This topic would have been better in the Programming Questions forum.

Have you looked at the example that comes with Processing V2.0a5. From the menus select File | Examples then in the window locate libraries | opengl | camera | OrthoVsPerspective if you can't find I have copied the code (minus comments) below.

1. void setup()
2. {
3.   size(640, 360, P3D);
4.   noStroke();
5.   fill(204);
6. }
7. 
   
8. void draw()
9. {
10.   background(0);
11.   lights();
12.  
13.   if(mousePressed) {
14.     float fov = PI/3.0;
15.     float cameraZ = (height/2.0) / tan(fov/2.0);
16.     perspective(fov, float(width)/float(height),
17.                 cameraZ/2.0, cameraZ*2.0);
18.   } else {
19.     ortho(0, width, 0, height, -200, +200);
20.   }
21.  
22.   translate(width/2, height/2, 0);
23.   rotateX(-PI/6);
24.   rotateY(PI/3);
25.   box(160);
26. }

" This topic would have been better in the Programming Questions forum."
No, and perhaps I moved it myself here...
Perhaps I am a bit too strict / systematic, but I consider that any "programming question" asked without code should go here. A bit arbitrary, but allows to distinguish generic / general questions from those about real code.
It is hard to put a limit between such question, indeed relatively simple and focused, and those like "help me to make a Pacman game, I have no idea where to start"...