A graphics processor includes a vertex shader 20 that processes input attribute values from a vertex buffer 26 to generate output vertex shaded attribute values 28 to be used by a rasteriser/fragment shader 22 of the graphics processor when processing an image for display. Vertex shader output attributes for which the vertex shader input attributes that the vertex shader output attribute depends on are defined solely on a per-vertex basis or solely on a per-instance basis are identified. Then, for such vertex shader output attributes, the vertex shader 20 stores, for use by the rasteriser/fragment shader 22 of the graphics processor when processing an image for display, only one copy of the vertex shader output attribute for a given vertex or instance, respectively, irrespective of the number of instances or vertices, respectively, that the output attribute value applies to.