1. A method of processing data defining a plurality of first polygons and texture data therefor, to generate data defining a texture map, the method comprising: determining a respective measure of the importance of the texture data for each first polygon; defining a respective second polygon in a two-dimensional area for each first polygon, such that each second polygon is defined with an area dependent upon the measure of the importance of the texture data determined for the corresponding first polygon; and generating texture data for the second polygons in dependence upon the texture data for the first polygons.