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Hi, I have written three simple sketches using geometry shaders and a short introduction, covering some use cases where they can be quite handy. I have used Processing 1.5 and GlGraphics, as Processing 2 doesn't support them for now. To keep shaders as simple as possible the lighting has been reduced to just a diffuse factor.
Find it here: www.nachocossio.com/geometry-shaders-in-processing/