Realistic human skin has long been one of the most difficult goals for CG. Not only does human skin have very detailed texture with freckles, pores, wrinkles, and even fine hairs on its surface but it’s also translucent, so a large amount of light penetrate the skin and react with all the layers underneath. These layers make light bounce and scatter in complex ways, resulting in the “soft glow” of skin. Variations in skin thickness must be taken into account as well, since these variations can effect the skin color and the way light interacts with the layers beneath it.

The latest 3D technology called “Sub Surface Scattering” makes simulating skin in CG possible. “G2 skin mode” makes realistic skin shading possible in LW! Even better, you can do it in G2's real-time interactive preview!

As you can see in Figure 1, there are not many parameters in G2's skin mode. But that doesn’t mean that it’s lacking power! It’s designed to be as intuitive and as simple as possible, even automatically setting some parameters that are usually annoying for artists to define. G2’s amazing real-time preview( Figure 2 )makes tweaking a pleasure, not a chore.. With G2, your LightWave world is changed forever.

Setting the thickness of skin from texture mapping

First of all, what G2’s skin mode helps us with is shading. What is required for making realistic human skin is not only G2’s skin mode, but also good texture mapping and light setting skills and so on.

Let’s talk about parameters in G2 skin mode! Human skin varies in thickness so the way light interacts with it different as well. So if you don't account for the thickness of skin at every point, you won't get very accurate shading. But G2 does this automatically if you have a textured skin map,deriving skin thickness by reading textured color values. If you paint a reddish color on the surface of the skin, G2 understands that the thickness of the skin is thinner at that point.

G2 has its PhotoMap tool, which is very useful when using a photograph to make a texture map. A photograph includes natural lighting so it’s not usually convenient to use it as a texture map, but you can get rid of much of the poor shading using G2’s PhotoMap tool. (Figure 3 and Figure 4)

Before getting started with G2, I would like to talk about modeling and texturing, which are very important for making your character look photorealistic. In terms of modeling, I started the face symmetrical at first, and then worked each side of the face separately so that the character was not unrealistically symmetrical, a common fault in CG. (Figure 5). When it came time to texture, I used David Beckham’s photo as a base and cut it into parts that were pasted and tweaked until they formed a single skin map. I said that G2’s skin mode calculates shading based on the colors on the surface but I also assisted it with my own intuition. (Figure 6) . Next I applied G2’s skin mode and tweaked the parameters.

First, I defined the main parameter, Multilayer Subsurface Diffusion. The higher number you put, the softer the surface looks. Compare it to the LightWave standard shading, and you can see the difference between LW and what G2 skin mode can do (Figure 7). The edges of the ears and cheeks are brighter, and the highlights are clearer, and over all, the contrast became softer and it adds a human touch onto the skin. I used 100% this time. The thickness of the skin is still equal at this point so next I increased the Epidermis Visibility to vary the thickness. You can see the edges of the surface became extremely bright. The best value depends on the lighting setup, I set it to 100% here. Epidermis Scattering allows further control of the way light reacts to the outer layer of skin. I set it to 50% (Figure 8)

Skin color is also effected by the G2's "Blood Color" control. When you change the blood color brighter, the outer skin color becomes darker, and when you set very reddish blood color, the skin color becomes blue greenish (figure 9).

With Follow Bump Mapping, you can define how blood color reacts to bump maps you might add, like pores and wrinkles.

G2's Rough Surface Oren/Nayar Diffusion is similar to the Diffuse Roughness that Gaffer had. It’s usually very effective on rough appearances but I found it not so effective for creating realistic skin.
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Lighting in G2

Since G2’s skin mode is specialized for skin, there is a big difference in appearances between G2 and the LW’s standard shader. The quality of skin renders will improve by using G2’s skin mode, but the contrast will decrease since Multilayer Subsurface Scattering can make the renders become too soft, and Epidermis Visibility and Epidermis Scattering can make the edges around objects too bright. You can fix those problems by tweaking G2’s parameters as well as scene lighting. You can see the big differences with and without the shader, lit in front with the same lighting ( Figure 10). My point is that you should get the hang of what the shader does before setting the lights.

The key point for lighting is to use back light effectively (Figure 11). With the back lighting, soft-glow effects occur and the edges become bright so the contrast becomes clearer. This effect adds depth to the scene by separating the subject from the background. The object's edges become especially lifelike! Please note that you should balance the edge's brightness with lighting setting and G2’s Epidermis Control. Luckily, G2's interactive real-time preview makes this easy to do. Not only does G2's preview show the effect of the G2 skin shader, But you get instant feedback when you move/add lights or change their intensity.! (Figure 12)

I usually don’t use radiosity because of the long render times but I tried an HDRI based radiosity render as a G2 test (Figure 13). I didn’t use the highest quality settings since the rendering is too time consuming but as you can see the shading becomes soft and natural.. I’m used to using my own lighting to create realistic effects but there is something in the renders that radiosity creates that is beyond normal lighting. I need to explore it more! But my time is up! I’d like to check more on G2 and radiosity later for my own research.

Syunichi Shirai
After working for Square and Imagica, he became a free-lance 3D CG artist and founded the company "mix core" in 2002 with his friends. Their business includes CG movies for games, commercials, movies, illustrating, web design, and multimedia CD-ROM. They are way too busy with a lot of job offers,
and need to find more staff!

The Japanese version of this tutorial was printed in the Japanese Computer Graphics World magazine in January 2003.