Like assembly, mosaic balancing is normally automatic
and painless. You may sometimes have problems if the
image is very large, or needs dramatic corrections:
Each VIPS image file has an associated history,
recording the operations on that image since it was
loaded from a file. You can view an image’s history
by clicking on View / Image header in an image view
window.
The automatic balancer uses the history to work
out how you built your mosaic. The balancer
knows about left-right and top-bottom joins, but
nothing else! If the history has other stuff recorded
in there, you’ll see unhelpful error messages like
unable to open tmp/xxx.v, or more than one root.
If you need to perform corrections to any of your
sub-images, do them, save the image, load it again,
and then build the mosaic. This will make sure the
history of the image you are trying to balance only
contains mosaic operations.
On some systems the balancer can run out of
memory or out of file descriptors on very large
mosaics. If your mosaic is made up of more than a
few hundred images, and you are having balancing
problems you may have hit one of these limits.
The solution (as with mosaic assembly) is to
assemble and balance your mosaic in smaller
pieces.
If your grey-card correction is not accurate, you
will find that the balancer will magnify any
problems you have.
Suppose your lighting and camera set-up always
produces images which are brighter on the right
than the left, and suppose, due to some problem with your grey-card correction, this effect is not
completely removed. You will find that when you
balance a mosaic, the small differences between
left and right edges of your sub-images will have
been smoothed out, but they will have caused a
large difference in brightness between the extreme
left edge of your final image and the extreme right.
nip2 includes several functions which can help to
fix this problem, the most commonly used being:
Tasks / Mosaic / Tilt Brightness / Left to Right and Tasks
/ Mosaic / Tilt Brightness / Top to Bottom.