These functions calculate
projective_coordinates. proj indicates which projection is to be performed. If it isProjections::PERSP, then f indicates whichFocusis to be used (in the first version), or the global variabledefault_focusis used (in the second). IfProjections::PARALLEL_X_Y,Projections::PARALLEL_X_Z, orProjections::PARALLEL_Z_Yis used, f is ignored, since these projections don't use aFocus. Currently, no other projections are defined. The x and y coordinates inprojective_coordinatesare multiplied by factor with the default being 1.