this, it had to be at university. By the way, this is how I feel when I read your comment after 12 years:

Thanks for the fast reply!!!

It's the do what you want license :). I don't even know when I wrote this, it had to be at university.

Great license choise!!! =)

By the way, this is how I feel when I read your comment after 12 years:

I hope the cat is crying for a good reason =(

P.S.: I think I spotted an error; shouldn't line 91

 if(0 == s) {
    result.r = result.g = result.b = l; // achromatic
  }

be:

 if(0 == s) {
    result.r = result.g = result.b = l * 255; // achromatic
  }

?

Ten years ago, a StackOverflow response came to the aid of a legacy system maintainer. It's unclear how many individuals benefitted from this code snippet. Should @alpheratz0 and others engage in analytics? Time is of the essence; we must act quickly. Our efforts will ultimately be forgotten, our loved ones will pass away, and our identity will fade into obscurity. However, we can make a difference by helping someone today. Thank you for not being indifferent; your actions can have an impact.

@Wdestroier what are you talking about?

@alpheratz0 It's possible there is a mistake, does you code works for you? I can correct it.

@ciembor it works if I change that line, the comment says...

 * Assumes h, s, and l are contained in the set [0, 1] and
 * returns RGB in the set [0, 255].

but when the saturation is 0, the function returns an RGB color in the range [0, 1]

This is what I've been searching for, I can't thank you enough! ;))