Hi!

I'm scattering a bunch of trees. All the points have a normal pointing upward. Now I would like each tree to randomly rotate around the Y axis.

I thought the @up was for that but I read that it is a vector, not a float… I'm confused.

Any help is apreciated!

-Olivier

Long version: A Long-Winded Guide to Houdini Instancing [www.toadstorm.com]

Basically any orientation of a copy is determined by a 3x3 transform matrix. Imagine a little gnomon for each of those copies. The matrix is three vectors that describe which way is “up”, which is “front” and which is “side”. To build this matrix you need at least two vectors, from which you can compute the third using the cross product. This is why N and up are both vectors, and both required to get a steady orientation. If “N” is pointing straight up, then your “up” vector would need to be pointing anywhere along the XZ plane to get random rotation around the “N” vector.

If you want the easy answer, I'll just toot my own horn here and recommend MOPs [www.motionoperators.com]. The MOPs Instancer and MOPs Randomize SOPs will get you this effect a little more intuitively.

If you are lazy like me, you can use an orient attribute to align them and attribute randomize to pick the angle.

Attribute randomize:
name: theta
distribution: Uniform
dimensions: 1
min value: 0.0
max value: 1.0

Then use an attribute wrangle to create a quaternion:


or if Eulers with degrees are more your thing:

My point @N were already pointing up. All I needed was to make that last rotation. I multiplied my random value (fitrand) from 0 to 6.28. I used that in a quaternion function.


float fitrand = rand(@ptnum)*6.2831;

@orient = quaternion(fitrand, @N);

-Olivier