User manual

Small Vision System User Manual 45
4 Calibration
N
OTE: There is a Calibration Addendum manual that details the exact steps necessary to perfor
m
calibration, and includes troubleshooting information. Please consult that manual for more detaile
d
information about the calibration procedure.
Most stereo camera setups differ from an ideal setup in which the cameras are perfect pinhole imagers
and are aligned precisely parallel. The divergence from ideal causes problems in the quality of the stereo
match since epipolar lines are not horizontal. In addition, if the camera calibration is unknown, one does
not know how to interpret the stereo disparities in terms of range to an object. Camera calibration
addresses these issues by creating a mathematical model of the camera.
SVS incorporates a simple automatic procedure for calibration, using a planar object that can be
printed on a standard printer. The calibration is preformed by fitting a model to a number of images taken
of a planar calibration object. The user presents the object to the stereo rig in five different (arbitrary)
poses. The calibration procedure finds model features in the images, and then calculates a best-fit
calibration for the rig. The procedure works for many different combinations of imagers, baselines, and
lenses, including wide-angle lenses with severe distortion.
When is it necessary to perform calibration? In general, whenever an action changes the camera
intrinsics (lens focal length and center axis) or extrinsics (the cameras move with respect to each other).
Here are some actions that would necessitate re-calibration:
• Changing lenses
• Screwing the lenses in or out of their mount
• Zooming, if the lenses are zoom lenses
• Changing the baseline of the cameras
• Any movement or rotation of one camera independent of the other, e.g., severe vibration or shock
can change the cameras’ relative position
A rigid mount that keeps the cameras stable with respect to each other is a necessity for a stereo rig. For
example, the MEGA-D uses an extruded aluminum frame to stabilize the cameras. There are some actions
that do
not require re-calibration:
• Changing the lens focus with a focusing ring on the lens
• Changing the lens aperture
The next section reviews the calibration procedure, detailing the steps required to generate a
calibration file.