CODEWITHSUNDEEP
three.js
Rylan Schaeffer
Rylan Schaeffer

Reputation: 2705

Three JS - Access ArrowHelper direction?

Suppose I create an arrow helper, e.g. something like:

const arrowPosition = new THREE.Vector3(0, 0, 0);
const arrowPosition = new THREE.Vector3(2, -2, 0);

arrowDirection.subVectors( scene.position, new THREE.Vector3(1, 1, 1)).normalize();

arrow = new THREE.ArrowHelper( arrowDirection, arrowPosition, arrowLength, 0xffff00, arrowHeadLength, arrowHeadWidth);

After creating it, how do I access its direction?

Upvotes: 1

Views: 779

Answers (2)

Don Hatch
Don Hatch

Reputation: 5527

Just for educational purposes (please don't do this in production code)...

The setDirection source code (shown in @RobertSilver 's answer) sets arrow.quaternion to be the quaternion that rotates the y axis (0,1,0) to the arrow direction dir. So, if you know arrow.quaternion and you want to deduce dir, just rotate (0,1,0) by arrow.quaternion, and that's your answer:

const dir = new THREE.Vector3(0,1,0).applyQuaternion(arrow.quaternion);

It would be a bad idea to actually do this in production code, though, since it relies on the undocumented internal implementation of THREE.ArrowHelper which may change in the future (probably not, but it's best to simply refrain from making this kind of gamble so that you don't get occasionally bitten). If your code is the code that sets the arrow direction in the first place, then it would be better to just save that value when you set it (in the arrow helper object's .userData, if you like, as suggested by @prisoner849 ).

Upvotes: 0

Robert Silver
Robert Silver

Reputation: 72

In short, I'm not sure there is an easy way to access the direction, at least in a vector3 format. The value dir used in the ArrowHelper constructor (as described in the docs) is never assigned directly to a vector property of ArrowHelper or the Object3D parent class, but is instead used to set the quaternion property of the ArrowHelper, inherited from the parent class.

See the code snippet below from the helper source code:

setDirection(dir) {
  // dir is assumed to be normalized

  if (dir.y > 0.99999) {
    this.quaternion.set(0, 0, 0, 1);

  } else if (dir.y < - 0.99999) {
    this.quaternion.set(1, 0, 0, 0);

  } else {
    _axis.set(dir.z, 0, - dir.x).normalize();
    const radians = Math.acos(dir.y);
    this.quaternion.setFromAxisAngle(_axis, radians);
  }
}

As listed in the Object3D docs, the quaternion describes the helper's rotation (quaternion docs). A better discussion on the math might be found on this unity forum, but to get the vector direction you'll need to use the quaternion field

Hope this helps and if someone can improve this explanation please do, my experience with Eulers and quaternions is limited

Upvotes: 1

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