Realistic skin is hard to fake because it's both an artistic and a technical challenge. It's artistic because you can almost feel a character's personality through the look of its skin; its wrinkles, colours and beauty spots. But it's also a technical test, because skin isn't like a homogeneous material that has simple physical properties - it's composed of several different layers. However, we always model it as a perfectly even thin surface, which is why it's so hard to get good results. To make matters worse, until recently, 3D software didn't include appropriate rendering tools to make human skin, such as subsurface scattering shaders.
To create realistic skin, you have to use some tricks. When I started the image on the right, my aim was simply to create a realistic portrait. With work and good anatomical references, I created the model in about two weeks with 3ds max. I then started to paint the texture maps with Paint Shop Pro, which isn't too hard with hi-res photos as references - but only as references, as I'll discuss later.
