The up
vector is basically a vector defining your world’s “upwards” direction. In almost all normal cases, this will be the vector (0, 1, 0)
i.e. towards positive Y. eye
is the position of the camera’s viewpoint, and center
is where you are looking at (a position). If you want to use a direction vector D
instead of a center position, you can simply use eye + D
as the center position, where D
can be a unit vector for example.
As for the inner workings, or more details, this is a common basic function for building a view matrix. Try reading the docs for gluLookAt() which is functionally equivalent.