Which, as you've found, is not true.
What is true, is if all the bounding box points are *in* the frustrum,
then the object is in the frustrum. That's the "trivial accept"
condition. "Trivial reject" occurs if all the points are completely to
one side of one of the frustrum's *planes*. i.e. completely above,
completely below, completely left, completely right, completely in
front of, or completely behind the frustrum.
If it isn't trivial accept or trivial reject, then you've got a more
complicated clipping problem to deal with.
Well, clearly, you're gonna have to do more complicated geometry tests
for these cases. Quite a lot of ink has been spilled on this subject.
Don't the basic Computer Graphics textbooks cover this sort of thing
nowadays? Can't remember if mine did or not, it's been a long time
since I worried about frustrum clipping. Most people just use the 3D
HW nowadays to take care of this, so I take it this is a homework
problem, not a practical problem. Thus, what exactly to do is left as
an exercise to the reader.
No problemo. I'd offer up my ancient Free3d code for your edification,
but seeing as how I haven't compiled it in a decade, I think that would
be irresponsible. :-) Also I don't know if the solutions I employed
are good ones. Maybe you'd be better off poking at the source code of
the Mesa rendering library?
Cheers,
Brandon Van Every