how to implement complex models in aframe - three.js

Question

I'm pretty new to aframe and the ECS-modeling technique, so I probably didn't fully grasp how the architecture should be used.
I want to model something like a robotic arm: in a simplified version that is a base, on top of that a rotator and the arm itself. The model is loaded from a single json-file and consist of several nested objects for the different parts.
How would something like this be implemented in aframe if I want to be able to control the different degrees of freedom independently (which means setting object.rotation-values on the different childs of the object itself)?
One thing I thought of was to implement the loading of the model-file as one component and each degree-of-freedom as a seperate component. So basically something like this:
<a-entity robot-model="..." base-rotation="123" arm-pitch="10" />
Or would it be a better way to use registerPrimitive for something like this?
My first take on it looks like this:
registerComponent('robot', {
schema: {type: 'asset'},
update() {
// - load and parse model using THREE.ObjectLoader
// - once ready, assign property this.parts with the various
// parts of the robot-arm
}
});
registerComponent('dof-1', {
schema: {type: 'number'},
dependencies: ['robot'],
init() {
this.robot = this.el.components.robot;
},
tick(t, dt) {
if (!this.robot.parts) { return; } // not ready yet
// update values (left out here: acceleration etc)
this.robot.parts.dof1.rotation.x = this.data;
}
});
// more parts / dof implemented likewise

Answer 1

I'm assuming you've already created and rigged a 3D model using software like Blender, Maya, or Cinema4D. If not, the article Animation from Blender to three.js is a good starting point.
Once you've done that, you can import the model into A-Frame with any format that supports skinning/rigging. THREE.ObjectLoader (.json) or THREE.GLTFLoader (.gltf) are good options, and there are already A-Frame components that wrap these loaders. Assuming you're using JSON and the object-model component from A-Frame Extras, you could do:
<a-entity object-model="src: url(my-model.json)"></a-entity>
At this point you should see a model in the scene, without having written any JavaScript, but it won't be animating yet. If you know what animation you want up front, you can create the animations in keyframes or morph targets using the same modeling software: Blender, Maya, or Cinema4D. Assuming you included the animations when you exported the model, you can use the animation-mixer component (also from A-Frame Extras) as follows:
<a-entity object-model="src: url(my-model.json)"
animation-mixer="clip: *;"></a-entity>
This will play all animations at once. You could use a clip name, instead of *, to play a specific animation.
If your animations need to be computed at runtime, and can't be baked into the model, you'll need to write a custom component. This gets complicated quickly, but the basics aren't too bad:
<a-entity object-model="src: url(my-model.json)"
custom-animation></a-entity>
And the JS:
AFRAME.registerComponent('custom-animation', {
tick: function (t, dt) {
var mesh = this.el.getObject3D('mesh');
// With complex models, you may need to loop over `mesh.children`
// in case the mesh you want to animate is a child of another
// object in your model file.
if (!mesh || !mesh.isSkinnedMesh) { return; }
mesh.traverse(function (node) {
if (node.isBone && node.name === 'arm') {
node.rotation.x += dt * Math.PI / 1000;
}
});
}
});