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This task shows you how to align a cloud of points with another cloud
of points. |
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If you want to align a cloud of points with points, use
the Align with Points action. |
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Open the
Reposition1.CATPart model the from the samples directory. It consists
of two clouds of points with three spherical tags on each. These tags
have been created during digitizing in order to align the two clouds in
future operations. |
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Click the Align with Cloud icon
 . A selection
dialog box is displayed.
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 | To activate an area by picking elements, select them and click
OK to confirm the activation and close the dialog box. |
| To create a single activation area by trap, draw the trap, modify it
when necessary and click OK to confirm the activation
and close the dialog box. |
| To create several activation areas by traps, draw the first trap,
modify it when necessary, click Valid Trap to validate this
trap. Repeat these steps for each trap then click OK to
confirm the activation and close the dialog box. |
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Select first the
cloud to move (in fact the output cloud is a moved copy of the initial
cloud, which can then be hidden if necessary).
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Then select the target cloud.
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Define significant common areas on each cloud (source and
target) to align using the Alignment dialog box (its operating
mode is the same as
Activating a Portion of Cloud). Click Valid Trap between
each area definition, and OK when all areas have been defined,
on the source clouds first, then on the target cloud.
These areas are the basis of the computation. They may have any shape, be
in any number. In our example, the significant area are the
circular tags.
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As long as you have not double-clicked to end
the polygonal or spline trap, you can undo/redo each pick of the
polygonal trap.
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Use
the Selective Display option to make the definition of the
computation areas easier: when this option is active, only the cloud
where you should define computation areas is displayed. Once you are
finished with the first cloud, it is hidden and the second one is
displayed.
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After the last OK, Digitized Shape Editor proposes to
compute the first move:
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Answer Yes: Digitized Shape Editor aligns the
center of gravity and the inertia axes of the parts, then the
position of each part. |
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Answer No: Digitized Shape Editor aligns only the
position of each part. This is recommended when the alignment of the
parts is already almost correct. |
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The input cloud is duplicated
and this copy is moved towards the target. This new cloud appears in the
specification tree. It has the name of the input cloud, with its index
increased by one. An element Axis_Systems is also created in the
specification tree, that contains the reference axis system and the target
axis system. These two axis systems can be used in an
Axis to Axis transformation with other geometrical elements (i.e.
replay the alignment). |
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Statistics on the alignment operation are available.
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Push the More button.
The Statistics are displayed as soon as the computation
is started.
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This is the first section displayed. It indicates:
| the type of alignment performed (here it is a best fit
registration), |
| the number of points found in the mobile cloud, i.e. the cloud
to move, |
| the number of points found in the reference, i.e. the target
cloud. |
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This section is displayed when the computation is done. It
indicates:
| the number of iterations done, |
| the maximum deviation found between the points of the cloud
moved and the target cloud, |
| the mean deviation found between the points of the cloud moved
and the target cloud. This deviation should be as small as
possible. |
| the standard deviation, i.e. the dispersion of the points
around the mean deviation. A small standard deviation indicates
that most points are within the mean deviation, i.e. that there are
only few outliers. |
| the percentage of points of the cloud moved that are below the
mean deviation. |
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You can select the text and copy it to a word or data processing
software.
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| You can repeat the process to improve the alignment, but this time,
do not accept the automatic first move. |
| The cloud to align should not contain a large amount of points, |
| The shape of the traps on each cloud should be similar and contain
coincident points |
| The trap(s) on the target should contain more points and be larger
than the trap(s) of the source cloud. |
| You can use the function
Distance analysis to check the output accuracy. The target will be
the output cloud. Since a new output cloud is generated at each
alignment, you should repeat the distance analysis with each new output
cloud. |
| The result entity has the same structure as the input entity: scans,
grids or polygons. |
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