no-roll virtual camera

what is the minimum number of real numbers required to define the position and orientation of a “no-roll” virtual camera?why?
How many floating point parameters does the gluLookAt function take?Why is this number not the same as in the above question?

void gluLookAt (
GLdouble eyex,
GLdouble eyey,
GLdouble eyez,
GLdouble centerx,
GLdouble centery,
GLdouble centerz,
GLdouble upx,
GLdouble upy,
GLdouble upz);

First three parameters are the camera postion, next three are camera looking at position and the last three are the up vector.


i started to answer, but it sounds like a homework question.