Does a render region affect SSS?

[evanerichards]

I have a model. I have a fastSSS shader applied to it. Although all the maps should match, somehow the arm doesn't match the body...unless I make a smaller render region. If I render a smaller region around the troubled area it renders fine.
Any idea why this might be the case? I change nothing about the render except the region and yet the area rendered looks significantly different.

Is this normal?
See the attached picture for an example.

[vlado]

Yes, it's normal (depending on the sss scatter radius); you can try to set the "prepass blur" parameter to 0.0 and see if it works better. We are working on a more stable solution to the problem though (by distributing the samples for the illumination map directly on the surface, rather than with a prepass).

Best regards,
Vlado

[Morbid Angel]

it feels that your sss scale is too large in general.

[evanerichards]

I'm pretty new to SSS. How can you tell?

[Morbid Angel]

if you look at his tail on the first render, you can see the red color in the deepest part of the tail, same for the torso. usually you wont have that unless the object is really thin. The way to measure proper sss radius is to take your scene unit, which typically = 1 grid unit, for example 1 cm = 1 grid unit, and see what your sss scale is (Default is 1), which means that the furtherest trace of the sss will equal to 1 unit. So if your object is way too small, for example 0.1 of the grid unit, then the sss depth will be big, which is what it appears to be in your scene. What I would do is go to the thinnest area of your object, for example ear, or nose, and measure the thickness of the front and back of the surface, and usually base the sss scale of of that.

[evanerichards]

Excellent info. Thanks for the reply.