User manual

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8.2. Problems with Alignment / Fusion
Cause / SolutionsProblem
DAVID aligns the scans such that the overlapping areas are maximized. That
maybe causes wrong solutions if there are more than one matching overlap-
ping areas.
Create more scans, so that neighboring scans have more overlap and thus can
be aligned more precisely.
Especially for smooth objects, a texture can help to find a good alignment.
Make sure that your scans are not distorted. This can easily happen if some-
thing was changed on the scanner after calibration. Calibrate again in case of
doubt.
Make sure that all around, all scans are aligned very accurately to their
respective neighbors, otherwise an error is easily continued. Align a new scan
only when all previous scans are precisely aligned.
Start the alignment again. The algorithm is in part randomized and therefore it
can find another solution next time.
Use the ”Contact Pair Selection“ to define by mouse a pair of regions which
are supposed to overlap.
The scans should overlap at least about 30%. The more overlap there is, the
more likely the correct alignment is found.
If the object has color transitions or markings, take a texture with each scan,
and activate “Use Texture” when aligning.
If you have rotated the object only about the vertical axis (parallel to the axis of the
calibration corner) and if you have not tilted the scanner after calibration, you can
choose the ”Around Y-Axis“ alignment mode.
If you know the approximate angle of rotation between scans, enter it in the
”Rotation Angle“ field. Depending on the direction of rotation you may have to
enter a negative value.
In difficult cases, you can manually align the scans: Click on the scan to be
moved, then hold down the Ctrl key and move/rotate the scan with the mouse
(left/right mouse button). Release the Ctrl button temporarily to change your
viewing direction. With practice, this can be achieved quite quickly. When
you have reached an approximate alignment with some overlap, use the mode
”Pairwise fine registration“ in order to perfect your solution.
Shape Fusion: The scans are aligned
completely wrong
Shape Fusion: The alignment is
inaccurate / the scans do not match
exactly together
Other possible problems and their solutions can be found on our website in the FAQ:
http://www.david-3d.com