Ballistic Imaging (2008) / Chapter Skim
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7 Three-Dimensional Measurement and Ballistic Imaging
7 Three-Dimensional Measurement and Ballistic Imaging
Pages 186-196
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From page 186... ...
The basic objective of any ballistic image database is to collect some accurate representation of cartridge cases and bullets, derived so that entries can be compared and scored for similarity with others. The presence of an electronically coded representation of the physical objects obviates the need for direct access to the physical objects for comparison (though they would have to be directly examined for final confirmation)
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From page 187... ...
using a prototype three-dimensional sensor on cartridge cases. This chapter provides basic background for that discussion, beginning with a discussion of the conceptual differences between two-dimensional and three-dimensional image acquisition technologies (Section 7–A)
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From page 188... ...
in a standard imaging device: • position and strength of the light source; • physical extent of the light source (assuming it is not a point source) ; • use of multiple light sources; • geometry relating light source positions to the surface material of the object being sensed; • geometry of the object itself (see below)
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From page 189... ...
This is especially true if there can be self-occlusion, that is, that some light rays may not reach part of the object because they are reflected by other parts of the object -- casting self-shadows. Given all of these factors, one can see that there is a fundamental issue in comparing an image of a probe object against an image of a target object -- one needs to ensure that the comparison of intensities in an image, or of some other feature extracted from the intensities, is actually reflecting the shape of the underlying object, and not some other factor.
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From page 190... ...
A contact probe, such as a stylus, can directly measure the three-dimensional position of a point on a surface relative to a fixed coordinate frame. Repeated passes of such a contact probe can be used to more fully reconstruct a three-dimensional shape, and these direct surface measurements can be directly compared against other exemplars.
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From page 191... ...
7–B Past Efforts in Three-Dimensional Imaging of BallisticS Evidence Seeking a way to reinforce firearms identification with a quantifiable and objective basis, Davis (1958) developed an instrument he called a "striagraph." The striagraph recorded measurements from a stylus riding around the circumference of a recovered bullet as the bullet was rotated, providing three-dimensional surface measurement of the path around the bullet.
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From page 192... ...
. To compare bullets, they constructed a correlation matrix consisting of the correlations between feature vectors for each of the land impressions (the bullets they studied had six land engraved areas)
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From page 193... ...
A middle range of barrels and manufacturers worked best for toolmark deposition. At one extreme were relatively cheap firearms barrels whose less precise manufacturing standards added randomness to the observed markings and precluded easy matching; at the other were extremely high-end barrels that were so finely polished and machined as to render toolmarks too subtle to readily distinguish.
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From page 194... ...
entry that requires operators to specify and image the separate land engraved areas. Graphically, this image data can be rendered onscreen in layers and, notably, as a planar surface that can be rotated and lit (altering both direction and type of simulated lighting)
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From page 195... ...
touted BulletTRAX-3D (and its companion MatchPoint Plus display stations) as "the latest configuration of IBIS" -- suggesting a replacement of IBIS -- the system was originally positioned as a counterpart to IBIS.
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