Not sure where to put this, it’s already on Tocpe’s skin shading forum - link - but I want it on CGTalk too in case Tocpe closes his… and I also want more feedback than I’m getting there.
To figure out how to shade realistic skin, I think the first step is all about nailing as precisely as we can the HSV variations across the surface, in different lighting conditions. This may sound obvious, but I don’t think anyone’s really nailed it yet… maybe it can’t be totally ‘nailed’, but I think it should be possible to get closer than we are today no?
Below, 2 test renders with a Phong sphere and 1 spotlight, default shader except for the Color, and default light except slightly higher intensity.
Next, the idealised version (retouched by me)
I retouched this in PS to get as close as possible to the ‘ideal’ or ‘norm’ of (caucasian in this example because it’s the hardest) skin, on the same shape and with the same lighting setup, for better comparison.
We can see that the default Phong needs a cooler darker Dayside, hotter brighter Terminator and perhaps a warmer Nightside (terms explained in my skinshade-tute on my site).
On the right, we see the glancing highlight needs to be wider and cooler, and the skin color overall much cooler - less saturated - in such low-key setups. (In fact, if the light level is low enough skin will lose all color.)
Some may not agree this is what skin behaves like, if so I encourage you to post and discuss it.
Below are some more images to illustrate my thinking, but please post your own if you want.
First the Hue and Saturation of the skin from Dayside to Terminator, in side lighting:
Saturation has been increased for clarity, and the image blurred to remove irrelevant detail.
The numbers refer to the saturation; as you can see the Terminator is redder and more saturated than any part of the Dayside. In fact the skin grows progressively cooler and less saturated towards the brightest point. You will see this effect in almost any photo that hasn’t been processed or filtered too much, the stronger and more directional the light the more clearly you see it.
Another, this time concerning highlights and their Hue:
Again I’ve oversaturated and blurred the image for clarity. The Hue of the ‘flat’ highlight is very different from that of the Hue of the glancing highlight. Plus, if you color-pick it you’ll see the same effect mentioned above with the lower saturation in the brighter spots.
Please post your own reference images for discussion. Note that your color picking will be more accurate if you blur it first. Also only select source images that don’t have too much of a color-cast, and are correctly exposed. (Saturating it is optional and in some cases not a good idea, if you do, don’t overdo it because it might clamp all the colors to max saturation.)