From - Mon Apr 13 20:37:55 1998
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From: "Doug Chism" <chismdm@earthlink.net>
Newsgroups: comp.graphics.algorithms
Subject: Re: Matrix inverse algo
Date: Tue, 7 Apr 1998 14:23:20 -0500
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I can help. I have done this in my 3D engine. The idea is to post multiply
the inverse of the matrix in question every time that objects matrix is
transformed. For instance in my engine the lowest drawable object is called
a "node". So when I want to rotate the node I create a temporary inverse
matrix which is the inverse of the rotational matrix used in the rotation of
that particular node. I then post multiply the nodes inverse matrix by this
inverse matrix to get the result of that particular transform on the node's
inverse matrix. I also keep an inverse rotational matrix around. This helps
me perform directional lighting very easily. The Inverse matrix is used for
all point type lights, and for backface culling. The downside is that this
adds transformation time for complicated heirarchically animated objects.
The way I solve this problem is to override the rotate functions for these
objects to use lookup tables for sin and cos. Also I sometimes store
matricies as animation data for animations that do not need to be
interpolated ( for objects that are far from the camera).

void node::rotateX( float theta )
{
 Matrix invRot;

 myMatrix.rotateX(theta);
 invRot.rotateX(-theta);
 myInverseMatrix = myInverseMatrix*invRot;
 myInverseRotMatrix = myInverseRotMatrix*invRot;
}

for more help concerning the lighting and culling themselves email me.


J-Réginald Louis wrote in message <352952C2.515E3247@teccart.qc.ca>...
>Can someone here can refer me to some info on how to find the inverse of
>a matrix?
>
>
>BTw, if you can give me some doc or http on doing lighting abd back face
>culling on object space.
>