Placing some offset directional arrows along a path, take 4.
This method is definitely overkill. It calculates one offset point per three pixels along the original path, and then walks that list of points one at a time to ensure that the resulting arrows are roughly evenly spaced and roughly the same length, regardless of the angles of their curves. You could make this slower + more precise, or faster + less precise, by changing the number of pixels per step.
marker-segment and marker-pattern should make this easier eventually.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<style>
path {
fill: none;
stroke: #fc0;
stroke-width: 2px;
}
.arrow {
stroke-width: 1px;
stroke: #444;
}
#arrowhead path {
stroke: none;
fill: black;
}
</style>
</head>
<body>
<svg version="1.1" xmlns="//www.w3.org/2000/svg" width="960" height="500">
<path id="loop" d="M636.5,315c-0.4-18.7,1.9-27.9-5.3-35.9
c-22.7-25-107.3-2.8-118.3,35.9c-7,24.4,20.6,37.2,16,71c-4,29.6-30.8,60.7-56.5,61.1c-30.8,0.4-32.9-43.8-81.7-70.2
c-50.9-27.6-110.1-12.9-125.2-9.2c-66.1,16.4-82.2,56.9-109.2,47.3c-38-13.6-55.9-112.1-19.8-143.5c39-34,121.2,27.7,148.1-3.8
c18-21.1,3.1-74.3-25.2-105.3c-31.1-34.1-70.1-32.4-105.3-76.3c-8.2-10.2-16.9-23.8-15.3-39.7c1.2-11.4,7.5-23.3,15.3-29
c33.8-25,101.6,62.6,193.1,59.5c40.1-1.3,38.7-18.5,99.2-38.9c126.2-42.6,242.4-4.9,297.7,13c54.7,17.7,105.4,35,129.8,82.4
c13,25.3,22.9,67.7,4.6,87c-11.6,12.3-25.1,5.1-46.6,20.6c-2.8,2-28.9,21.4-32.1,49.6c-3.1,27.4,18.7,35,29,70.2
c8.8,30.1,8.5,77.8-18.3,99.2c-32.3,25.8-87,0.6-100-5.3c-69.6-32-67.2-88.4-73.3-109.2z"/>
<defs>
<marker id="arrowhead" viewBox="0 0 10 10" refX="1" refY="5" markerWidth="6" markerHeight="6" orient="auto">
<path d="M 0 0 L 10 5 L 0 10 z" />
</marker>
<path id="test" />
</defs>
</svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
var path = document.getElementById("loop"),
test = document.getElementById("test"),
length = path.getTotalLength(),
desiredArrowLength = 40,
offset = 15,
numArrows = 20;
var line = d3.line();
d3.select("svg").selectAll(".arrow")
.data(arrows(20))
.enter()
.append("path")
.attr("class", "arrow")
.attr("marker-end", "url(#arrowhead)")
.attr("d", line);
function arrows(numArrows) {
var points = [],
point,
previousLength = outerLength = 0;
test.setAttribute("d", "");
for (var i = 0; i < length; i += 3) {
points.push(point = offsetPoint(i));
test.setAttribute("d", line(points));
previousLength = outerLength;
outerLength = test.getTotalLength();
point.push(outerLength - previousLength);
}
var interval = outerLength / numArrows,
arrows = [[]],
position = 0;
points.forEach(function(point){
position += point[2];
if (position >= interval) {
arrows.push([]);
position = point[2];
}
if (position <= desiredArrowLength) {
arrows[arrows.length - 1].push(point);
}
});
return arrows.slice(0, numArrows);
}
function offsetPoint(l) {
var angle = angleAtLength(l) - Math.PI / 2,
point = pointAtLength(l);
return [
point[0] + offset * Math.cos(angle),
point[1] + offset * Math.sin(angle)
];
}
function pointAtLength(l) {
var xy = path.getPointAtLength(l);
return [xy.x, xy.y];
}
// Approximate tangent
function angleAtLength(l) {
var a = pointAtLength(Math.max(l - 0.01,0)), // this could be slightly negative
b = pointAtLength(l + 0.01); // browsers cap at total length
return Math.atan2(b[1] - a[1], b[0] - a[0]);
}
</script>