A graphics processing system performs a final gather process so as to generate final gather lighting data for a scene. The final gather process comprises casting sampling rays from a final gather point within the scene. Radiosity data provided for the scene is sampled using the sampling rays cast from the final gather point. Final gather lighting data is then generated from the sampled radiosity data. The sampling rays are cast from the final gather point in an informed manner based on directional irradiance data provided for the scene. The final gather process can therefore be carried out by the graphics processing system more efficiently and effectively.