Modeling layered materials can be incredibly complicated, especially when you have to take into account how much light they scatter, as well as how much light they reflect. Here is a really interesting paper on the subject of rendering layered materials: http://www.cs.cornell.edu/projects/layered-sg14/layered.pdf
There is quite a lot of math in here, but it is worth a read to see how many different considerations need to be taken into account in order to render a nice-looking layered material!
Modeling layered materials can be incredibly complicated, especially when you have to take into account how much light they scatter, as well as how much light they reflect. Here is a really interesting paper on the subject of rendering layered materials: http://www.cs.cornell.edu/projects/layered-sg14/layered.pdf There is quite a lot of math in here, but it is worth a read to see how many different considerations need to be taken into account in order to render a nice-looking layered material!