In this paper we address the problem of geometric calibration of video projectors. Like in most previous methods we also use a camera that observes the projection on a planar surface. Contrary to those previous methods, we neither require the camera to be calibrated nor the presence of a calibration grid or other metric information about the scene. We thus speak of geometric auto-calibration of projectors (GAP). The fact that camera calibration is not needed increases the usability of the method and at the same time eliminates one potential source of inaccuracy, since errors in the camera calibration would otherwise inevitably propagate through to the projector calibration. Our method enjoys a good stability and gives good results when compared against existing methods as depicted by our experiments.