firstly, i'm not sure you need to in processing, does it even cull faces? maybe if you are using opengl. then there are probably opengl flags that'll let you specify this.*

traditionally this is done by working out the surface normal for the polygon being drawn, which is the cross product of the two vectors joining three adjacent points and then working out the sign of the dot product of that with the view vector. BUT the points for the face have to be numbered anti-clockwise whilst looking into the object for this to work.

to subvert this just draw all your points in a clockwise order when looking into the object (so the back walls will be shown)

* there are - glCullFace(GL_FRONT) and glEnable(GL_CULL_FACE)