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Small Vision System User Manual 39

4.1 Calibration Procedure

An automatic calibration procedure using five image pairs of a planar calibration target is included as

part of the smallv program. Given the image pairs of the calibration object, the system automatically

locates corner features in the target, fits a model of the target to the images, and finally produces an

estimate for the left and right camera intrinsics, the stereo head extrinsics, and the rectification matrices

P ,

H , and

H . These values are then used by smallv, and can also be saved as a parameter file

for later re-use. More information about the

calibration procedure can be found in the

Calibration Addendum to the User’s Manual.

4.1.1 Calibration procedure steps

1. Create the calibration object. Print out a

copy of the file data/check.pdf

(Figure 4-1) and paste it to a surface that

is as flat as possible. We use a wooden

cutting board as a backing surface at SRI.

2. Start the smallv application and start

capturing video. It is recommended that

you set the video resolution to at least

320x120 in order to get enough detail of

the calibration object. A calibration

computed when capturing video at a

higher resolution can be used for future

video captured at any resolution with the

same cameras.

3. Bring up the calibration window by

pressing the Calibrate… menu button.

Fig. 4-4 shows the calibration dialog

window (the figure shows the dialog after

an image has been captured and processed).

4. Determine the appropriate characteristics of the camera imagers and enter them into the four

boxes in the middle of the dialog. If you have one of the Videre Design stereo heads, check the

appropriate box and the parameters are loaded automatically.

5. Acquire five stereo pairs of the calibration object at different rotations and translations. Try to

avoid views that differ by a simple translation, as they are less informative than views with

variation in rotation. As shown in Fig 4-4, there is a tab control that shows only one pair at a

time; choose a tab to select another pair. To capture the current video feed into a stereo pair box,

simply press the capture button. You can also save and load images to and from disk using the

load and save buttons.

6. Detect the corner features in all views by pressing the features button in the lower bar of

buttons. This cycles through all the images, displaying the corners in green as they are detected.

If the feature finder fails on an image, please re-capture the image and redetect the features.

When redetecting features for a single stereo pair, use the features button in the stereo pair

box.

7. Compute the calibration parameters (intrinsics, extrinsics) and rectification matrices by pressing

the calibrate button in the lower button bar. This operates in three phases:

a) Calibrates individual views using a planar model of the calibration object. The projection of

these model features is shown in red.

b) Calibrates all the views jointly using nonlinear optimization over all the intrinsic and

extrinsic parameters. This phase usually takes a few minutes, and when finished, the

projected model features are shown in yellow.

Figure 4-1 Checkerboard calibration object.