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<h3 class="sectionHead"><span class="titlemark">3.2 </span> <a
id="x12-230003.2"></a>Assembling the mosaic</h3>
<!--l. 299--><p class="noindent" ><a name="nip_label_sec:mosaicing"></a>
<!--l. 301--><p class="indent" > The tutorial has a section on mosaic assembly with <span
class="phvr7t-x-x-80">nip2</span>:
see <span
class="cmsy-10"></span><a
href="nipguidese2.html#x8-70002.2">2.2<!--tex4ht:ref: sec:irtut --></a>.
<!--l. 303--><p class="indent" > Mosaic assembly is normally painless. There are
a few factors you should bear in mind when you are
deciding how to assemble an image (particularly a large
image):
<ul class="itemize1">
<li class="itemize">You can open up a mosaic join and change a few
options, such as the blend width. If you want to
change the defaults for a whole workspace, change
the <span
class="phvr7t-x-x-80">Mosaic defaults </span>options in your <span
class="phvr7t-x-x-80">Preferences</span>.
</li>
<li class="itemize">If two images just won’t join correctly, try using
<span
class="phvr7t-x-x-80">Tasks </span>/ <span
class="phvr7t-x-x-80">Mosaic </span>/ <span
class="phvr7t-x-x-80">One Point </span>/ <span
class="phvr7t-x-x-80">Manual Left to Right</span>
instead. These functions operate in the same way as
the usual mosaic functions, but do not do a search.
This is useful when the overlap is too small for the
search to work correctly, or when the overlap area
is very smooth and contains too few features for
the search to find the exact overlap for you.
</li>
<li class="itemize"><span
class="phvr7t-x-x-80">nip2 </span>does not do sub-pixel interpolation. As a
result, each join will on average cause a positioning
error of about 0.5 pixels, even if your input
images contain no geometric distortion. If there
are distortions in your input images (there usually
are distortions in infrared images), then errors can
be as much as 1–2 pixels per join. These errors
accumulate as the number of images you join
becomes larger.
<!--l. 331--><p class="noindent" >If you join a strip of 10 images together with <span
class="phvr7t-x-x-80">Tasks</span>
/ <span
class="phvr7t-x-x-80">Mosaic </span>/ <span
class="phvr7t-x-x-80">One Point </span>/ <span
class="phvr7t-x-x-80">Left to Right</span>, on average you can
expect a total error of about 5 pixels. If you join
two strips like this together top-bottom, you can
therefore expect a mismatch of about 2.5 pixels at
each end of the join. <!--l. 337--><p class="noindent" >You can minimise the effect of these errors if you
assemble your images differently. Suppose you
have a 10 by 10 mosaic to build. Instead of making
and joining 10 strips of 10 images each, make four
5 by 5 sub-mosaics (one for each quadrant) and
then join these four quadrants together. The errors
will now be more evenly spread over the image and
therefore will be less visible.
</li>
<li class="itemize"> <a name="nip_label_sec:pieces"></a> Some operating systems limit the number of files
a program can have open at once: this in turn limits
the size of the mosaics you can assemble in one go.
If you are having problems putting together very
large mosaics, try building your image in sections
(<span
class="phvr7t-x-x-80">top</span>, <span
class="phvr7t-x-x-80">middle </span>and <span
class="phvr7t-x-x-80">bottom</span>, perhaps), and later load up
and join these larger pieces.
</li></ul>
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