example/gradient-links/index.html

<head>
  <style> body { margin: 0; } </style>

  <script src="//unpkg.com/3d-force-graph"></script>
  <!--<script src="../../dist/3d-force-graph.js"></script>-->
</head>

<body>
  <div id="3d-graph"></div>

  <script type="module">
    import * as THREE from 'https://esm.sh/three';
    import { scaleOrdinal, schemeRdYlGn, color as d3Color } from 'https://esm.sh/d3';

    // Random tree
    const N = 25;
    const gData = {
      nodes: [...Array(N).keys()].map(i => ({ id: i })),
      links: [...Array(N).keys()]
        .filter(id => id)
        .map(id => ({
          source: id,
          target: Math.round(Math.random() * (id-1))
        }))
    };

    const nodeColorScale = scaleOrdinal(schemeRdYlGn[4]);

    const Graph = new ForceGraph3D(document.getElementById('3d-graph'))
        .nodeColor(node => nodeColorScale(node.id))
        .linkThreeObject(link => {
          // 2 (nodes) x 3 (r+g+b) bytes between [0, 1]
          // For example:
          // new Float32Array([
          //   1, 0, 0,  // source node: red
          //   0, 1, 0   // target node: green
          // ]);
          const colors = new Float32Array([].concat(
            ...[link.source, link.target]
              .map(nodeColorScale)
              .map(d3Color)
              .map(({ r, g, b }) => [r, g, b].map(v => v / 255)
            )));

          const material = new THREE.LineBasicMaterial({ vertexColors: true });
          const geometry = new THREE.BufferGeometry();
          geometry.setAttribute('position', new THREE.BufferAttribute(new Float32Array(2 * 3), 3));
          geometry.setAttribute('color', new THREE.BufferAttribute(colors, 3));

          return new THREE.Line(geometry, material);
        })
        .linkPositionUpdate((line, { start, end }) => {
          const startR = Graph.nodeRelSize();
          const endR = Graph.nodeRelSize();
          const lineLen = Math.sqrt(['x', 'y', 'z'].map(dim => Math.pow((end[dim] || 0) - (start[dim] || 0), 2)).reduce((acc, v) => acc + v, 0));

          const linePos = line.geometry.getAttribute('position');

          // calculate coordinate on the node's surface instead of center
          linePos.set([startR / lineLen, 1 - endR / lineLen].map(t =>
            ['x', 'y', 'z'].map(dim => start[dim] + (end[dim] - start[dim]) * t)
          ).flat());
          linePos.needsUpdate = true;
          return true;
        })
        .graphData(gData);
  </script>
</body>