RGB, HSV, and HSL color conversion algorithms in JavaScript

color-conversion-algorithms.js

/**
 * 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 h, s, and l in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSL representation
 */
function rgbToHsl(r, g, b) {
  r /= 255, g /= 255, b /= 255;

  var max = Math.max(r, g, b), min = Math.min(r, g, b);
  var h, s, l = (max + min) / 2;

  if (max == min) {
    h = s = 0; // achromatic
  } else {
    var d = max - min;
    s = l > 0.5 ? d / (2 - max - min) : d / (max + min);

    switch (max) {
      case r: h = (g - b) / d + (g < b ? 6 : 0); break;
      case g: h = (b - r) / d + 2; break;
      case b: h = (r - g) / d + 4; break;
    }

    h /= 6;
  }

  return [ h, s, l ];
}

/**
 * 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 r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  l       The lightness
 * @return  Array           The RGB representation
 */
function hslToRgb(h, s, l) {
  var r, g, b;

  if (s == 0) {
    r = g = b = l; // achromatic
  } else {
    function hue2rgb(p, q, 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;
    }

    var q = l < 0.5 ? l * (1 + s) : l + s - l * s;
    var p = 2 * l - q;

    r = hue2rgb(p, q, h + 1/3);
    g = hue2rgb(p, q, h);
    b = hue2rgb(p, q, h - 1/3);
  }

  return [ r * 255, g * 255, b * 255 ];
}

/**
 * Converts an RGB color value to HSV. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSV_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns h, s, and v in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSV representation
 */
function rgbToHsv(r, g, b) {
  r /= 255, g /= 255, b /= 255;

  var max = Math.max(r, g, b), min = Math.min(r, g, b);
  var h, s, v = max;

  var d = max - min;
  s = max == 0 ? 0 : d / max;

  if (max == min) {
    h = 0; // achromatic
  } else {
    switch (max) {
      case r: h = (g - b) / d + (g < b ? 6 : 0); break;
      case g: h = (b - r) / d + 2; break;
      case b: h = (r - g) / d + 4; break;
    }

    h /= 6;
  }

  return [ h, s, v ];
}

/**
 * Converts an HSV color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSV_color_space.
 * Assumes h, s, and v are contained in the set [0, 1] and
 * returns r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  v       The value
 * @return  Array           The RGB representation
 */
function hsvToRgb(h, s, v) {
  var r, g, b;

  var i = Math.floor(h * 6);
  var f = h * 6 - i;
  var p = v * (1 - s);
  var q = v * (1 - f * s);
  var t = v * (1 - (1 - f) * s);

  switch (i % 6) {
    case 0: r = v, g = t, b = p; break;
    case 1: r = q, g = v, b = p; break;
    case 2: r = p, g = v, b = t; break;
    case 3: r = p, g = q, b = v; break;
    case 4: r = t, g = p, b = v; break;
    case 5: r = v, g = p, b = q; break;
  }

  return [ r * 255, g * 255, b * 255 ];
}

Finally note that the first algorithm is perfectly correct : it is written in Javascript using the "number" type for each component whose precision is a 64-bit IEEE double-precision floatting point: this keeps all the precision needed (its 56-bit mantissa is 7 times the precision of the 8-bit input), so it does not need ANY use of Math.round() or Math.floor(). It is made externally when needed, so it does not have to "denormalize" the output to another limited range than [0, 1].

The comment saying "to be more accurate, multiply by 256 then floor" is then completely false: it would actually make the accuracy WORSE than what it gives now (dropping 48 bits of precision in each component to keep only 8 with some arbitrary rounding limit asssuming a symetric use of all integer values oin 0-255, while in fact msot videos and photo use "rounding", not flooring, to range 16-240, where clamping is not even necessary to suppress the values 1-15 and 241-254, and where 0 and 255 values are also NOT used in 24-bit sRGB input).

Would you guys consider HSL or HSV better for conversion to humanly sensible text via conditions (a la pale dark blue etc) rather than choosing specific color names via proximity comparisons? I currently do both, but most exact color names are incomprehensible to the intended audience, so I need something simpler ("what the heck is mauve?" etc). In the first case currently via HSL, with ranges for each attribute + some special conditions for black, white, gray etc, currently lacking distinction of brown, pink etc common color names.

Someone correct me if I'm wrong, but I believe hsvToRgb() is incorrect. Example:

hsvToRgb( .074, 1, 1 ) = r:255, g:113, b:0 when it should be r:195, g:255, b:0

About license, the docs tell that comes from an adaption of algorithms in Wikipedia. The texts under Wikipedia are covered by Creative Commons Attribution-ShareAlike 3.0 which give you the right to distribute adaptations only under the same license so this code must be under it too.

