How to determine vector of Object3D (where it is facing)

[casperjeff]

My goal is to create a ray with my object's position x,y.z as the origin and pointing ...well...out the 'front' of my object......
I know each Object3D has a "rotation vector" (x y and z are in radians I thnk) . How can I convert that to a 'directional' vector to use as the 'direction' in my ray? Or is there some other gem hidden in the Object3D that I can pull out that already has this vector?

[mrdoob]

I think this is what you're after:

var matrix = new THREE.Matrix4();
matrix.extractRotation( mesh.matrix );

var direction = new THREE.Vector3( 0, 0, 1 );
matrix.multiplyVector3( direction );

[casperjeff]

wow..thanks for the quick response...

Stupid question 1:
What exactly is stored in the mesh(Object3d) 'matrix' attribute that the extractRotation function can pull the rotational matrix from it?

Stupid question 2:
My end result is now a Matrix4....but Ray is expecting a Vector3....am I missing something?

[mrdoob]

Stupid question 1:
What exactly is stored in the mesh(Object3d) 'matrix' attribute that the extractRotation function can pull the rotational matrix from it?

matrix contains position, rotation and scale.

Stupid question 2:
My end result is now a Matrix4....but Ray is expecting a Vector3....am I missing something?

Ray expects a origin vector and a direction vector. You can use mesh.position as the origin vector. Use the snippet from before for getting the direction vector.

[casperjeff]

duh....I didn't realize that the multipleVector3 function returned results...I assumed it modified the original matrix.
Thanks...

[mrdoob]

Indeed. It wasn't too clear. Perhaps this makes it more obvious:

var matrix = new THREE.Matrix4();
matrix.extractRotation( mesh.matrix );

var direction = new THREE.Vector3( 0, 0, 1 );
direction = matrix.multiplyVector3( direction );

[casperjeff]

no worries..thanks again for a FANTASTIC library. I haven't had this much fun coding in years....

[mrdoob]

:D

[timstutts]

Related question... how can I set the direction an object is facing based on a Vector3? I am using cylinder primitives, making them into cones, translating them outward. I want them all to point to (0,0,0). Is there something like... geometry.applyMatrix( new THREE.Matrix4().makeRotationFromVector3(Vector3) ); ?

[timstutts]

Also remember an example of this in the docs that seems to have disappeared. A sphere would drift through a field of cones.

[mrdoob]

As stated in the guidelines, help requests should be directed to stackoverflow. This board is for bugs and feature requests.

[kontur]

This is a super old and also closed issue, but as a relative 3D newcomer this was the only real answer to this same problem I found after quite a while of searching and trying. Maybe it would make sense to have a helper function on Object3D getNormals() that gives the orientation as a Vector3 as extracted in above example.

[roboshoes]

This is an old thread but I thought I'd still give a little input for everyone finding this now. A clean solution based on quaternions would be this:

var direction = new Vector3( 0, 0, -1 ).applyQuaternion( mesh.quaternion );

Comes in very handy if you want the direction the camera is looking for instance. 📷

[WestLangley]

@MathiasPaumgarten

var vector = new THREE.Vector3(); // create once and reuse it!
...
camera.getWorldDirection( vector );

See this post.