Very large specimens (trackways,
excavations, mounted skeletons). The
calculation time for alignment, dense cloud
generation and mesh creation increases
exponentially with the number of photographs
used. If the vast majority of photographs
overlap with a significant portion of other
photographs nothing can be done to accelerate
the process. If, on the other hand, each
photograph overlaps only with a handful of
others, as is the case if a long sequence of
photographs documents a large area such as a
trackway, the set can be split into chunks that
each calculate quickly. If the alignment is
performed in chunks, these can later be
merged based on markers, as described above
for the multi-chunk method, which takes little
time. To ease this task it is useful to place
numbered markers before the photography.
Tracks, for example, may be photographed
with scale bars next to them and a piece of
paper carrying a number. Later, distinct points
on the scale bars can easily be used to create
in-program markers for chunk alignment.
Additionally, for large numbers of photographs
it is usually worthwhile to turn on generic pair
selection which filters out those photos that
likely overlap for alignment and discounts all
other photo pairs, thus reducing the calculation
time significantly.
For dense point cloud and polygon mesh
generation it is similarly advisable to select
only a section of the entire model, calculate the
dense cloud and mesh, save to a new file name
(e.g., the name of the entire file with ‘_part01’
appended), select the next part, calculate the
dense cloud and mesh and save with a new
name (‘_part02’), and so on (Figure 14).
Alternatively, in the program the box used for
selecting the volume for dense cloud
computation can be moved ahead by dragging
a corner point from the beginning of the model
past the points marking the end of the first,
already calculated dense cloud part, so that the
borders between dense clouds are an exact
touch without any gap or overlap. This process
results in manageable file sizes, but each
partial dense cloud is aligned with all others,
and most research tasks can be performed by
alternately using the individual files.
In order to scale large models at high accuracy
it is best to include markers at the far extremes
of the specimen and measure their physical
distance in the field with a measuring tape. For
aligning several versions of models based on
photograph series from different times, e.g. to
document the advance of an excavation and
the relative position of bones removed at
different times of the digging season, or even
across several seasons, it is advisable to place
several immobile and clearly visible markers
(e.g. chisels or poles cemented in drilled
holes).
If large complex specimens were photographed
in chunks, the multi-chunk method described
above can also be used for alignment. For
segmented specimens (e.g. skeletons) it is
advisable to calculate the dense point clouds
for the separate chunks after alignment, as
editing is much faster when done on smaller
overall point clouds. Instead of merging all
chunks (step 7 above), the chunks can be kept
separate and only the finished polygon mesh
models of the individual segments should be
combined into one file, usually in the separate
CAD software used for further processing.