SS scale

When working in Blender I try to maintain a scale of 1BU(Blender Unit)=1M(Meter) as this is most commonly advised in the forums. This gives me good results when working with Ambient Occlusion and simulations like cloth.

With Subsurface Scattering the scale parameter ranges from 0.000 to 1000.000 so there should be a lot of room to tweak you would think. But this is not the case when you maintain the 1BU=1M rule, I always end up tweaking the SS scale somewhere between 0.010 and 0.001. So you can imagine I feel a bit cheated out of a very large scope that the scale parameter has to offer. I cant make very subtle changes to my settings every little tweak gives me a large change.

So am I doing something wrong here, is the 1BU=1M a bad rule of thumb when it comes to SS or am I just missing something?

Did you try checking the blender manual?

Usually, in SSS implementation, the RGB values are the extinction coefficients in micrometers so you can expect that for normal SSS, the scale would be quite small for usual type of materials. The extinction coefficient is the distance in micrometer that light can travel inside the material before it looses half of its energy. It is specified in RGB because light can travel different distances depending on the wavelength.

yes

This underlines the problem I had with the small effective range I was able to tweak the scale factor with.

I was under the impression with the presets in the SS panel that setting the scale correctly would give me the same results regardless of the (geometry) scale. I’ve tested this with the suzanne head by scaling the head and then rendering it, scaling it again adapting the SS settings accordingly rendering it again and then comparing the renderings. Unfortunately the renderings don’t match.

I have been able to get better results through multiplying the Radius RGB settings by a factor of 10 or 100 and combining it with the proper scale settings.

Well, to be honest, I’ve often had problems figuring what the number truely mean in Blender implementation of SSS. Sometime, the logic seems reversed from what I expect (and not only for SSS). For instance, I recently did the candles lighting challenge in Blender and after fidling with the SSS scale for two hours, I just abandonned any hope of figuring it and just picked the setting I had at this time. To me, the SSS implementation is broken because it is barely usable as it is now. There are settings that works but one can only fall on them by pure experimentation and luck. I know what the numbers are supposed to mean because I implemented the SSS feature in Animation:Master but that does not help me when I try to use SSS in Blender. Multiplying the RGB values themselves I never tried. That is a good idea and I will try it.