ofxStencil

Simple addon for using stencil buffer. Code adapted from:
http://www.videotutorialsrock.com/opengl_tutorial/reflections/video.php

You need to enable stencil buffer in main.cpp, for example:

ofAppGlutWindow window; window.setGlutDisplayString("stencil~2 rgba double samples>=4 depth");

Usage:

ofxStencil stencil; stencil.beginDrawingStencil(); //draw your mask stencil.beginUsingStencil(); //draw what you want masked stencil.endUsingStencil();