Adjust the images of the range data in each window, using the zoom and translate mouse
features, until you have a clear view of some overlapping point in the two scans. Hold
Ctrl
and
the cursor changes to a cross. Select a point in either range window. Then, find the
corresponding point in the other window and select it in the same way. You can select in either
window first, and you can change the order for future selections – but you must complete a pair
before selecting the next point. Figure 54 shows correspondence points selected in our sample
data. The points will appear in the scene tree – and can be moved or deleted as with any other
points.
In addition, when a correspondence point is selected it can be moved up, down, left or right
using the keyboard’s arrow keys. This provides very precise control over point selection.
Look for points that are easy to identify – corners of objects, noticeable markings, etc. You also
will want to spread the points out to include different parts of the data. To save the points
you’ve selected, just save the scene file since they are simply another type of annotation.
Continue in this manner until you have at least 3, and perhaps as many as 6 correspondence
pairs. Range files with 4 pairs of correspondence points are shown in Figure 53.
Note: Because the scans were taken from different locations, the angle of view of
corresponding points will usually be different. This does not affect the alignment.
You just need points that are visible in each window.
Figure 53 Range align correspondence points
2:Calc Fit
After you’ve selected your correspondence points choose
2:Calc Fit
. This will execute the
alignment process which will end with a statement of the RMS (root mean square) error. The
error should be less than 1 – and is often less than 0.5. (Currently, this number is in inches.
Future versions will produce a number in the units specified in
Preferences
.)
At times, you may wish to exclude a region of data from consideration during the alignment
process. For example, a person or object in the scene may have moved from one scan to the next
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