Method 2: multiple-chunk. Summary o

Method 2: multiple-chunk. Summary of
photography method suitable for multiplechunk
method: one set of photos (with or
without turntable), flip specimen over, take
next set (repeat if necessary). Include scale bar
in each set. For walk-around method of large
specimens: Take one set of one end of the
specimen, take a separator photograph, take
set of next section, ensuring that the two
sections overlap. Repeat if necessary.
See Figure 12.
The multi-chunk method allows using the
background for the alignment step within each
chunk (those parts of the background that do
not move relative to the specimen during the
photography of one set of photos), thus it is a
good method for specimens where few points
for alignment will be found on the specimen
itself. A background well suitable for point
detection should be chosen (newspaper,
Persian rug, etc.). Also, using the multi-chunk
method means less care is needed to set up
the turntable than for the one-chunk method,
and the preparation in the photogrammetry
program is easier, as background blanking or
masking is not needed. However, much fiddling
may later be necessary to adjust the alignment
between the two chunks, or the fit between the
two model halves. The method also makes
handling very large projects easier, by reducing
the overall calculation time and improving the
chance at good alignment. The method works
with more than two series of photos and shells
as well, but more parts usually mean more
trouble aligning them.
As in the one-chunk method, it is possible to
mask the background for a cleaner model
provided the features detected on the specimen
itself suffice for alignment within each image
set. In the example shown in Figure 12 this is
the case; but barely: several images are not
entirely correctly aligned (visible by the handful
of ‘floating’ points in the lower left quadrant
next to the bone shaft; caused by slight
misalignment of several images). If the same
data set is run via the one-chunk method, the
alignment is less good (Figure 13).
In order to ameliorate the fiddling necessary to
align the chunks, it is advisable to mark the
specimen with points that can be used to place
markers with high accuracy (Figure 12A). Three
markers are the minimum necessary, more are
advisable. However, the markers will be visible
in the texture of the finished model. If two
separate dense point clouds are generated and
later combined in a different CAD program,
there typically will be artifacts from a subperfect
fit of the two halves and a lot of manual
correction is needed.
1. Add each photograph series to a separate
chunk in one overall file.
2. Add markers to the scale bar in one or both
sets, create the scale bars in the program
and set the appropriate length. Note that the
markers used to create the scale bars should
have different names in each chunk, as they
will otherwise be included for a markerbased
alignment.
3. Mask in all chunks only those parts of the
background that move relative to the
specimen, or mask the full background if you
expect the specimen to deliver sufficient
features for alignment.
4. Align the images in all sets (use batch
process).
5. Place markers on the photographs on the
physical markers you put on the specimen.
Rename the markers so that the markers
corresponding to the same physical marker
have the same name in all chunks. Leave
one physical marker unused; it can later be
used to check the chunk alignment.
6. Align the chunks, marker-based.
Alternatively, align them point-based, if all
background in all photos has been masked.
7. Merge the chunks.
8. Add a marker to the previously unused
physical marker in two photographs, and
check its position in all other photographs. If
there are systematic divergences, one chunk
is not well aligned. Note that such
misalignment may be obscured on the
model!
9. Remove background points in the sparse
point cloud of the merged chunks.
10. Calculate the dense point cloud.
11. Remove any remaining background points
from the dense point cloud.
12. Calculate the polygon mesh.