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Subject: Occlusion Passes and Innermouth darkening techniques


servo ( ) posted Fri, 15 October 2004 at 3:14 PM · edited Sat, 27 July 2024 at 6:49 PM

This a cross post from an ongoing poser forum thread -- I was hoping some hi-tech guys here would jump in with ideas. "Nostril Glow" and Poser "BrightMouth" have been talked about a lot, and there are various freestuff fixes and techniques, but I'm pressing a little deeper (especially for the inner mouth area). At Pro FX houses, CG creatures are often rendered in several passes, one of which being an "occlusion pass". This pass of the render uses a clever surface shader all over the model and a "one-bounce-only-to-camera" ray trace. The result is a frame where the "concave" and generally non-camera facing regions (tight crevices, skin folds, etc) are grey to black, and the other surfaces remain white. This image is used in a composite with the main render as a key to darken those regions proportionally, and it produces great results on the inner mouth, nostrils, armpits, cracks, etc. Generally this is done with Renderman or Mental Ray. (When you see a CG creature with and without occlusion darkening, you'd be amazed at the comparative extra level of realism you get from a relatively subtle change like this; The shadowing in general seems 1000% more realistic.) I personally have tried and failed to do this kind of occlusion pass with Firefly and Poser material settings: I would LOVE to hear success stories (and see tutorials)if you've had better luck. With Poser, we have some standard tricks for nostril and mouth glow, but none are quite satisfactory for photoreal closeups: The trouble with just darkening mouth textures is that unless you specifically remap the mouth, teeth and tongue in variable sections, you have to darken them uniformly; When you're going for true photoreal shadowing in a closeup image, the front facing teeth, the front of the tongue, and sometimes even the front-facing back wall of the throat need to be brighter, with a gradient intensity falloff to the innermost non-facing concave portions of the inner mouth -- This becomes even more tricky when the light is coming from side, high, or low angles relative to the head position. That's why I thought maybe using some kind of a semi-translucent, layered, and/or positionable "darkmouth blob" object parented inside the mouth might be the thing to use, if I could figure out a useable implementation of it. Any and all thoughts on this subject are welcome. I'm hoping by posting this we can all learn about some possible solutions and workarounds.



diolma ( ) posted Fri, 15 October 2004 at 4:24 PM

Never thought about this (I don't do close-ups as a rule), but 1 idea springs to mind (I haven't tried it - I'm making this up as I'm typing).. Would plugging a U or V (probably U) coordinate node in there somewhere help? Perhaps into the texture strength of the image map? Or maybe running the output from the image map into 3 colour (oops - color) component nodes, extracting the RGBs from them, runnning those through 3 U coordinate nodes, then plugging them all back together via the user-defined colo(u)r node, finally plugging that into the diffuse colour... Alternatively: do -ve values work with ambient? If so, running U coord through a math "subtract from 1 node" might achieve something... I don't have P5 on this machine so I'm just tossing balls (no innuendo intended..) into the air.. Cheers, Diolma



servo ( ) posted Fri, 15 October 2004 at 4:55 PM

Thanks, those are interesting thoughts... I hadn't considered digging that deep into node juggling and coordinates -- I guess I was fixated on the idea of utilizing some kind of auxilary shadowing prop (which might still have merit when used in combination with this, or maybe other possible fix ideas). I'll look into this idea (to the degree I can grasp your clever concepts) but I hope others will also for the purpose of comparing results and settings here. I'm also still hoping someone has toyed with an actual occlusion pass attempt/workaround as I mentioned above, since that would help deal with far more than just the mouth in terms of general shadowing improvement. --servo



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