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3.3 Assembling the mosaic

There are two main stages to mosaicing: first, the process of actually making the joins to assemble the image (this is semi-automatic), and a second optional and fully-automatic stage which searches the assembled mosaic for differences in brightness and balances them out.

`nip2` includes a set of infrared images you can use to practise assembly. Click on the `Open` button to get an open file dialog (see §4.3 for an explanation of all the features of the file dialog). Double Click on the second or lower `data` directory listed in the left hand column to enter `nip2`'s main data directory. The `Examples` directory should appear in the directories list. Double click on it and then double click on `1_point_mosiac` directory as it appears. The `1_point_mosiac` directory contains a set of eight example infra-read images and a complete workspace showing how they are mosaiced together. Select them all of the jpeg images and click OK. Figure 3.2 shows how these images fit together.

Figure 3.2: How the sample images join up

As was explained above, mosaics are assembled by repeatedly joining pairs of images. Pop up image view windows for `cd1.1.jpg` and `cd1.2.jpg`, and position them side-by-side on the screen -- see Figure 3.3. You should be able to make out the overlapping area.

Figure 3.3: Ready to join cd1.1.jpg and cd1.2.jpg

Mark a point on each image by Ctrl-left-clicking on a feature you can see in both images, see Figure 3.4. Move a point after you've marked it by dragging on the label. You don't need to be exact: `nip2` just uses the point you select as the start point for a search. It can cope with misses of up to about 10 pixels. See §4.2 to see how to create and edit points. You can use the display control bar to brighten the image if the overlap is very dark -- see §4.1.

Figure 3.4: Marking a tie-point

Now click on `Tasks` / `Mosaic` / `One Point` / `Left to Right`. `nip2` will use the two points you marked to determine the approximate overlap, and then search for the best way to join the two images. Open a viewer for the output image, and check that it joined them together correctly -- see Figure 3.5. See the manual page for `im_lrmosaic(3)` if you are interested in a technical description of the method used by the mosaic functions to find the exact overlap.

Figure 3.5: Joining cd1.1.jpg and cd1.2.jpg

Try moving one of your regions slightly -- you should find that the output image does not change. Despite the error in selection, `nip2` was still able to find the correct overlap. Now try moving one of them a long way -- suddenly, the output image will jump and will obviously mismatch. `nip2` has found the best overlap it could, within the 10 pixel margin of error, but it is not correct, and there is no error message.

Delete the joined image (right click on the thumbnail and select `Delete`), and click on `Tasks` / `Mosaic` / `One Point` / `Manual Left to Right` instead. This functions operate in the same way as the previous mosaic function, but does not do a search. Try moving the tie-points again -- the joined image will change every time you move a point. The manual mosaic functions are 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.

Delete the forced images, and mosaic them again. Now pop up viewers for the next pair, `cd2.1.jpg` and `cd2.2.jpg`, and join them too. Pop up viewers for the two rows, and position them one above the the other. Again, you should be able to see the overlapping area. Ctrl-left-click again to mark a pair of tie-points, and click on `Tasks` / `Mosaic` / `One Point` / `Top to Bottom`.

Mosaicing proceeds in this manner: you make repeated left-right joins to assemble each row (or repeated top-bottom joins, if the data was grabbed in columns). When two rows have been assembled, you then make a top-bottom join to link the rows together. Eventually, you will have assembled the whole mosaic. There is a workspace file, `$VIPSHOME/share/nip2/data/examples/1_point_mosaic/1pt_mosaic.ws`, which you can load to mosaic all the images. See Figure 3.6.

Figure 3.6: The entire mosaic

Subsections

- 3.3.1 Assembly hints and tips

Next: 3.3.1 Assembly hints and Up: 3. Assembling infrared mosaics Previous: 3.2.5 Correcting within nip2 Contents