[Algorithms] Up Vector in Projection Matrix

[Pai-Hung C. <pa...@ac...>]

Hi,

I am manipulating projection matrix in my D3D program so that the player can
lift up/down his head to view the world.  I do modify the viewing-direction
component of the projection matrix but always leave the "up-direction"
component as (0, 1, 0).  I think this should not work theoretically (at
least introduce some weird effect).  However, it DOES work even I don't
modify the "up-direction" component (such that it is perpendicular to the
"viewing-direction").  I do know different up-vector, when setup reasonably,
can have different effect with the same direction-vector.  But what if
up-vector is setup unreasonably (impossible to happen in the real world)?
Could someone give me some idea?

Thanks in advance,

Pai-Hung Chen