Turntable method: using physical markers
on specimens
If at least three markers are placed on the
specimen (see Figure 4) so that each is visible
in two photos, the markers can be used to align
models calculated from each of the two (or
more) separate sets of photographs required to
cover the whole surface (required for the multichunk
method). It is generally a good idea to
place markers even if the use of the one-chunk
method is intended, as they greatly ease the
task of alignment of parts if the one-chunk
method fails.
Because all markers placed on the specimen will
be visible in the final model, markers should be
as small as possible. Tiny pieces of white selfadhesive
labels marked with the letter X (use
the crossing point of the two lines for the inprogram
marker) and a number (for easier
identification on the photos) work well, if care is
taken that the markers do not fall off when the
specimen is flipped upside down. Make sure to
use only materials from which no chemicals can
leach into the specimen, and which do not leave
any residue when removed.
Markers must be well visible in all positions that
the specimen will be placed in during
photography. It is advisable to turn the
specimen over after placing them, to check if
they really are visible in the alternate positions.
It is possible to produce a marker-free model
with added effort, by taking additional
photographs with the markers removed, and
masking the markers in all photographs. Care
must be taken to take a sufficient number of
images, so that each spot that was covered by a
marker is well represented in sufficient detail in
at least two additional photographs. The markerfree
images then provide the surface information
for those parts of the surface that is under the
marker. However, because this approach
involves talking near-duplicate photographs, the
calculation time for the model will rise
significantly.