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:

• a matched pair of high sensitivity CCD cameras,
  
• a laser scanner all mounted on an optical bench to reduce vibration.

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

• determing the 3D position of them relative to some world coordinates
  
• focal length and lens distortion of the camera (+ lens etc.).
  
• Camera Calibration is described in my book.

Depth maps extracted from the scene by :

• Moving the laser stripe across the scene to obtain a series of vertical columns of pixels
  
• Triangulate Pixels to give the required dense depth map. The depth of a point is measured as the distance from one of the cameras, chosen as the master camera.
  
• Knowing the relevant geometry and optical properties of the cameras the depth map is constructed using the following method:

 

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 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 and 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 , proceed to Step 6.
   
6. Find the point on line l which corresponds to . Calculate the (x,y,z) coordinates of , and store the z value at position (i,j) corresponding to x and y in the depth map.

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

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

 

Fig. 10 Depth Map/Image Overlay