Top tip: Make the most of specularity

Anders Langlands, CG supervisor at MPC, explains why specular properties are important for creating realistic materials

In the ’150 CG secrets’ article in issue 150, Anders Langlands discusses the need to prioritise specularity in order to achieve realistic materials.

Anders explains: The most important thing with any material is to get the specular properties right. The specular/glossy response of a surface (or lack of it) tells us whether it’s made of metal, plastic or rubber. You know you’ve got a shader right when you can look at an image and instantly know what each surface would feel like to the touch.

This means getting two things right:

1. Your Fresnel settings determine what your surface is actually made of. If you don’t apply a Fresnel falloff, you’re going to be limited to something that looks like metal. Otherwise apply a Fresnel falloff that’s appropriate to your material.

Water: 1.34
Skin: 1.4
Plastic and glass: 1.5-1.7.

If you’re unsure, just set it to 1.5: better to have something slightly wrong than no Fresnel at all. 

For the best metal effects, use measured reflectance data (‘nk data’) to get the subtle shifts in the colour and brightness of reflections as the incidence angle changes. If your renderer doesn’t support this, you can often use a very high IOR with a standard Fresnel to get some of the same effect.

 
2. If specular reflections are the most important part of a shader, bump and displacement maps are the most important part of your texture stack. 

If you’re using ZBrush or Mudbox for overall displacement, make sure you’re happy with your base model and displacement before you start shading.

If you’re going to try extracting bump maps from your colour textures as a starting point, do so immediately after you get a first colour pass. Once you have your bump and displacement down, move on to specular maps. If you’ve done your displacement well these can usually be quite simple. What you want to paint is the ‘exponent’, ‘glossy’ or ‘shininess’ map, depending on what your software calls it – in other words, the one that controls the shape of the specular highlight. You should always use an energy-conserving specular BRDF so that as the highlight gets broader, it also gets dimmer. If your software does not support an energy-conserving BRDF, you can plug the inverse of your shininess map into the specular strength parameter to fake this effect.

Once you’ve got your displacement right and your Fresnel settings correct, you can just tweak your shininess settings until your CG material matches your reference images.