README.md

The above gradients compare interpolation in RGB, HSL, CIE Lab and CIE LCH color spaces. For more on color spaces, see Gregor Aisch’s post, How To Avoid Equidistant HSV Colors.

This example is built with D3. Code and documentation are available as the d3.cie plugin.

cie.js

(function(d3) {
  var cie = d3.cie = {};

  cie.lab = function(l, a, b) {
    return arguments.length === 1
        ? (l instanceof Lab ? lab(l.l, l.a, l.b)
        : (l instanceof Lch ? lch_lab(l.l, l.c, l.h)
        : rgb_lab((l = d3.rgb(l)).r, l.g, l.b)))
        : lab(+l, +a, +b);
  };

  cie.lch = function(l, c, h) {
    return arguments.length === 1
        ? (l instanceof Lch ? lch(l.l, l.c, l.h)
        : (l instanceof Lab ? lab_lch(l.l, l.a, l.b)
        : lab_lch((l = rgb_lab((l = d3.rgb(l)).r, l.g, l.b)).l, l.a, l.b)))
        : lch(+l, +c, +h);
  };

  cie.interpolateLab = function(a, b) {
    a = cie.lab(a);
    b = cie.lab(b);
    var al = a.l,
        aa = a.a,
        ab = a.b,
        bl = b.l - al,
        ba = b.a - aa,
        bb = b.b - ab;
    return function(t) {
      return lab_rgb(al + bl * t, aa + ba * t, ab + bb * t) + "";
    };
  };

  cie.interpolateLch = function(a, b) {
    a = cie.lch(a);
    b = cie.lch(b);
    var al = a.l,
        ac = a.c,
        ah = a.h,
        bl = b.l - al,
        bc = b.c - ac,
        bh = b.h - ah;
    if (bh > 180) bh -= 360; else if (bh < -180) bh += 360; // shortest path
    return function(t) {
      return lch_lab(al + bl * t, ac + bc * t, ah + bh * t) + "";
    };
  };

  function lab(l, a, b) {
    return new Lab(l, a, b);
  }

  function Lab(l, a, b) {
    this.l = l;
    this.a = a;
    this.b = b;
  }

  Lab.prototype.brighter = function(k) {
    return lab(Math.min(100, this.l + K * (arguments.length ? k : 1)), this.a, this.b);
  };

  Lab.prototype.darker = function(k) {
    return lab(Math.max(0, this.l - K * (arguments.length ? k : 1)), this.a, this.b);
  };

  Lab.prototype.rgb = function() {
    return lab_rgb(this.l, this.a, this.b);
  };

  Lab.prototype.toString = function() {
    return this.rgb() + "";
  };

  function lch(l, c, h) {
    return new Lch(l, c, h);
  }

  function Lch(l, c, h) {
    this.l = l;
    this.c = c;
    this.h = h;
  }

  Lch.prototype.brighter = function(k) {
    return lch(Math.min(100, this.l + K * (arguments.length ? k : 1)), this.c, this.h);
  };

  Lch.prototype.darker = function(k) {
    return lch(Math.max(0, this.l - K * (arguments.length ? k : 1)), this.c, this.h);
  };

  Lch.prototype.rgb = function() {
    return lch_lab(this.l, this.c, this.h).rgb();
  };

  Lch.prototype.toString = function() {
    return this.rgb() + "";
  };

  // Corresponds roughly to RGB brighter/darker
  var K = 18;

  // D65 standard referent
  var X = 0.950470, Y = 1, Z = 1.088830;

  function lab_rgb(l, a, b) {
    var y = (l + 16) / 116, x = y + a / 500, z = y - b / 200;
    x = lab_xyz(x) * X;
    y = lab_xyz(y) * Y;
    z = lab_xyz(z) * Z;
    return d3.rgb(
      xyz_rgb( 3.2404542 * x - 1.5371385 * y - 0.4985314 * z),
      xyz_rgb(-0.9692660 * x + 1.8760108 * y + 0.0415560 * z),
      xyz_rgb( 0.0556434 * x - 0.2040259 * y + 1.0572252 * z)
    );
  }

  function rgb_lab(r, g, b) {
    r = rgb_xyz(r);
    g = rgb_xyz(g);
    b = rgb_xyz(b);
    var x = xyz_lab((0.4124564 * r + 0.3575761 * g + 0.1804375 * b) / X),
        y = xyz_lab((0.2126729 * r + 0.7151522 * g + 0.0721750 * b) / Y),
        z = xyz_lab((0.0193339 * r + 0.1191920 * g + 0.9503041 * b) / Z);
    return lab(116 * y - 16, 500 * (x - y), 200 * (y - z));
  }

  function lab_lch(l, a, b) {
    var c = Math.sqrt(a * a + b * b),
        h = Math.atan2(b, a) / Math.PI * 180;
    return lch(l, c, h);
  }

  function lch_lab(l, c, h) {
    h = h * Math.PI / 180;
    return lab(l, Math.cos(h) * c, Math.sin(h) * c);
  }

  function lab_xyz(x) {
    return x > 0.206893034 ? x * x * x : (x - 4 / 29) / 7.787037;
  }

  function xyz_lab(x) {
    return x > 0.008856 ? Math.pow(x, 1 / 3) : 7.787037 * x + 4 / 29;
  }

  function xyz_rgb(r) {
    return Math.round(255 * (r <= 0.00304 ? 12.92 * r : 1.055 * Math.pow(r, 1 / 2.4) - 0.055));
  }

  function rgb_xyz(r) {
    return (r /= 255) <= 0.04045 ? r / 12.92 : Math.pow((r + 0.055) / 1.055, 2.4);
  }
})(d3);

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
}

</style>
<body>
<script src="http://d3js.org/d3.v2.min.js?2.9.5"></script>
<script src="cie.js"></script>
<script>

var interpolators = {
  "HSL": d3.interpolateHsl,
  "LCH": d3.cie.interpolateLch,
  "Lab": d3.cie.interpolateLab,
  "RGB": d3.interpolateRgb
};

var width = 960,
    height = 500;

var y = d3.scale.ordinal()
    .domain(d3.keys(interpolators))
    .rangeRoundBands([0, height], .1);

var values = d3.range(960 - 28);

var x = d3.scale.ordinal()
    .domain(values)
    .rangeRoundBands([14, width - 14]);

var color = d3.scale.linear()
    .domain([0, values.length - 1])
    .range(["hsl(62,100%,90%)", "hsl(228,30%,20%)"]);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

var g = svg.selectAll("g")
    .data(d3.entries(interpolators))
  .enter().append("g")
    .attr("transform", function(d) { return "translate(0," + y(d.key) + ")"; });

g.each(function(d) {
  color.interpolate(d.value);

  d3.select(this).selectAll("rect")
    .data(values)
  .enter().append("rect")
    .attr("x", x)
    .attr("width", x.rangeBand())
    .attr("height", y.rangeBand)
    .style("fill", color);
});

g.append("text")
    .attr("x", 28)
    .attr("y", y.rangeBand() / 2)
    .attr("dy", ".35em")
    .text(function(d) { return d.key; });

</script>