[Sidefx-houdini-list] i am looking for the best way to get my head into matrix math.

Edward Lam edward at sidefx.com
Thu Jan 14 09:34:16 EST 2010


Andrew D Lyons wrote:
> From:
> http://www.sidefx.com/docs/houdini10.0/hom/hou/Matrix4
> 
> "Most mathematical notations assume matrices are stored in
> column-major format and that points are stored as column vectors. They
> will often use A*B*C (or simply ABC) to refer to a combined transform
> that first applies C's transform, then B's, and then applies A's. To
> represent the same matrix expression in Houdini, you need to
> concatenate the transforms in the reverse order. So, you would instead
> write C*B*A."

ACK, that' wrong. Mathematical notation doesn't care whether matrices 
are row-major or column-major. The row-major vs column-major issue is 
simply how the matrix is internally stored, an implementation detail. 
More info can be read here: http://en.wikipedia.org/wiki/Row-major_order

What's important is whether matrix transforms are created such that they 
assume whether points are column or row vectors. The matrix transforms 
created for row vectors (Houdini) are transposed from the matrix 
transforms created for column vectors (most math textbooks).

I'll fix the docs.

Regards,
-Edward



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