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2.3 `nip2` for reflectogram mosaics

This section quickly builds an infrared reflectogram mosaic using the sample images that come with `nip2`. See Chapter 3 for detailed coverage.

Click on `File` / `Open` and doubleclick on the second `data` bookmark. You should see a directory called `IR`. Go into that directory and select `VIPS Image Files` from the file type chooser. Eight files should appear. Click on the first file, shift-click on the last, and click OK. See Figure 2.8.

Figure 2.8: Loading the sample images

The images have been named to match their positions in the mosaic, so for example `cd2.1.v` is the first image in row two. Open up viewing windows for the first two images by double clicking on the thumbnails. Move the two opened images viewers so that they are side by side. Adjust the zoom (using the `i` and `o` keys) and the pan so that the overlap area is visible in both images.

Mark a tie-point on each image by Ctrl-left-clicking on a feature you can see in both images, see Figure 2.9. 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. To mosaic the two images together, click on `Toolkits` / `Tasks` / `Mosaic` / `One Point` / `Left to Right`. See Figure 2.10.

Figure 2.9: Ready to join

Figure 2.10: Joined images

Picking items deep in the toolkit menu is fiddly, so `nip2` has several shortcuts. First, you can tear off any toolkit menu by clicking on the dotted line at the top. Torn-off menus float on your screen for easy access. Secondly, you can assign any keyboard accelerator to any menu item. Navigate to the menu item and while it is selected, press the key combination you want to use as a shortcut (for example, Ctrl-L might be good for `Mosaic` / `One Point` / `Left to Right`). Now whenever you press Ctrl-L with the keyboard focus in the main window, you will do a left-right mosaic join. Finally, you can use the toolkit browser to display a selection of the tools in a pane on the right-hand side of the main window. Click on `View` / `Browse Toolkits`, then type ``mosaic'' into the search box at the top. The toolkit browser will display all items related to mosaicing.

Join the rest of the pairs of sample images together left-right. Figure 3.2 shows how they should fit together. Once you have made all the rows, join the rows together in turn to make the complete image using `Mosaic` / `One Point` / `Top to Bottom`. Chapter 3 has some tips for doing this quickly.

When you've built the whole thing you'll see that there are differences in brightness between the tiles that make up your composite image. You can fix most problems like this automatically by selecting your final mosaiced image and clicking on `Mosaic` / `Balance`. This operation takes your mosaic apart, examines the overlap areas for differences in brightness, calculates a set of adjustment factors to minimise these differences, and then rebuilds the mosaic.

There can be some problems left even after mosaic balance. Use `Mosaic` / `Tilt Brightness` to remove any left-right or up-down graduations in brightness. The mosaic balancer outputs a floating point image (each pixel is a large floating point number), which you can't save as PNG or JPEG. Therefore as a final step click on `Image` / `Number Format` / `8 bit unsigned` to turn the image back into something you can save.

Save your mosaic workspace for future reference by clicking on `File` / `Save Workspace`. To save just the mosaiced image, right click on the thumbnail and select `Save As`.

Next: 2.4 nip2 for nerds Up: 2. Tutorial Previous: 2.2 Quick interface tour Contents