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Poser F.A.Q (Updated: 2016 Nov 29 4:50 pm)
Subject: Ambient vs. Translucence?
Hi, I was studying some of the "advanced" G2 shaders and noticed that a few of them used the Translucence (color & value) nodes. I had wondered what exactly this was, since I've seen so few materials use it. I had hoped that it would be kind of a subsurface scattering node, but no such luck. In fact, I can't tell any difference between the Translucence channels and the Ambient channels at all! Here is a picture demonstrating. There are two lights with raytraced shadows. The ball on the left has an ambient value, the one on the right a translucence value. The cylinder is there just to cast shadows and to see if either of these are see-through. I don't know about you, but when I hear "translucence" I would expect, at a minimum, that it would cast a colored shadow (if the light is set for raytraced shadows and RT is turned on), but it doesn't. I might expect something translucent to be at least partially transparent, but it isn't. To my eye, these balls look exactly the same in the render. The only difference is that the ambient ball looks "lit" in the preview, but the translucent one doesn't. So, I'm asking: is there any real difference between the ambient and translucence nodes that I'm not understanding? Is Translucence just a more logical name for people rendering stained-glass windows? Or is it just a way of making more complex ambient values by having two top-level inputs?
Transluceny works differently from any other part of the Poser shader system. Specifically contrary to the general "downstream only" model of the shading system, connecting things to the translucency channel creates "up-stream" effect.
The channel itself is identical to the ambient channel, but diffuse lighting nodes (diffuse, clay, skin etc.) connected to the translucency channel act as if the normals of the surface are reversed, so that lights behind the surface cause it to be illuminated.
A few things to note:
i) The diffuse nodes need to be exclusively attached to the translucency channel. If they are part of a tree that attaches to any other channel (e.g. Ambient) the effect is nullified.
I presume that the direction of the diffuse normals for the specific node are determined at some time before the sampling for the entire Poser surface is carried out and the reversed state of the normals is marked as a flag on the node data object.
ii) In P6 and P7 (?) (but not P5) the "normals forward" checkbox of the diffuse node must be checked (it also works with other diffuse type nodes if they have this checkbox).
iii) When using shadows the shadow bias must be great enough for backlights to penetrate a solid object far enough to illuminate the front (camera facing) surface. i.e. if an object is too thick the front of an object won't get illuminated. It's easy enough to demonstrate the effects of the translucency channel with shadows disabled, but tweaking it to get good effects in practise (e.g. backlit ears) can be a lot harder.
Williamshell, thank you for the explanation. Here's the same thing except with spots, and a diffuse node is plugged in to the translucence channel, and the spots are plugged into it. The spots are plugged into the ambient channel of the other ball. Now I can see a difference! It's going to take me a while to wrap my mind around this one. I tried with the shadows turned on but it appeared nearly black. I think that turning the shadow bias up enough to make it right could easily mess up the rest of the scene... So I'm not sure how useful this node is. Plus, this shows that the spots on the back side do not actually appear on the front. I don't know, maybe some ray trace nodes would be easier to use and give better results. (?) Your explanation sounds similar to how I had imagined the FastScatter node works. I assumed that FastScatter would ignore the faces facing the light, and if it hit the back side of a face, it would light up the front side. Am I right?
Willber, I agree, the ball does kind of look like the back side shining through to the front. I think it has a lot to do with the gray line across the top, even though I'm not sure where it came from! Stewer, your image is the ideal illustration of this concept. That looks just like you would expect a sheet or something to look in that situation. I could see myself using this technique on curtains, trees, and yes... stained glass windows! And all it took was plugging a diffuse node into the translucence color channel. Very interesting. Did you use ray traced or depth mapped shadows for that image, by the way? The Poser manual says that translucence is "the color of the light passing through the object." That's about all it says! I don't know how you folks figured it out, but I would not have guessed. There are a lot of things in the Poser manual that get this minimalist (or copy and paste) description. Oh well!