Advanced Diffuse Shading

Gaffer allows two new diffuse shading effects.

The first is known as "diffuse transmission," an effect which can be seen in thin materials like leaves, lampshades, or bat wings. When a surface is thin and translucent, you can backlight the surface and see the light diffusely transmit through to the front. Gaffer allows you to add this behavior to your surfaces with a simple control.

The second ability changes the behavior of normal diffuse shading. One characteristic of real objects is that real surfaces which are rough simply don't have the same appearance as smoother objects. An excellent example is clay or brick, which has a very matte appearance even when lighting angle is changed significantly. This behavior is due to the light interaction and reflection effects at the rough surface.

Gaffer has a physical model of this diffuse behavior to use on rough surfaces. It has the effect of reducing the shading contrast of a material, and can be tweaked to get a full range of surface responses. It's especially useful for rendering rock, sand, cloth, airless planets, and even skin. The effect is subtle, but can change the feel of a surface to give that extra bit of extra realism, our ultimate goal.

The algorithm used for this technique is actually fairly involved, and the subject of a 1994 SIGGRAPH paper by Oren and Nayer.

The two images to the left show the effect on a simple sphere lit from the front. The upper image has normal diffuse shading; notice the contrast in intensity from the center to the edges, which fade off to black. The lower image shows the rough surface attribute set to a large value. The shading contrast is reduced, making the surface appear more like a flat disk. This is exactly the true behavior of the full moon at night, which is also evenly lit across its full surface.