By treating projectors as pin-hole cameras, we show it is possible to calibrate the projectors of a casually-aligned, multi-projector display wall using the principles of planar auto-calibration. We also use a pose estimation technique for planar scenes to reconstruct the relative pose of a calibration camera, the projectors and the plane they project on. Together with assumptions about the pose of the camera, we use the reconstruction to automatically compute the projector-display homographies needed to render properly scaled and oriented imagery on the display wall. The main contribution of this paper is thus to provide a fully automated approach to calibrate a multi-projector display wall without the need for fiducials or interaction.