Houdini - Orientation

How Houdini determines orientation

Houdini searches for these attributes, in this order. It goes down this list until none is found.

  1. @orient attribute. Use this to orient copy/instances.
  2. Using @N as the Z-axis and +Y is up.
  3. Use @v e.g. velocity.
  4. If @rot exists, apply it to the above.

Copy to Points

We have a point with @N = {0, 1, 0} e.g. its normal pointing up (the yellow trail). When the girl gets copied to the point, this happens because her z-axis is being aligned with @N (according to rule #2 above).

To get her standing up from this position, we have to rotate the top of her head (her y-axis) to align with @N.

To get this rotation matrix – e.g. the matrix that rotates her y-axis to @N – we use the dihedral function.

vector from = {0, 1, 0};
vector to = @N;
@orient = dihedral(from, to);

Here’s a “platform” – yellow is @N and green is @P visualized as a vector. When we copy the girl to the platform we get this (as expected).

What we want is her standing up and facing out, along the @P vector. In this case, we want to rotate her z-axis to align with @P.

vector from = {0, 0, 1}; // her z-axis
vector to = @P;
@orient = dihedral(from, to);

Which brings us to:

We have a box with @N vectors extending straight out from the center of each face.

If we want her standing on those faces, we have to align the top of her head – her y-axis – to @N.

@orient = dihedral({0,1,0}, @N);

Which gets us: