Parallel Projections

When a Picture is projected onto the x-y plane, the x and y-values from the world_coordinates of the Points belonging to the objects on the Picture are copied to their projective_coordinates, which are used in the MetaPost code written to out_stream. If a Picture p contains an object in the x-y plane, or in a plane parallel to the x-y plane, then the result of p.output(Projections::PARALLEL_X_Y) is more-or-less equivalent to just using MetaPost without 3DLDF.

     Rectangle r(origin, 3, 3, 90);
     Circle c(origin, 3, 90);
     c *= r.shift(0, 0, 5);
     r.draw();
     c.draw();
     current_picture.output(Projections::PARALLEL_X_Y);
     

Fig. 53.

If the objects do not lie in the x-y plane, or a plane parallel to the x-y plane, then the projection will be distorted:

     current_picture.output(Projections::PARALLEL_X_Y);
     

Fig. 54.

Picture::output() can be called with an additional real argument factor for magnifying or shrinking the Picture.

     Rectangle r(origin, 4, 4, 90, 60);
     Circle c(origin, 4, 90, 60);
     c *= r.shift(0, 0, 5);
     r.filldraw(black, gray);
     c.unfilldraw(black);
     current_picture.output(Projections::PARALLEL_X_Y, .5);
     current_picture.shift(2.5);
     current_picture.output(Projections::PARALLEL_X_Y);
     current_picture.shift(1);
     current_picture.output(Projections::PARALLEL_X_Y, 2);
     

Fig. 55.

Parallel projection onto the x-z and z-y planes are completely analogous to parallel projection onto the x-y plane.