In a graphics processing system, a bounding volume 20 representing the volume of a scene to be rendered and a cube texture 30 representing the transparency of the surface of the bounding volume are defined. Then, when the scene is to be rendered for output, a colour to be used to represent a first sampling point 25 as seen from a viewpoint position 24 for the scene is determined by determining, for each of plural second sampling points 27 along a vector 26 from the first sampling point 25 to the viewpoint position 24, a transparency parameter indicative of the amount of light that falls on the second sampling point 27, and then using the determined transparency parameter values for each of the plural second sampling points 27 to determine the colour. Each transparency parameter is determined by determining a vector 29 to be used to sample a graphics texture 30 that represents the transparency of the surface of the bounding volume 20 in the scene, and then using the determined vector 29 to sample the graphics texture 30 to determine the transparency parameter value for the light source 22 for the second sampling point 27.