1. A method of determining local irradiance estimates for a scene in an image processing system, the method comprising: obtaining initial irradiance estimates for sample positions on one or more surfaces in the scene, the initial irradiance estimates being based on a global irradiance estimate for the scene; obtaining visibility indications which indicate the visibility of the sample positions in the scene in dependence on viewing angle; calculating an angle-dependent radiance estimate for the scene using the obtained initial irradiance estimates and the obtained visibility indications; and determining local irradiance estimates at the sample positions using the radiance estimate and the visibility indications.