Color Interpolate (RGB, HSL & CubeHelix)

Comparando la interpolación de colores con RGB, HSL y CubeHelix

<!DOCTYPE html>
<html>
<head>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js"></script>
<title>Color Interpolate (RGB, HSL, CubeHelix)</title>
</head>
<body>

(function() {
  var radians = Math.PI / 180;

d3.scale.cubehelix = function() {
    return d3.scale.linear()
        .range([d3.hsl(300, 0.5, 0), d3.hsl(-240, 0.5, 1)])
        .interpolate(d3.interpolateCubehelix);
  };

d3.interpolateCubehelix = d3_interpolateCubehelix(1);
  d3.interpolateCubehelix.gamma = d3_interpolateCubehelix;

function d3_interpolateCubehelix(γ) {
    return function(a, b) {
      a = d3.hsl(a);
      b = d3.hsl(b);

var ah = (a.h + 120) * radians,
          bh = (b.h + 120) * radians - ah,
          as = a.s,
          bs = b.s - as,
          al = a.l,
          bl = b.l - al;

// if (isNaN(bs)) bs = 0, as = isNaN(as) ? b.s : as;
      // if (isNaN(bh)) bh = 0, ah = isNaN(ah) ? b.h : ah;

return function(t) {
        var h = ah + bh * t,
            l = Math.pow(al + bl * t, γ),
            a = (as + bs * t) * l * (1 - l),
            cosh = Math.cos(h),
            sinh = Math.sin(h);
        return "#" + 
          hex(l + a * (-0.14861 * cosh + 1.78277 * sinh)) + 
          hex(l + a * (-0.29227 * cosh - 0.90649 * sinh)) + 
          hex(l + a * (+1.97294 * cosh));
      };
    };
  }

function hex(v) {
    var s = (v = v <= 0 ? 0 : v >= 1 ? 255 : v * 255 | 0).toString(16);
    return v < 0x10 ? "0" + s : s;
  }
})();

var a = "blue";
var b = "red";
var c1 = d3.interpolateRgb(a, b);
var c2 = d3.interpolateHsl(a, b);
// var c3 = d3.interpolateCubehelix("hsl(300,50%,0%)", "hsl(-240,50%,100%)"); 
//                                d3.hsl(300, .5, 0) ,d3.hsl(-240, .5, 1)
var c3 = d3.scale.cubehelix();  
    
  var width = 200, height = 200, radius = Math.min(width, height) / 2;
  var arc = d3.svg.arc()
    .outerRadius(radius - 10)
    .innerRadius(50);
  var data = [1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1];
  var pie = d3.layout.pie()
    .sort(null)
    .value(function(d) { return d; });

function draw(id, color) {
  var svg = d3.select(id).append("svg")
    .attr("width", width)
    .attr("height", height)
    .append("g")
    .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");
  var g = svg.selectAll(".arc")
      .data(pie(data))
    .enter().append("g")
      .attr("class", "arc");
  g.append("path")
   .attr("d", arc)
   //.attr("stroke", "#fff")
   .style("fill", function(d,i) {return color(i / data.length);}); 
}

draw("div#rgb", c1);
draw("div#hsl", c2);
draw("div#cubehelix", c3);

</script>

</body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js