A dynamic volumetric medium, such as hair, fog, or smoke, may be represented, for purposes of shadow mapping, by transmittance versus depth data for that medium. In one embodiment, the representation may take the form of a plot of transmittance versus depth, with nodes where the transmittance changes non-live linearly with respect of depth into the medium. The number of nodes in the representation may be reduced to reduce memory footprint and to enable the storage of the representation on the same chip doing the shadow mapping. In some embodiments, the number of nodes may be reduced, one node at a time, by removing the node whose underlying trapezoid has the least area of all the remaining nodes.