Perhaps, finally, a good test case for isosurface artefacts I've worked 
around for years! Confirmation Windows / OSX showing similar artefacts 
would be useful. Linux users not on Ubuntu 18.04 too.

Looks like something goes wrong when an isosurface's function has an 
inflection (local min/max) where rays cross the containing box/sphere 
surface. Also looks like the inflection can be well away from any shape 
created inside the container; It need not be near the threshold value.

Move the container sides away from the inflection and things work fine - 
also my long standing hack work-around. In the attached test case, three 
of the four containing box sides are at an inflection for the, adjusted, 
tiling pattern function. The last side isn't - by a tiny amount - and no 
artefacts on that side.

See attached image and scene file for v3.8.

Bill P.

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Attachments:
Download 'isoinflectioncontainersurface.png' (149 KB) Download 'isoinflectioncontainersurface.pov.txt' (4 KB)

Preview of image 'isoinflectioncontainersurface.png'