inverse matrix

OpenGL
1

I have 4x4 matrix (GL_MODELVIEW of the current object) …
How to find inverse matrix?
(i want to get object space position of light)

2

This worked well for me, and is reasonably simple :

fmatrix4 &fmatrix4::invert(void)
{
fmatrix4 D;
int i, j, k;

D.identity();

for( i = 0; i < 4; i++ ) {

  float tmp = 1.0f / data[i*4+i];
  for( j = 3; j >= 0; j-- ) {
  	D.data[i*4+j] *= tmp;
  	data[i*4+j] *= tmp;
  }

  for( j = 0; j < 4; j++ ) if( j != i ) {
  	tmp = data[j*4+i];
  	for( k = 3; k >= 0; k-- ) {
  		D.data[j*4+k] -= D.data[i*4+k] * tmp;
  		data[j*4+k] -= data[i*4+k] * tmp;
  	}
  }

}

*this = D;

return( *this );
}

Oh yes: and if your matrix is guaranteed to only encode a rotation in the upper 3x3 part, you could of course use a simple transpose. Optional translation can also be taken into account, without needing a full inversion. If you have scaling, shearing or even a projection (rather unlikely for the modelview matrix), then you need a full invert.

[This message has been edited by AlexH (edited 01-04-2004).]

3

Thanks! It work very well …