[help-3dldf] Re: [metafont] Re: button-hole problem

[Laurence Finston]

> Ah, so if I understand well, you are using parallel projection?
> This is indeed a special case of projective projection, where the
> "eye" point is far away (strictly speaking: at infinity).

I have implemented perspective projection using a `Focus' and parallel
projection onto the three major planes.  For example:

focus f;

set f with_position (-5, 5, -10) with_direction (-5, 5, 10) 
   with_distance 10;

picture p;

beginfig(1);
circle c;
c := unit_circle scaled 2 rotated (15, 30, 60);
draw c;
p := current_picture;
endfig with_focus f; %% perspective projection is the default.

beginfig(2);
output p with_projection parallel_x_y;
endfig;

beginfig(3);
output p with_projection parallel_x_z;
endfig;

beginfig(4);
output p with_projection parallel_z_y;
endfig;

Laurence