1. A method for generating bump map data substantially in real time for use in a 3-dimensional computer graphics system comprising the steps of: receiving data defining an area to which a texture is to be applied; receiving texture data to apply to the area, the data including surface height data; filtering each of a set of partially overlapping samples of the texture data; deriving surface tangent vectors from the filtered samples; and deriving a bump map surface normal from the surface tangent vectors.