A computer-implemented method of determining a bounce light map to represent lighting in a scene for use in rendering the scene as a frame in a graphics processing system, wherein the bounce light map is used to approximate global illumination for the scene, the method comprising: determining initial lighting indications representing lighting within the scene, wherein the initial lighting indications are determined as an initial light map, and wherein the initial light map either: (i) is a direct light map, or (ii) is determined by combining a direct light map with a bounce light map determined for a previous frame in a sequence of frames; for each of a plurality of texel positions of the bounce light map: identifying one or more positions (516 1 and 516 2) within the scene based on values of corresponding elements of the initial light map (508 1 and 508 2) which correspond to said one or more positions within the scene, wherein identifying one or more positions comprises using an importance sampling technique, and the likelihood of identifying a given element is based on an importance value associated with each element of the initial light map; tracing one or more sampling rays (512 1 and 512 2) between a position in the scene corresponding to the texel position of the bounce light map and the respective one or more identified positions (516 1 and 516 2) within the scene; and determining a lighting value for the texel position of the bounce light map using results of the tracing of the one or more sampling rays. determining initial lighting indications representing lighting within the scene, wherein the initial lighting indications are determined as an initial light map, and wherein the initial light map either: (i) is a direct light map, or (ii) is determined by combining a direct light map with a bounce light map determined for a previous frame in a sequence of frames; for each of a plurality of texel positions of the bounce light map: identifying one or more positions (516 1 and 516 2) within the scene based on values of corresponding elements of the initial light map (508 1 and 508 2) which correspond to said one or more positions within the scene, wherein identifying one or more positions comprises using an importance sampling technique, and the likelihood of identifying a given element is based on an importance value associated with each element of the initial light map; tracing one or more sampling rays (512 1 and 512 2) between a position identifying one or more positions (516 1 and 516 2) within the scene based on values of corresponding elements of the initial light map (508 1 and 508 2) which correspond to said one or more positions within the scene, wherein identifying one or more positions comprises using an importance sampling technique, and the likelihood of identifying a given element is based on an importance value associated with each element of the initial light map; tracing one or more sampling rays (512 1 and 512 2) between a position in the scene corresponding to the texel position of the bounce light map and the respective one or more identified positions (516 1 and 516 2) within the scene; and determining a lighting value for the texel position of the bounce light map using results of the tracing of the one or more sampling rays.