Gradient Along Stroke

This example demonstrates how to create a gradient that follows a stroke. This technique is sometimes used to indicate directionality along a curved edge, such as with hierarchical edge bundling.

To start, take any SVG path element and uniformly sample points along the path using getPointAtLength. (This method can also be used for path tweening.) Then, for each segment between adjacent points, compute the miter joint via line-line intersection. Lastly fill each segment by interpolating the start and end colors, here green to red, using the normalized length t along the path. Although each segment is a constant color, there are many segments to give the appearance of a continuous gradient.

This example uses a thin stroke in addition to filling the segments. This avoids antialiasing artifacts due to most web browsers not implementing full-scene antialiasing.

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
<script>

var points = [
  [86, 388],
  [788, 40],
  [805, 447],
  [93, 72]
];

var width = 960,
    height = 500;

var color = d3.interpolateLab("#008000", "#c83a22");

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

var line = d3.svg.line()
    .interpolate("basis");

svg.selectAll("path")
    .data(quad(sample(line(points), 8)))
  .enter().append("path")
    .style("fill", function(d) { return color(d.t); })
    .style("stroke", function(d) { return color(d.t); })
    .attr("d", function(d) { return lineJoin(d[0], d[1], d[2], d[3], 32); });

// Sample the SVG path string "d" uniformly with the specified precision.
function sample(d, precision) {
  var path = document.createElementNS(d3.ns.prefix.svg, "path");
  path.setAttribute("d", d);

var n = path.getTotalLength(), t = [0], i = 0, dt = precision;
  while ((i += dt) < n) t.push(i);
  t.push(n);

return t.map(function(t) {
    var p = path.getPointAtLength(t), a = [p.x, p.y];
    a.t = t / n;
    return a;
  });
}

// Compute quads of adjacent points [p0, p1, p2, p3].
function quad(points) {
  return d3.range(points.length - 1).map(function(i) {
    var a = [points[i - 1], points[i], points[i + 1], points[i + 2]];
    a.t = (points[i].t + points[i + 1].t) / 2;
    return a;
  });
}

// Compute stroke outline for segment p12.
function lineJoin(p0, p1, p2, p3, width) {
  var u12 = perp(p1, p2),
      r = width / 2,
      a = [p1[0] + u12[0] * r, p1[1] + u12[1] * r],
      b = [p2[0] + u12[0] * r, p2[1] + u12[1] * r],
      c = [p2[0] - u12[0] * r, p2[1] - u12[1] * r],
      d = [p1[0] - u12[0] * r, p1[1] - u12[1] * r];

if (p0) { // clip ad and dc using average of u01 and u12
    var u01 = perp(p0, p1), e = [p1[0] + u01[0] + u12[0], p1[1] + u01[1] + u12[1]];
    a = lineIntersect(p1, e, a, b);
    d = lineIntersect(p1, e, d, c);
  }

if (p3) { // clip ab and dc using average of u12 and u23
    var u23 = perp(p2, p3), e = [p2[0] + u23[0] + u12[0], p2[1] + u23[1] + u12[1]];
    b = lineIntersect(p2, e, a, b);
    c = lineIntersect(p2, e, d, c);
  }

return "M" + a + "L" + b + " " + c + " " + d + "Z";
}

// Compute intersection of two infinite lines ab and cd.
function lineIntersect(a, b, c, d) {
  var x1 = c[0], x3 = a[0], x21 = d[0] - x1, x43 = b[0] - x3,
      y1 = c[1], y3 = a[1], y21 = d[1] - y1, y43 = b[1] - y3,
      ua = (x43 * (y1 - y3) - y43 * (x1 - x3)) / (y43 * x21 - x43 * y21);
  return [x1 + ua * x21, y1 + ua * y21];
}

// Compute unit vector perpendicular to p01.
function perp(p0, p1) {
  var u01x = p0[1] - p1[1], u01y = p1[0] - p0[0],
      u01d = Math.sqrt(u01x * u01x + u01y * u01y);
  return [u01x / u01d, u01y / u01d];
}

</script>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js