Re-memeing this because this is the project used at Phreaknic this year (Friday night) out on the balcony. Neat stuff!
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This is truly awesome. The folks at Graffiti Research Labs created a rig that facilitates painting with light on the side of a building. It uses a high lumens projector to project the light, a green laser pointer to do the writing, and a security/astrononmy camera to detect where the green laser was pointed. They have made all the code available under the GPL.