Apparatus for controlling a spatial light modulator (106) of an image projector (100), the image projector further comprising an illumination source (102) arranged to illuminate the spatial light modulator (106), the apparatus comprising an input (126) for receiving image data, an output for outputting image data and a controller (110) in communication with the input and the output, the controller(110) being configured to: determine an estimated thermal state of the projector; use at least the estimated thermal state of the projector to obtain an expected illumination pattern of light incident on the spatial light modulator (106) when illuminated by the illumination source (102); use the expected illumination pattern to adjust the received image data so as to compensate for thermal changes in one or more components of the image projector; and output the adjusted image data to the spatial light modulator (106).