The Use of 3D Acquisition Techniques of Surfaces for Forensic Image Databases

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The Use of 3D Acquisition Techniques of Surfaces
for Forensic Image Databases

• AAFS 2004
• Dallas
• Zeno Geradts zeno_at_holmes.nl

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Outline

• Introduction
• Methods
• Results with Toolmarks
• Results with Drug pills
• Conclusion and discussion

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Methods

• - Stereo Photogrammetry - Laser Scanning
  Triangulation - Moiré Fringe Contouring- Structu
  red light

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Triangulation
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Stereo Photogrammetry

• combining two (or more) images from cameras with
  a different viewpoint.
• similar in the way that we see depth with two
  eyes.
• An algorithm and computer can exploit this offset
  to perform the same operation. 

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Laser Scanning

• involves projecting a stripe of laser light onto
  the object of interest and viewing it from a
  camera.
• The stripe is then scanned across (or around) the
  scene to produce 3D data for the rest of the
  object.

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Moiré Fringe

• a grating is projected onto an object and an
  image is formed in the plane of some reference
  grating.
• The image then interferes with the reference
  grating to form Moiré fringe contour patterns
  which appear as dark and light stripes.
• Analysis of the patterns.

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Structured Light

• patterns of light (grids, stripes, elliptical
  patterns etc.) are projected onto an object.
• Surface shapes are then deduced from the
  distortions of the patterns that are produced on
  surface of the object.
• With knowledge of relevant camera and projector
  geometry, depth can be calculated by
  triangulation

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Phase shift method
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Phase-code combination method
graycode

• High resolution

phase shift
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Projection system

• Digital Mirror Device
• matrix met 600x800 mirrors
• 16 x16 ?m
• On/off in 20 ?sec

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Research project Toolmarks

• Continuation of Previous research in 1995 for
  side light
• User interactive signature selection
• Calculating the standard deviation of the
  difference after normalization steps

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Selecting a part and previewing

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Slope compensation of striation mark
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Slope in z-direction

Compensate by selecting the edges and normalize
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Experiment

• 6 screwdrivers
• Striation marks of 45 degrees to the surface
• Both gray values / 3D values

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Conclusion

• Higher correlation factors
• Less sensitive to the lighting of the surface

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Research project XTC tablets

• Use of 3D equipment in XTC-investigation
• Comparison stamp / tablet

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Tablets and stamps

• Toolmark investigation

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3D measurement
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Side view
50 0 -50 -100 -150 -200 -250 -300 -350 -400 -450
?m
418 ?m
112.9
0 1 2 3 4 5 6 mm
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translations
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occlusion

• Triangulation angle ? 30
• Not all angles are possible
• Combining images
• Use of special pacman holder

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Example occlusion
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Preventing occlusion
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Conclusions

• With the structured light technique, heights of 5
  micron can be measured.
• The matching of the different positions is
  possible with the Fourier Mellin algorithm
• Structured light can be useful for filling
  databases in a higher resolution setting and
  prevention of occlusion

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• Questions ??