Using matrices to transform the Three.js scene graph

Question

I'm attempting to load a scene from a file into Three.js (custom format, not one that Three.js supports). This particular format describes a scene graph where each node in the tree has a transform specified as a 4x4 matrix. The process for pushing it into Three.js looks something like this:

// Yeah, this is javascript-like psuedocode
function processNodes(srcNode, parentThreeObj) {
    for(child in srcNode.children) {
        var threeObj = new THREE.Object3D();

        // This line is the problem
        threeObj.applyMatrix(threeMatrixFromSrcMatrix(child.matrix));

        for(mesh in child.meshes) {
            var threeMesh = threeMeshFromSrcMesh(mesh);
            threeObj.add(threeMesh);
        }

        parentThreeObj.add(threeObj);

        processNodes(child, threeObj); // And recurse!
    }
}

Or at least that's what I'd like it to be. As I pointed out, the applyMatrix line doesn't work the way that I would expect. The majority of the scene looks okay, but certain elements that have been rotated aren't aligned properly (while other are, it's strange).

Looking through the COLLADA loader (which does approximately the same thing I'm trying to do) it appears that they decompose the matrix into a translate/rotate/scale and apply each individually. I tried that in place of the applyMatrix shown above:

var props = threeMatrixFromSrcMatrix(child.matrix).decompose();
threeObj.useQuaternion = true;
threeObj.position = props[ 0 ];
threeObj.quaternion = props[ 1 ];
threeObj.scale = props[ 2 ];

This, once again, yields a scene where most elements are in the right place but meshes that previously were misaligned have now been transformed into oblivion somewhere and no longer appear at all. So in the end this is no better than the applyMatrix from above.

Looking through several online discussions about the topic it seems that the recommended way to use matrices for your transforms is to apply them directly to the geometry, not the nodes, so I tried that by manually building the transform matrix like so:

function processNodes(srcNode, parentThreeObj, parentMatrix) {
    for(child in srcNode.children) {
        var threeObj = new THREE.Object3D();

        var childMatrix = threeMatrixFromSrcMatrix(child.matrix);
        var objMatrix = THREE.Matrix4();
        objMatrix.multiply(parentMatrix, childMatrix);

        for(mesh in child.meshes) {
            var threeMesh = threeMeshFromSrcMesh(mesh);
            threeMesh.geometry.applyMatrix(objMatrix);
            threeObj.add(threeMesh);
        }

        parentThreeObj.add(threeObj);

        processNodes(child, threeObj, objMatrix); // And recurse!
    }
}

This actually yields the correct results! (minus some quirks with the normals, but I can figure that one out) That's great, but the problem is that we've now effectively flattened the scene hierarchy: Changing the transform on a parent will yield unexpected results on the children because the full transform stack is now "baked in" to the meshes. In this case that's an unacceptable loss of information about the scene.

So how might one go about telling Three.js to do the same logic, but at the appropriate point in the scene graph?

(Sorry, I would dearly love to post some live code examples but that's unfortunately not an option in this case.)

Yseulte

Answer 1

You can use matrixAutoUpdate = false to skip the Three.js scenegraph position/scale/rotation stuff. Then set object.matrix to the matrix you want and all should be dandy (well, it still gets multiplied by parent node matrices, so if you're using absolute modelview matrices you need to hack updateMatrixWorld method on Object3D.)

object.matrixAutoUpdate = false;
object.matrix = myMatrix;

Now, if you'd like to have a custom transformation matrix applied on top of the Three.js position/scale/rotation stuff, you need to edit Object3D#updateMatrix to be something like.

THREE.Object3D.prototype._updateMatrix = THREE.Object3D.prototype.updateMatrix;
THREE.Object3D.prototype.updateMatrix = function() {
  this._updateMatrix();
  if (this.customMatrix != null)
    this.matrix.multiply(this.customMatrix);
};

See https://github.com/mrdoob/three.js/blob/master/src/core/Object3D.js#L209

Laudation

Same answer, one minute apart. :) Thanks anyway, though! The updateMatrix bit is very interesting. – Yseulte

Answer 2

Sigh...

Altered Qualia pointed out the solution on Twitter within minutes of me posting this.

It's a simple one-line fix: Just set matrixAutoUpdate to false on the Object3D instances and the first code sample works as intended.

threeObj.matrixAutoUpdate = false; // This fixes it
threeObj.applyMatrix(threeMatrixFromSrcMatrix(child.matrix));

It's always the silly little things that get you...

Yseulte