A method and apparatus for simulating the illumination effects of a vehicle-mounted light source in an image displayed on a screen. The image is intended to represent the appearance of a model defined in a database in world space coordinates from and eyepoint position in world space, the model being defined in terms of a plurality of features each having predetermined attributes. The vehicle-mounted light source is defined in terms of an origin and a direction in eyepoint space. For each feature potentially contributing to the image, the angular position of a point on that feature relative to the light source is calculated as the angle between the light source direction and a line drawn to that point from the light source. An illumination intensity for that point is determined by reference to the calculated angle, and the illumination intensity is reduced as a function of the distance from the eyepoint to the said point to provide a resultant intensity. The attributes of the feature at the said point are modulated as a function of the resultant intensity.