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 Virtual
Caliper Tools
Working
Virtual Calipers
- written in Java -
caliperV3.html - Very Large Download
(Both
require Netscape 4.08 to 4.77 as well as the CosmoPlayer
2.1 and latest Java JRE's - Available from our Plug-in's
page. Internet Explorer is not supported. The Mac platform
is not supported either.)
The
following is a description of the function of the measuring
tools referred to as "virtual calipers." Virtual calipers
allow for the extraction of 3D point (vertex) data from a
digitized 3D model within the confines of a 3D model viewer
supporting a scripting language. This vertex data can be collected
according to specified rules in order to produce number values
that translate into meaningful measurement data for archaeological
research. The inclusion of 3D digitized artifacts and digital
caliper tools in the Java2/Java3D archive-browsing applet
will allow researchers access to a multitude of previously
unavailable research data, whether due to artifact unavailability
or the difficulty in obtaining these meaningful measurements
from the physical artifact.
The
Vivid 700 outputs its vertex data to a ratio of 1 millimeter
per 3D unit of measure. This ratio is what the ATL refers
to as the "measurement ratio". Because of this, all virtual
caliper software authored by the ATL outputs its data in millimeters.
However, through the use of Polyworks Modeler, the measurement
ratio of any model could be easily scaled to any unit of measurement.
Since virtual calipers only deal with the number values received
from the model (through a scripting language), measurements
taken by a virtual caliper are based on the model's measurement
ratio.
Simple
virtual calipers, are measuring tools that only require the
3D location data of each vertex selected in order to produce
a meaningful measurement. These tools are easy to construct.
A simple cord or "straight-line" measurement can be obtained
through the selection of two vertices. The 3D coordinates
of each of these vertices can then be put through a simple
formula based on the Pythagorean theorem to produce a distance
value. The "Simple Straight-Line (or SSL)" virtual caliper
can be altered in a slight manner to produce a "Simple Surface
Contour (or SSC)" virtual caliper. With the SSC virtual caliper,
the researcher selects the vertex from where he/she wishes
the measurement to begin, after which the researcher selects
a successive number of vertices in the order that he/she wishes
the caliper's contour path to follow. The SSC virtual caliper
then uses the straight-line formula to calculate the straight-line
distance for the length between the first and second vertices,
the second and third vertices, and so on until it has calculated
the straight-line distance between each vertex, keeping track
of the total length as it calculates each individual length.
The SSC virtual caliper can also be used in conjunction with
a slicing plane to produce a "Simple Circumference Contour
(or SCC) digital caliper.
Complex
virtual calipers, are measuring tools that not only require
the 3D location data of each vertex selected, but scripting
access to the 3D model's vertex and polygon arrays. In VRML
2.0 these arrays are respectively known as the "point" field
of a Coordinate node and the "coordIndex" field of an "IndexFaceSet"
node (http://www.web3d.org/).
These tools require a good understanding of triangular and
polygonal geometry to construct, and more computer power to
implement. However, complex virtual calipers offer the researcher
more user-friendly tools, as well as more accurate results.
Where a SSC or SCC virtual caliper may require anywhere from
5 to over 100 vertices selected by the researcher, "Complex
Surface Contour (or CSC)" and "Complex Circumference Contour
(or CCC)" virtual calipers only ever require the researcher
to select three vertices in order to obtain a meaningful measurement.
This is because any three vertices create a plane, and this
plane can be used to judge the 3D locations of all vertices
in a vertex array relative to this plane. Thus, complex virtual
calipers can traverse the vertex and polygon arrays to find
which polygons are bisected by the researcher-created plane,
and then the caliper can calculate the appropriate distances
using formulas based on the Pythagorean theorem. This functionality
can also be extended to a "Complex Surface Area" virtual caliper,
so that an area measurement value can be easily returned for
a selected surface area of a 3D artifact for the 3D Archive
user.
A
researcher-created plane may also be used to take automatic
measurements. The first two vertices selected create a cord
that represents a new z-axis for the 3D digitized artifact.
The model is thus re-aligned so that the entire model is rotated
and reposition to align the new z-axis of the 3D digitized
artifact to the z-axis of the 3D coordinate system, at the
same time moving the 3D digitized artifact so that the halfway
point of the new z-axis is located on the origin of the 3D
coordinate system. Once this is done the first vertex selected
will be located on the positive side of the origin along the
z-axis and the second vertex selected will be on the negative
side of the origin along the z-axis. Next a third vertex is
selected in order to rotate the 3D digitized artifact around
the z-axis so that the model is properly oriented in 3D space.
Where this third vertex is located depends on the type of
artifact to be measured. Once oriented properly, a series
of artifact-type-specific scripts can be run to automatically
calculate measurements.
The
method of delivering 3D digitized artifacts via the Internet
has raised some concerns about the usefulness virtual calipers.
3D digitized models created with laser digitizers, though
accurate, are quite large in both file size and polygon count.
Such 3D models are viewable through an Internet2 connection
on computers that have 3D accelerated video cards. However,
many researchers do not have the equipment or Internet connection
to handle such large models. In an attempt to solve this problem,
the ATL also delivers compressed 3D digitized artifact models
via regular Internet that are relatively suitable for computers
that do not use 3D accelerator video cards. However, compression
also involves loss of accuracy.
In
a recent test conducted by the ATL using an endocast provided
by Dr. Ralph Holloway of Columbia University, a tolerance
of 300 microns was used to compress the digitized La Chapelle
endocast model in IMCompress. The original digitized La Chapelle
model has a file size of 18.2MB with 683,470 polygons. The
compressed version of the same model (using a tolerance of
300 microns) has a file size of 395KB with 14,122 polygons.
Volume measurements were taken using IMEdit for each of these
models. The original digitized La Chapelle measured 1578.72
cc. The compressed digitized La Chapelle measured 1577.39
cc. From this we can see that use of IMCompress has a negligible
effect on measurements taken from compressed digitized models.
*According
to Jim Foley at (http://www.talkorigins.org/faqs/homs/specimen.html),
the official volume measurement for La Chapelle is 1620 cc.
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