[Plib-users] culling problems w/ off-axis projection

[<p.l...@ci...>]

Dear all,

I'm doing off-axis projection (the most general form of projection) by
translating the four corners of the screen into a projection matrix, for which
I added a new method to the sgFrustum class and to the Context too. The only
difference with giving the six frustum extrema values is that no calculation
occurs. Culling seems to rely only on the projection matrix values, so,
apparently, no difference w/ the on-axis standard case.

As far as the projection is not far from the on-axis case the culling gives no
problem. Instead when screen position definition results in a large rotation in
the projection matrix objects suddenly disappear and reappear by apparently no
rule.

Are culling calculations based on a particular form of the projection matrix?
should culling planes be extracted in a different way to support the more
general case?

Thanks a lot,

Paolo Leoncini