Rainbow Perceptual Distance

This attempts to measure the perceptual uniformity of three rainbow color scales by computing the CIE76 color difference at various points along the scale. The flatness of the curve, not its absolute value, shows the uniformity of each scale. The HCL rainbow is perfectly flat by definition. Cubehelix is better than HSL, but is not uniform.

It would be better to use CIEDE2000 to compute the difference, but I’m lazy.

forked from mbostock's block: Rainbow Perceptual Distance

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

.axis {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.axis--y path {
  display: none;
}

.line path {
  fill: none;
  stroke-width: 1.5px;
  stroke-linejoin: round;
}

.line text {
  font: bold 12px sans-serif;
  text-shadow: 1px 1px 0 #fff, -1px -1px 0 #fff, 1px -1px 0 #fff, -1px 1px 0 #fff;
}

</style>
<body>
<script src="//d3js.org/d3.v4.0.0-alpha.29.min.js"></script>
<script>

var margin = {top: 20, right: 20, bottom: 30, left: 30},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.scaleLinear()
    .domain([0, 360])
    .range([0, width]);

var y = d3.scaleLinear()
    .domain([0, 80])
    .range([height, 0]);

var z = d3.scaleCategory10();

var line = d3.line()
    .curve(d3.curveBasis)
    .x(function(d) { return x(d.input); })
    .y(function(d) { return y(d.delta); });

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("g")
    .attr("class", "axis axis--x")
    .attr("transform", "translate(0," + y(0) + ")")
    .call(d3.axisBottom(x));

svg.append("g")
    .attr("class", "axis axis--y")
    .call(d3.axisLeft(y))
  .selectAll(".tick:last-of-type")
  .append("text")
    .attr("class", "axis-title")
    .attr("x", 3)
    .attr("dy", ".32em")
    .text("Color Difference at ±10° (CIE76)");

var g = svg.selectAll(".line")
    .data(data)
  .enter().append("g")
    .attr("class", "line");

g.append("path")
    .attr("d", function(d) { return line(d.deltas); })
    .attr("id", function(d, i) { return "path-" + i; })
    .style("stroke", function(d, i) { return z(i); });

g.append("text")
    .attr("x", function(d) { return d.labelOffset; })
    .attr("dy", -5)
    .style("fill", function(d, i) { return d3.lab(z(i)).darker(); })
  .append("textPath")
    .attr("class", "textpath")
    .attr("xlink:href", function(d, i) { return "#path-" + i; })
    .text(function(d) { return d.name; });

// CIE76 per https://en.wikipedia.org/wiki/Color_difference#CIE76
// Not as good as CIEDE2000 but a lot easier to implement.
function delta(a, b) {
  var dl = (a = d3.lab(a)).l - (b = d3.lab(b)).l, da = a.a - b.a, db = a.b - b.b;
  return Math.sqrt(dl * dl + da * da + db * db);
}

</script>

https://d3js.org/d3.v4.0.0-alpha.29.min.js