When rendering a scene for output that includes a light source that could cast shadows in a graphics processing system, the world-space volume for the scene to be rendered is first partitioned into a plurality of sub-volumes, and then a set of geometry to be processed for the scene that could cast a shadow from a light source to be considered for the scene in the sub-volume is determined for any sub-volume that is lit by a light source. The determined sets of geometry for the sub-volumes are then used to determine light source visibility parameters for output samples, such as vertex positions and/or screen space sampling positions, for the scene. The determined light source visibility parameter for an output sample is then used to modulate the effect of the light source at the output sample when rendering an output version of the output sample.