The main issue with the partial images is that Povray writes the image file in a
non-standard way in order to keep the created image files small, this prevents
most image manipulation programs from reading the file.
The file declares itself to contain the data of the complete image (the image
headers width and height information is set to the full image size), but in
reality it contains just the number of rows having been rendered by the partial
image. Most image manipulation programs are trying to read as many data as
declared in the image header, resulting in an unexpected-end-of-file while



behaviour:
8<==================================================
When rendering a subset of *columns* (+sc/+ec) POV-Ray generates a full width
image and fills he not rendered columns with black pixels. This should not be a
problem for any image reading program no matter what file format is used.

When rendering a subset of *rows* (+sr/+er) POV-Ray writes the full height into
the image file header and only writed those lines into the image that are
rendered.
This can cause problems with image reading programs that are not checking the
file while reading and just read over the end.

If POV-Ray wrote the actual height of the partial image into the image header
there would be no way to continue the trace in a later run.
===================================================>8

The main problem is to find a image manipulation program being able to handle

file.

Cheers,
Wolf

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