I have just dug up the source code (and the instructions for compiling it
on Mac OS X) again and can confirm that removing the two lines

  Image->width--;
  Image->height--;

from Make_Pattern_Image(...) in parstxtr.cpp fixes the problem.

One thing that still makes me wonder is that the function evaluation point
(for the pixel that controls the height_field vertex at <x, 0, z>) is
<x, 1-z, 0>, i.e. the image is upside down. Is there a reason for this?
Wouldn't it be more logical (and consistent with e.g. image_maps) to have
<x, z, 0>? Or would that break anything?

I believe that this is a bug which was introduced because the author
didn't consider that image scanlines are usually (and apparently also in
POV's IMAGE structure) numbered from top to bottom, while y coordinates
(in POV's standard coordinate system) increase from bottom to top.

A way to fix this, should one agree that it is a bug, would be replacing

  gray16_line = Image->data.gray16_lines[i] = ...

by

  gray16_line = Image->data.gray16_lines[Image->iheight-1-i] = ...

in Make_Pattern_Image(...).

 -Christian

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