Full auto-orient via expression

Question

Hi!
I'd like a 3D Null to get the orientation of 3D a layer. (without parenting, it would be too easy )
I've tried a lot of solutions, through orientation, rotations, both combined... some results are close, but never perfect.
If I'm right, orientation is computed before rotations...
I think the simplest way would be with auto-orientation, or the lookAt() expression on orientation, wich gives me correct X and Y orientations, and then get an expression on zRotation to get a "full" orientation.
But what would it be?
How can I get the Z orientation in layer space?
Any idea will be appreciated!
François

Answer 1

So here's what I tried:
- scale expression
  var layer = thisComp.layer(  /* name of the parent layer */  );
  var scl = [1, 1, 1];
  while (layer != null)
    for (var j=0; j<3; j++)
      scl[j] *= layer.scale[j]/100;
    try
      layer = layer.parent;
    catch (err)
      layer = null;
  scl*100;
- orientation expression
  function sign(x)
    return x>=0  ?  1 : -1;
  function getOrientation(u, v, w)
      // still the same thing
  layer = thisComp.layer(  /* name of the parent layer */  );
  u = layer.toWorldVec([1,0,0])*sign(scale[0]);
  v = layer.toWorldVec([0,1,0])*sign(scale[1]);
  w = layer.toWorldVec([0,0,1])*sign(scale[2]);
  getOrientation(u, v, w);
I haven't tested it that much but the idea seems fine to me. In the scale expression note that I start the loop from the parent layer because in my case I want my parented layer to have the exact same scale.