1. A method for simulating the effects of non-homogeneous fog in an image displayed on a screen, the image being intended to represent the appearance of a model defined in a database in world space coordinates from an eyepoint position in said world space coordinates, and the model being defined in terms of a plurality of features having predetermined attributes, said method comprising the following steps: a) defining a non homogeneous fog structure in world space coordinates as a series of parallel strata of predetermined extinction coefficients, b) determining a position relative to the defined parallel strata of the eyepoint, c) determining a position relative to the defined parallel strata of a feature which may contribute to the image, d) determining a distance from the eyepoint to the feature, e) calculating an average value of the extinction coefficient between the eyepoint and the feature from the defined parallel strata located between the eyepoint and the feature, f) modulating said attributes as a function of said distance and a calculated average value of the extinction coefficient, the average value of the extinction coefficient between the eyepoint and the feature being calculated by: calculating and storing average extinction coefficient values for each of a series of distances from the eyepoint in a direction perpendicular to the parallel strata, calculating the distance from the eyepoint to the feature in the direction perpendicular to the parallel strata, and looking up the stored average extinction coefficient corresponding to said distance from eyepoint to feature, the non-homogeneous fog structure being defined in the model by a series of sample extinction coefficients, each corresponding to the extinction coefficient at a respective one of a series of parallel equally spaced sample planes, the non-homogeneous fog structure being redefined with reference to the eyepoint by a series of average extinction coefficients each corresponding to the average extinction coefficient between the eyepoint and a respective resample plane, the resample planes being parallel to and equally spaced to the said sample planes and one of the resample planes passing through the eyepoint, and each resampled average extinction coefficient being stored at an address corresponding to a distance from the eyepoint to the resample plane to which that coefficient is related.