1. A method of rendering a scene comprising a first volume and a second volume, the method comprising: determining a first transmissivity function associated with the first volume, wherein the first transmissivity function indicates a cumulative transmissivity of the first volume at a first plurality of depth values along a ray associated with a pixel; determining a second transmissivity function associated with the second volume, wherein the second transmissivity function indicates a cumulative transmissivity of the second volume at a second plurality of depth values along the ray; determining a combined transmissivity function using the first transmissivity function and the second transmissivity function, wherein the combined transmissivity function indicates a cumulative transmissivity of the first volume and the second volume at a third plurality of depth values along the ray; determining a cumulative density function using the combined transmissivity function, scattering information for the first volume, and scattering information for the second volume; sampling the cumulative density function to determine a plurality of points along the ray, wherein each point is associated with a depth value and a source volume; for each sampled point, casting a shadow ray to a light source to determine a contribution to the pixel; combining the determined contributions to the pixel for the sampled points; and determining a combined contribution to the pixel based on the combining.