There are many ways to do this, see the code snippet below for some useful methods. Try searching for 2D/3D and you will most likely find more solutions, such as using PGraphics. This topic has been discussed many times.

Code Snippet

1. void draw() {
2.   // 3D code
3.  
4.   hint(DISABLE_DEPTH_TEST);
5.   camera();
6.   noLights();
7.   // 2D code
8.   hint(ENABLE_DEPTH_TEST);
9. }