When rendering a shot with mosaic preview enabled, preview end size set to 2,
and anti-aliasing turned off, file output will contain artifacts.

See this thread for an example:

http://news.povray.org/povray.binaries.images/thread/%3Cweb.49ce235734f66dc822390e420%40news.povray.org%3E/

The bug has been present at least since beta.29. Using anti-aliasing, focal
blur, or a preview end size greater than 2 will circumvent the problem. Image
size has no effect on the general problem, except for changing the pattern
slightly. Image file format seems to have no effect either.

The bug is obviously related to a speedup mechanism kicking in under these
preconditions, causing re-use of all trace results gathered during the preview.
As it seems, the results from the rightmost cells at each pretrace level are
lost for some reason.


There is also a superficially similar issue with mosaic preview *display*; as it
seems both the right- and bottom-most cells are not displayed if the image size
is not an integer multiple of the mosaic cell size, leaving blank areas as
pixels are not re-rendered on the next step. However, these artifacts are not
written to disk, and are therefore probably unrelated, and also less
problematic.

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