# Viewspace->Object Space??

Hi,

I want to transform some vectors from view space back to object space. Usually I would use the ModelViewInverse to do this with points but how can I do this with normals? The normal needs to be transformed by the inverse transposed of a matrix. But what exactly is the inverse transpose matrix of the modelviewinverse matrix?

Can anyone help me?

Case

Right, normals are transformed by the inverse transpose of the modelview matrix:
n’ = (M^-1)T * n;
To reverse that transformation you actually need the inverse of that which is
n = ((M^-1)T)^-1 * n’;
not what you said the inverse transpose matrix of the modelviewinverse which is ((M^-1)^-1)T.

That’d be only equivalent if you have just rotations in the normal matrix! Which you normally should!
Translations do not affect vectors, so only the 3x3 matrix is needed.
The inverse of a rotation is its transpose.
n = ((M^-1)T)^-1 * n’; <=> n = ((MT)T)T * n’; <=> n = MT * n’;

Well, looks almost too simple to be true. But makes sense because the inverse transpose of a rotation is the original rotation.

(M^-1)T == MT^-1 for each invertible matrix.

So it doesn’t matter if there are only rotations or not, multiplying the transpose of the upper-left 3x3 part of the modelview matrix with the normal should always work.

Thanks, looks like my linear algebra lessons have been too long ago. Now that simplifies the matter drastically.

Thanks, that helps a lot 