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<h3 class="sectionHead"><span class="titlemark">3.3 </span> <a
id="x13-240003.3"></a>Balancing the mosaic</h3>
<!--l. 354--><p class="noindent" ><a name="nip_label_sec:balance"></a>
<!--l. 356--><p class="indent" > Like assembly, mosaic balancing is normally automatic
and painless. You may sometimes have problems if the
image is very large, or needs dramatic corrections:
<ul class="itemize1">
<li class="itemize">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 <span
class="phvr7t-x-x-80">View </span>/ <span
class="phvr7t-x-x-80">Image header </span>in an image view
window.
<!--l. 367--><p class="noindent" >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
<span
class="phvr7t-x-x-80">unable to open tmp/xxx.v</span>, or <span
class="phvr7t-x-x-80">more than one root</span>.
<!--l. 373--><p class="noindent" >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.
</li>
<li class="itemize">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.
<!--l. 384--><p class="noindent" >The solution (as with mosaic assembly) is to
assemble and balance your mosaic in smaller
pieces.
</li>
<li class="itemize">If your grey-card correction is not accurate, you
will find that the balancer will magnify any
problems you have.
<!--l. 391--><p class="noindent" >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.
<!--l. 399--><p class="noindent" ><span
class="phvr7t-x-x-80">nip2 </span>includes several functions which can help to
fix this problem, the most commonly used being:
<span
class="phvr7t-x-x-80">Tasks </span>/ <span
class="phvr7t-x-x-80">Mosaic </span>/ <span
class="phvr7t-x-x-80">Tilt Brightness </span>/ <span
class="phvr7t-x-x-80">Left to Right </span>and <span
class="phvr7t-x-x-80">Tasks</span>
/ <span
class="phvr7t-x-x-80">Mosaic </span>/ <span
class="phvr7t-x-x-80">Tilt Brightness </span>/ <span
class="phvr7t-x-x-80">Top to Bottom</span>.
</li></ul>
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