But, yes, better if the author adds it clearly.

This doesn't really work. I just implemented the wikipedia algorithm as is and it works:
https://gist.github.com/vahidk/05184faf3d92a0aa1b46aeaa93b07786

@isbyerley, came here to say the same thing. I think there's a mistake in the hsvToRgb. Here's my version:

function rgbFromHSV(h,s,v) {
  /**
   * I: An array of three elements hue (h) ∈ [0, 360], and saturation (s) and value (v) which are ∈ [0, 1]
   * O: An array of red (r), green (g), blue (b), all ∈ [0, 255]
   * Derived from https://en.wikipedia.org/wiki/HSL_and_HSV
   * This stackexchange was the clearest derivation I found to reimplement https://cs.stackexchange.com/questions/64549/convert-hsv-to-rgb-colors
   */

  hprime = h / 60;
  const c = v * s;
  const x = c * (1 - Math.abs(hprime % 2 - 1)); 
  const m = v - c;
  let r, g, b;
  if (!hprime) {r = 0; g = 0; b = 0; }
  if (hprime >= 0 && hprime < 1) { r = c; g = x; b = 0}
  if (hprime >= 1 && hprime < 2) { r = x; g = c; b = 0}
  if (hprime >= 2 && hprime < 3) { r = 0; g = c; b = x}
  if (hprime >= 3 && hprime < 4) { r = 0; g = x; b = c}
  if (hprime >= 4 && hprime < 5) { r = x; g = 0; b = c}
  if (hprime >= 5 && hprime < 6) { r = c; g = 0; b = x}
  
  r = Math.round( (r + m)* 255);
  g = Math.round( (g + m)* 255);
  b = Math.round( (b + m)* 255);

  return [r, g, b]
}

+1, used this for a coding bootcamp assignment, with attribution

This minified variation takes integer r,g,b values (0 to 255) and returns an object of integers (°,%,%) like {h:360, s:100, l:50} :

function rgbToHsl(a,r,e){a/=255,r/=255,e/=255;var t,h,n=Math.max(a,r,e),s=Math.min(a,r,e),c=(n+s)/2;if(n==s)t=h=0;else{var o=n-s;switch(h=.5<c?o/(2-n-s):o/(n+s),n){case a:t=(r-e)/o+(r<e?6:0);break;case r:t=(e-a)/o+2;break;case e:t=(a-r)/o+4}t/=6}return{h:Math.round(360*t),s:Math.round(100*h),l:Math.round(100*c)}}

If you have been looking for a variation that converts RGB to HSV with the hue ranging from 0 to 360 instead of 0 to 1, and the saturation and value ranging from 0 to 100, I have adapted this function, with this range re-mapping algorithm.

function mapMinMax(value,oldMin,oldMax,newMin, newMax) {
  return  (newMax-newMin)*(value-oldMin)/(oldMax-oldMin)+newMin;
}

function rgbToHsv(r, g, b) {
    r /= 255, g /= 255, b /= 255;

    var max = Math.max(r, g, b),
        min = Math.min(r, g, b);
    var h, s, v = max;

    var d = max - min;
    s = max == 0 ? 0 : d / max;
    if (max == min) {
        h = 0; // achromatic
    } else {
        switch (max) {
            case r:
                h = (g - b) / d + (g < b ? 6 : 0);
                break;
            case g:
                h = (b - r) / d + 2;
                break;
            case b:
                h = (r - g) / d + 4;
                break;
        }

        h /= 6;
    }

    h = mapMinMax(h,0,1,0,360);
    return [h, s * 100, v * 100];
}

working with css string here did:

const _rgbToHsl = rgbStr => {
  const [r, g, b] = rgbStr.slice(4, -1).split(',').map(Number)
  const max = Math.max(r, g, b)
  const min = Math.min(r, g, b)
  const l = Math.floor((max + min) / ((0xff * 2) / 100))

  if (max === min) return [0, 0, l]
  const d = max - min
  const s = Math.floor((d / (l > 50 ? 0xff * 2 - max - min : max + min)) * 100)

  if (max === r) return [Math.floor(((g - b) / d + (g < b && 6)) * 60), s, l]
  return max === g
    ? [Math.floor(((b - r) / d + 2) * 60), s, l]
    : [Math.floor(((r - g) / d + 4) * 60), s, l]
}

It takes a css rgb string and returns parts in a css ready scale.

I got results similar to chrome conversion.

