1. A head mountable display (HMD) system comprising: an eye position detector comprising one or more cameras configured to detect the position of each eye of a user of the HMD; a dominant eye detector configured to detect a dominant eye of the HMD user; and an image generator configured to generate images for display by the HMD in dependence upon the HMD user's eye positions, the image generator being configured to apply a greater weight to the detected position of the dominant eye than to the detected position of the non-dominant eye.