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Rendering with Arnold in CINEMA 4D using the C4DtoA plug-in.
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Randomwalk SSS and scale in C4DtoA

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Anonymous
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Randomwalk SSS and scale in C4DtoA

Greetings to all.

I'm having a couple of issues with randomwalk SSS in C4DtoA.

For whatever reason, it almost seems like I can't turn down the scale quite enough to get the look I want. Now, it's entirely possible that I'm doing something wrong- so I'd love to get some clarification on a couple of things if possible.

1) My C4D display units are set to millimeters, and my project scale is always set to 1cm. Arnold's export scale is set to 0.01. Is this correct? It appears like a 1m^3 cube centered around the scene origin exports as having vertices at 0.5 in each quadrant, and since 1.0 is equal to 1 meter in Arnold, I'm assuming this is correct.

2) The SSS radius parameter in C4D seems to differ from that of Maya in that our units are tagged using the current display units (ie, radius is a tuple of measurements in C4DtoA instead of raw numbers). Does this mean that scale should be set to 1.0 when setting the radius through the standard_surface UI directly? Does C4DtoA automatically convert the radius values on export based on your current display preferences and scene scale?

3) If randomwalk SSS has a technical limitation that limits the radius to >= 1mm, why does the "skin 1" preset in C4DtoA show up with the radius 3.67mm, 1.37mm, 0.68mm? Doesn't this imply that the blue channel is going to get clamped to 1mm right off the bat?

I've been trying to use randomwalk for a while now because I can clearly see that the results are generally way better than diffusion- but it always seems like I can't quite get the radius small enough and I'm not sure why. What's weird is that a lot of the Maya tutorials I've seen on the internet seem to suggest it works fine and the radius limitation isn't really an issue, but that doesn't seem to line up with what I'm seeing in the IPR and at render.

Cheers,

-CMPX

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Message 2 of 3
peter.horvath6V6K3
in reply to: Anonymous

See this thread: https://answers.arnoldrenderer.com/questions/7832/subsurface-randomwalk-depth-limit.html

The C4D Project Scale and the Arnold Export Scale defines how the length units are exported. By default the Project Scale is 1 cm and the Export Scale is 0.01 (side not: the Export Scale parameter was changed to a unit type in C4DtoA 2.4.0, so actually it's now 1 m), so 100 cm is exported as 1 unit in Arnold.

The Randomwalk SSS has a technical limit of clamping the radius below 0.0001. I recommend to change the Export Scale to 1 (or 1 cm in 2.4.0). Note that the light intensity has to be scaled as well if you change the export scale. From 2.4.0 it's automatic, but you have to do it manually in prior versions.

MtoA translates 1 cm to 1 Arnold unit, which could explain why it works fine there.

Message 3 of 3
Anonymous
in reply to: Anonymous

Aaaaah.

Okay, that makes perfect sense- C4D is running at 1/100th the scale as MtoA by default.

Thank you so much for the answer- I've been bashing my head against the wall for a while now wondering why all the MtoA tutorials didn't seem to mention anything about this, but C4DtoA never seemed to work in the same way.

Cheers,

-CMPX

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