Prim's V Color Brewer

Another variation of color-cycling a spanning tree generated by Prim’s algorithm. Here the periodicity of the color scale by tree depth is varied over time, alternating between emphasis of micro and macro structure. This idea was suggested in a Hacker News comment.

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

#wait {
  padding: 10px;
  position: absolute;
}

canvas {
  position: relative;
}

</style>
<div id="wait">Generating maze; please wait…</div>
<canvas width="960" height="500"></canvas>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script>

var canvas = d3.select("canvas"),
    context = canvas.node().getContext("2d"),
    width = canvas.property("width"),
    height = canvas.property("height");

var brewer = ["#ffffd9","#edf8b1","#c7e9b4","#7fcdbb","#41b6c4","#1d91c0","#225ea8","#253494","#081d58"];

var brewerupdown = brewer.slice(0);
brewerupdown = brewerupdown.concat(brewer.reverse().slice(1))

var color = d3.scale.linear()
  .domain(d3.range(0,360,360/brewerupdown.length))
  .range(brewerupdown)
  .interpolate(d3.interpolateLab);

var colors = d3.range(360)
    .map((function() {
      return function(i) {
        return d3.rgb(color(i));
      };
    })());

var worker = new Worker("generate-prims.js");
worker.postMessage({width: width, height: height});
worker.addEventListener("message", function(event) {
  worker.terminate();

var N = 1 << 0,
      S = 1 << 1,
      W = 1 << 2,
      E = 1 << 3;

var d = 0,
      r = -1,
      n = width * height,
      cells = event.data,
      distance = new Array(n),
      frontier = [(height - 1) * width],
      image = context.createImageData(width, height),
      imageData = image.data;

distance[frontier[0]] = 0;

for (var i = 0, c, i4 = 3; i < n; ++i, i4 += 4) {
    imageData[i4] = 255;
  }

while (frontier.length) {
    var frontier1 = [],
        i0,
        n0 = frontier.length,
        i1;

++d;
    for (var i = 0; i < n0; ++i) {
      i0 = frontier[i];
      if (cells[i0] & E && distance[i1 = i0 + 1] == null) distance[i1] = d, frontier1.push(i1);
      if (cells[i0] & W && distance[i1 = i0 - 1] == null) distance[i1] = d, frontier1.push(i1);
      if (cells[i0] & S && distance[i1 = i0 + width] == null) distance[i1] = d, frontier1.push(i1);
      if (cells[i0] & N && distance[i1 = i0 - width] == null) distance[i1] = d, frontier1.push(i1);
    }

frontier = frontier1;
  }

d3.timer(function(elapsed) {
    for (var i = 0, c, i4 = 0, s = 1.1 - Math.cos(elapsed / 9000); i < n; ++i, i4 += 4) {
      c = colors[(c = Math.floor(distance[i] * s) % 360) < 0 ? c + 360 : c];
      imageData[i4] = c.r;
      imageData[i4 + 1] = c.g;
      imageData[i4 + 2] = c.b;
    }
    context.putImageData(image, 0, 0);
  });
});

</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

https://d3js.org/d3.v3.min.js