[Sidefx-houdini-list] HDK : Matrix Inverse Problem

Lucio Flores dogboy_l at yahoo.com
Fri Aug 1 14:05:34 EDT 2008





--- On Fri, 8/1/08, Mario Marengo <mario at axyzfx.com> wrote:

On Friday 01 August 2008 07:29:05 am Jaideep Khadilkar wrote:
> Hi.....
> My Matrix A is
>
> 1 0 0 0 0 1 0 0 0 0 -1 0 0 0 20 1
>
> I am using A.invert(B) to invert A and place the result in B.
>
> When I print B, it is same as A
>
> 1 0 0 0 0 1 0 0 0 0 -1 0 0 0 20 1
> Why I am not getting the right inverse???

I could be wrong, but I believe that's an orthogonal matrix (is that the
right 
name for these? -> det=-1) and is its own inverse. In your case, A*A=I.
So you might actually be seeing its inverse... it just looks the same as the 
original.
Yes, interseting. Its easier to see what the matrix is doing if you break it down to rows

1 0 0 0
0 1 0 0
0 0 -1 0
0 0 20 1

So you're scaling -1 by z, then transforming by 20. So its inverse is the same, flip it in z again, then transform by 20. By looking at the diagonals, you can see that the determinant is -1. 




      


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