Selecting piece of merged geometry in a mesh

Hello threejs community!
I’ve been learning this wonderful js library recently throught a project for which I’m trying to render a tree built recursively. My goal is to be able to select the single branches and show them selected by changing their color.
At the beginning I started by creating a mesh for each branch and adding each sub-branch as a child of the parent branch, but the performance started to drop with 1000 > branches. Therefore I decided to create only the geometries for the branches, update their matrices relative to the parent matrix, merge all of them in one using mergeBufferGeometries() and create a single mesh. Performance improved drastically, but I have no idea on how to “select” the single branches.
How would you takle this problem? It seems to me that it might be quite a common problem, expecially with big, complicated meshes.


Are you able to add the branch geometry to the tree geometry, without resorting to mergeBufferGeometries() ?

Maybe by using geometry.merge(), but how would that be useful?

I used InstancedMesh to make procedural trees, where each level of branches is a separate InstancedMesh, it allows you to have them picked (and selected, if you like, individually). The performance is pretty good, you can have up to 10k instances running at 60fps on a laptop.

The limitation is that instanced meshes have the same geometry (although they can be scaled, rotated, translated and colorized individually).

Thanks for your replies!
Yes, maybe for my intents and purposes I’ll be better using InstancedMesh and use the matrices to transform each branch instead of each geometry.

That said, I’m still curious on how you would go about taking a piece of a whole geometry, so basically a subset of vertices, and modifying it (let’s say by changing color or position).

You could add a per vertex attribute to the geometry - an id of the model each vertex came from originally, this way you could identify them later.

For many reasons. First would be efficiency.

OK, a simple logic would be to keep track of the indices. Each time a geometry is added, update the list. Something like Groups, but a simple array with branches index offset.

For non-indexed, the number of vertices.
For indexed, the number of indices.

Now, with start & end offset for branches in main geometry. the selection can be shown by:

  1. Changing vertex color of vertices between ranges.
  2. Create a new mesh with geometry of selected branch.

An excellent tutorial on BufferGeometry.

Thanks for the replies
@tfoller I have implemented a tree using InstancedMesh and it worked, but I’m struggling with another problem not related to this topic so I came back to a whole merged buffer geometry.

@rrrr_rrrr i discovered that mergeBufferGeometries has the capability to automatically add groups of indexes by setting the second parameter to true, which would track the start and offset for indexing into the index buffer attribute and look for the vertices of a specific geometry contained in the buffer.

It is possible anyway to keep track of them as you suggested without using groups, since groups would lead to separate draw calls as written in the docs.

The result code looks like this:

/* in the constructor  which is called recursively */
this.geometry.userData.indexLength =

    /* after all geometries have been created */
    const root = new TreeBuilder(params);
    const geos = root.fold((acc, branch) => acc.concat(branch.geometry), []);
    const treeGeo = mergeBufferGeometries(geos);
    const mesh = new THREE.Mesh(treeGeo, material);
    mesh.userData.indexGroups = treeGeo.userData.mergedUserData.reduce(
      (acc, current, i) => {
        const previous = acc[i - 1];
        const start = !previous ? 0 : previous.start + previous.count;
        const count = current.indexLength;
        return acc.concat({

    /* raycasting on click*/
    const intersects = raycaster.intersectObject(tree, false);
    const intersected = intersects[0];
    if (intersected) {
      const selectedIndex = intersected.face.a;
      const indexBufferSlices = =>
        tree.geometry.index.array.slice(g.start, g.start + g.count)
      const selectedIndexGroup = indexBufferSlices.find((group) =>
      selectedIndexGroup.forEach((index) => {
        tree.geometry.attributes.color.setXYZ(index, 1, 0, 0);
        tree.geometry.attributes.color.needsUpdate = true;

works like a charm

Sweet. good luck.