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### 3.2 Assembling the mosaic

The tutorial has a section on mosaic assembly with nip2: see §2.2.

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):

- 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 Mosaic defaults options in your Preferences.
- If two images just won’t join correctly, try using Tasks / Mosaic / One Point / Manual Left to Right 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.
- nip2 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.

  If you join a strip of 10 images together with Tasks / Mosaic / One Point / Left to Right, 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.

  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.

- 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 (top, middle and bottom, perhaps), and later load up and join these larger pieces.

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