I was browsing some time ago through Christoph Hormann's site, when my 
attention was caught by this:

http://www.imagico.de/pov/asia/making.php

Lo! Using splines in isosurfaces! Something I had never done before. So, 
I started testing and soon discovered that (1) Christoph's explanation 
was not complete (how do you /really/ add curves to the road?) and (2) 
using splines in isosurfaces was not really a trivial matter to get right.

On my way, I got the attached landscape where a spline controls the 
ravine cutting the landscape from the lower right hand corner to the 
upper left.

Rendering is slow, even with a far from optimal max_gradient, so the 
render was restarted using +C a couple of times. Total render time was 
about six to seven hours.

What puzzles me in the resulting image here is the visible boundary 
between the first partial render and the first restart at about one 
third down the image. You cannot miss it. I never experienced something 
like that before. From a couple of experiments, I guess that it has to 
do with the isosurface but I am at a loss about /why/. Any idea?

Using version 3.8 (also shows with 3.7 as far as I can tell), 6 threads, 
on a i5 laptop.

-- 
Thomas

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