RGB, HSL, XYZ, LAB, LCH color conversion algorithms (ES6 Modules)
https://gist.github.com/avisek/eadfbe7a7a169b1001a2d3affc21052e

@isbyerley, came here to say the same thing. I think there's a mistake in the hsvToRgb. Here's my version:

function rgbFromHSV(h,s,v) {
  /**
   * I: An array of three elements hue (h) ∈ [0, 360], and saturation (s) and value (v) which are ∈ [0, 1]
   * O: An array of red (r), green (g), blue (b), all ∈ [0, 255]
   * Derived from https://en.wikipedia.org/wiki/HSL_and_HSV
   * This stackexchange was the clearest derivation I found to reimplement https://cs.stackexchange.com/questions/64549/convert-hsv-to-rgb-colors
   */

  hprime = h / 60;
  const c = v * s;
  const x = c * (1 - Math.abs(hprime % 2 - 1)); 
  const m = v - c;
  let r, g, b;
  if (!hprime) {r = 0; g = 0; b = 0; }
  if (hprime >= 0 && hprime < 1) { r = c; g = x; b = 0}
  if (hprime >= 1 && hprime < 2) { r = x; g = c; b = 0}
  if (hprime >= 2 && hprime < 3) { r = 0; g = c; b = x}
  if (hprime >= 3 && hprime < 4) { r = 0; g = x; b = c}
  if (hprime >= 4 && hprime < 5) { r = x; g = 0; b = c}
  if (hprime >= 5 && hprime < 6) { r = c; g = 0; b = x}
  
  r = Math.round( (r + m)* 255);
  g = Math.round( (g + m)* 255);
  b = Math.round( (b + m)* 255);

  return [r, g, b]
}

if h is equal to 360 this code shows NaN because you are not checking hprime for value 6 (360/60 = 6)
if (hprime >= 5 && hprime <= 6) { r = c; g = 0; b = x} "hprime <= 6" instead of "hprime < 6" fixes it

It does work, but it is not good to do the comparison of floating point Number values as you do in your switch. If it works in your case, this is only a potentially unreliable special case. You could figure out the three cases of your switch before your /= expressions. With minimal modifications of your code, it would look like

const rgbToHsl = (r, g, b, a) => {
        let [r, g, b, a] = rgba;
        let max = Math.max(r, g, b), min = Math.min(r, g, b);
        const componentCase = max == r ? 0 : (max == g ? 1 : 2); // is max r g or b?
        r /= 255, g /= 255, b /= 255, min /= 255, max /= 255;
        let h, s, l = (max + min) / 2;
        if (max == min) {
            h = s = 0; // achromatic
        } else {
            let d = max - min;
            s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
            switch (componentCase) {
                case 0: h = (g - b) / d + (g < b ? 6 : 0); break;
                case 1: h = (b - r) / d + 2; break;
                case 2: h = (r - g) / d + 4; break;
            }
            h /= 6;
        }
        return [h, s, l, a];
};

Also, note that you should use const or let, but not var, to avoid the known risks related to the obsolete var.
Then, it is not nice that you accept separate r, g, and b on input but the function returns an array. Why RGB should be different from HSL? Note the line let [r, g, b, a] = rgba.

Finally, it is good to have the alpha channel as well. This work is done for the use in CSS, where the alpha channel is generally used everywhere.

Here's a version of hslToRgb for Clojure (and ClojureScript and whatever Clojure implementation):

(defn hsl->rgb [h s l]
  (let [hue->rgb (fn hue->rgb
                   [p q t]
                   (let [t (if (< t 0) (inc t) t)
                         t (if (> t 1) (dec t) t)]
                     (cond
                       (< t (/ 1 6)) (+ p (* (* (- q p) 6) t))
                       (< t (/ 1 2)) q
                       (< t (/ 2 3)) (+ p (* (* (- q p) (- (/ 2 3) t)) 6))
                       :else p)))
        [r g b] (if (= s 0)
                  [l l l]
                  (let [q (if (< l 0.5) (* l (+ 1 s)) (- (+ l s) (* l s)))
                        p (- (* 2 l) q)]
                    [(hue->rgb p q (+ h (/ 1 3))) (hue->rgb p q h) (hue->rgb p q (- h (/ 1 3)))]))]
    [(* r 255) (* g 255) (* b 255)]))

(comment
  (hsl->rgb (/ 120 256) 1.0 0.5)
  ;; => [0 255 207.18750000000003]
  :rcf)

It's a straight port. There may be dragons such as boxed math and things? I haven't considered.

Thank you, Peter!

Here's a version of hslToRgb for Clojure (and ClojureScript and whatever Clojure implementation):