Turn Point Pos/n/up Into Matrix

[andrewlowell]

Ok I'm still in the process of learning all about matrix-math and the python not-documented or not-implemented isn't really helping things.

I'm trying to reproduce the functionality of the copy-sop, but on object level, with copies of actual nodes/objects. Everything is working fine but I want to do more than just position, I'd like to use the normals/up as well instead of just setting positions.

So, my question is how I can turn the point position, N, and up into a hou.matrix4

Once I have the matrix 4 I think it should be an easy task of assigning that transform matrix to the object transform.

[Mario Marengo]

So, my question is how I can turn the point position, N, and up into a hou.matrix4

I guess the somewhat "standard"-ish method (for an "abc" frame with "a" aligned to the normal) would be:

up = normalize(UP)
a  = normalize(N)
b  = normalize(up - dot(up,a)*a)
c  = cross(a,b)

This frame will flip whenever the normal is identical, or directly opposite to the up vector... no way around it that I know of.

Another method avoids the flipping, but replaces it with a constantly (but smoothly) varying frame. It also doesn't use the up vector as it derives its own perpendicular to N:

a  = normalize(N)
b  = normalize(vector3(a.z-a.y,a.x-a.z,a.y-a.x))
c  = cross(a,b)

To put these in the form of a matrix (assuming row vectors, which I think is true throughout Houdini -- i.e: Hom, VEX, hscript, etc):

// Here, p[x,y,z] stands for the point's position (i.e: translation)
Matrix4 M = { a.x, a.y, a.z, 0,
			  b.x, b.y, b.z, 0,
			  c.x, c.y, c.z, 0,
			  p.x, p.y, p.z, 1  }

All of the above is in pseudo-code, but hopefully clear enough that you can translate it to "hom-speak" without much trouble.

HTH.

[andrewlowell]

Awsome! crystal clear thank you!!