1. A graphics system comprising: a texture memory configured to store a plurality of layers of texture data; a texture accumulation buffer; a span walking unit configured to traverse a first primitive with a rectangle of bins, wherein a union of successive areas visited by the rectangle in said traversal cover the first primitive, wherein, for each position P of the rectangle in said traversal, the span walking unit is configured to repeatedly walk over candidate bins in the rectangle, wherein each iteration I of said repeated walking over the candidate bins in the rectangle corresponds to a layer L 1 of the texture data layers and comprises computing a texture coordinate vector VEC P,I,B for the corresponding layer L 1 at each candidate bin B of the candidate bins in the rectangle at position P; a texture pipeline configured to receive each texture coordinate vector VEC P,I,B, to read one or more texels from the corresponding texture layer L 1 based on the texture coordinate vector VEC P,I,B and to generate a texture value TV P,I,B using the one or more texels; a texture environment unit configured to apply the texture value TV P,I,B to samples of the corresponding candidate bin B in the rectangle at position P and to store the textured samples in the texture accumulation buffer between the application of successive layers of the texture data layers.