See the attached image. Three columns left to right with images in the 
first two rows rendered with 1, 2 and 4 threads. The bottom row is an 8x 
multiple of the image pixel differences.

Using the subsurface.pov scene shipped with POV-Ray and the command:

povray -j -d -c -p +wt4 subsurface.pov

changing only the number of threads each time. Ubuntu 16.04. Master 
branch as of commit 9c67174 Sat Jan 6 11:58:42 2018 +0100 "Merge branch 
'release/v3.8.0'"

I've not seen any differences between single thread renders in a half 
dozen runs. Always some difference in multi-thread runs, but the 
signature is interesting in that POV-Ray seems to match on some of the 
sub blocks. More often ones at the top match, but I've seen render-image 
pairs with matching blocks better than half way down into the image.

I know the SSLT code is not in it's final form. Is this a known issue or 
should we open up a github issue for it?

Bill P.

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