Spatial augmented reality extends augmented reality by projecting virtual data directly on a target surface, but requires to calibrate a projector-camera system. This paper introduces a free-calibration projector-camera system for spatial augmented reality with a planar surface. A pattern is projected on the target surface that can be freely moved. The main difficulty is to make the projected image fitting the target surface. This is often achieved by calibrating the projector according to the camera's pose, but the calibration process is offline and restricts the configuration of the setup. Our proposed method allows to independently move the projector, the camera or the surface while the projected image continuously fit the target surface. The experimental results demonstrate the efficiency of our calibration-free spatial augmented reality approach when applied on a moving planar surface.