User manual

Small Vision System User Manual 36
Disparity:
5 4 3 2 1 0 -1 -2 -3
Figure 3-5. Planes of constant disparity for verged stereo cameras. A search range
of 5 pixels can cover different horopters, depending on how the search is offset
between the cameras.
The location and size of the horopter depends on the application. If an object falls outside the horopter,
then its true disparity will not be found, and instead it will get some random distribution of disparities.
Figure 3-4 shows what happens when the object's range falls outside the horopter. In the left image, the
disparity search window is correctly positioned so that objects from 1 meter to infinity are in view. In the
right image, the window has been moved back so that objects have higher disparities. However, close
objects are now outside of the horopter, and their disparity image has been "broken up" into a random
pattern. This is typical of the disparity images produced by objects outside the horopter.
For a given application, the horopter must be large enough to encompass the ranges of objects in the
application. In most cases, this will mean positioning the upper end of the horopter at infinity, and making
the search window large enough to see the closest objects.
The horopter is influenced not only by the search window and offset, but also by the camera
parameters and the baseline. The horopter can be made larger by some combination of the following:
• Decreasing the baseline.
• Decreasing the focal length (wider angle lenses).
• Increasing pixel width.
• Increasing the disparity search window size.
As the cameras are moved together, their viewpoints come closer, and image differences like disparity
are lessened. Decreasing the focal length changes the image geometry so that perceived sizes are smaller,
and has a similar effect. It also makes the field of view larger, which can be beneficial in many
applications. However, very small focal length lenses often have significant distortion that must be
corrected (see the section on calibration). Another way to change the image geometry is to make the pixels
wider. This can be done by scaling the image, e.g., from 320x240 to 160x120, which doubles the pixel
size. Note that it is only necessary to change the pixel width. Most framegrabbers have hardware scaling to
arbitrary resolutions.
Figure 3-4. Disparity image for all regions withing the horopter (left) and
some regions outside the horopter (right).