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Our Active Stereo Vision System

This Section describes the active stereoscopic subsystem which provides the three-dimensional data to our system for automatically inspecting mechanical parts.

NOTE: Whilst this Section considers some specific active stereo problems, many of the other issues discussed are not specific to any particular three-dimensional data acquisition technique, and will be of general interest.

The main components of the Vision System are illustrated by the schematic diagram in Fig. 8.


Fig. 8 Schematic diagram of vision system

The vision system consists of:

Initially the cameras of the system must be calibrated in order to

Depth maps extracted from the scene by :


Fig. 9 Measuring a depth value

  1. For each vertical stripe of laser light form an image of the stripe in the pair of frames from each camera.
  2. For each row in the master camera image, search until the stripe is found at point P(i,j), say.
  3. Form a three-dimensional line l passing through the centre tex2html_wrap_inline3054 of the master camera and P(i,j).
  4. Construct the epipolar line which is the projection of the line l into the image formed by the other camera. Do this by projecting two arbitrary points tex2html_wrap_inline3060 and tex2html_wrap_inline3062 into the image and constructing a line between the two projected points.
  5. Search along the epipolar line for the laser stripe. If it is found at tex2html_wrap_inline3064, proceed to Step 6.
  6. Find the point tex2html_wrap_inline3066 on line l which corresponds to tex2html_wrap_inline3064. Calculate the (x,y,z) coordinates of tex2html_wrap_inline3066, and store the z value at position (i,j) corresponding to x and y in the depth map.

The position of the point tex2html_wrap_inline3066 is easily found by projecting a line tex2html_wrap_inline3086 from the centre tex2html_wrap_inline3088 of the secondary camera passing through Q. The intersection of the lines l and tex2html_wrap_inline3086 gives the coordinates of tex2html_wrap_inline3066.

The depth map is formed by using a world coordinate system fixed on the master camera with its origin at tex2html_wrap_inline3054.


Fig. 10 Depth Map/Image Overlay

next up previous
Next: Image processing Up: 3D imaging Previous: Methods of Acquisition

tex2html_wrap_inline2984 David Marshall 1994-1997