The Skin Node
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The Skin Node
One way to understand individual nodes is to experiment with their basic settings. Here we will look at the Skin Node, and see what the various settings actually do.
Our scene contains a single white light, shining down on the top of a single white sphere. The skin node has been plugged into the alt_diffuse channel, and every other channel turned off. The skin node uses the default settings apart from colours. Skin Color has been set to white, Sheen Color to bright red. These renders have all been created in Poser 7.
First we look at the Ka, or Ambient attribute. This does the same here as with the normal ambient channel, creating colour regardless of the scene lighting. In this case the Skin Color is used to control the ambient colour.
The Kd setting controls the diffuse element of the skin shader. It acts in a somewhat different manner to the normal diffuse attributes in two ways. First, a high Kd setting doesn't just brighten those parts of the object being illuminated. If you are not using depth mapped shadows, it also darkens those parts that are in shade, as seen below on the Kd=5 setting (left half without shadows, right half with shadows). Second, the diffuse element of the skin node does not illuminate the edges of the object (this is more clear in the Thickness and ETA settings below).
The Ks or specular setting controls the strength of the skin sheen colour. Like the diffuse setting, it also affects those parts of the surface that are facing away from the light, subtracting the sheen colour from any ambient colour (set Ka to 1 and experiment with different sheen colours to see the results).
Thickness and ETA
The Thickness and ETA attributes only alter the diffuse element of the skin node. For this image I have set Ka and Ks to zero and Kd to 10 to exaggerate the impact of these two attributes. As you can see, high thickness and low ETA increases the impact of the diffuse colour, while low thickness or high ETA increase it. I have not yet been able to produce any translucent effects using these settings.
The image below shows how weak the diffuse element is with the normal settings. This image uses the normal Kd and eta settings, and a high thickness, with Ks and Kd turned off. The positive diffuse element is very faint.
One extra effect can be produced with the ETA setting. Values between 0 and 1 produce these bright lines across the surface of the object.