Artists can also draw very realistic textures. When does it make more sense cost wise (time, resources) to have a computer trace the lighting of a material's texture than to have an artist draw it?
how are we able to model real life materials so accurately? do we have to measure the physical properties of materials and then enter those parameters into our renderer or do we just try various ways until we get the rendered material close enough?
where can we get the BRDF data for these surfaces?