Download HDevelop User's Guide - Image Understanding and Knowledge

Possible Error Source
Solution
Did any camera settings (like aperture, focus
or pose of the camera) change during the calibration process?
Take new calibration images and do not change
any settings during calibration and later during
the application. If you decide to change anything you have to start a new calibration.
Did you acquire the calibration images the way
they are required?
Check if you have acquired 10 to 20 images,
if the calibration plate has been positioned everywhere in the field of view and if it has
been tilted in every direction. If you are unsure about how to take good calibration images, read the instruction (page 235).
Are you using an extreme wide angle lens?
The distortions that appear close to the image borders cause a higher Mean Error or can
even be responsible for the failure of the calibration. You must use another lens in this case.
Is the size of your camera chip compatible with
the lens?
Using a lens that is not compatible with your
camera chip size (this information should be
included in the instructions of the lens) will decrease the quality of your image.
Table 7.1: Checklist for Calibration Errors.
2 Handling Quality Issues
Under Quality Issues you find an evaluation of each image, which includes descriptions of the defective image features and a quality percentage that tells you how severe the problem is. A result of
0% indicates a very defective image feature whereas 100% equals ideal quality. This can help you to
improve your calibration result by deleting images which are not good enough and might lead to a higher
error rate during the calibration process. If you need a certain quality level you can set a Warn Level
and the defects will be listed under Quality Issues. The quality issues are detected by image tests
and sequence tests. If you want the program to run faster or if you do not need quality feedback, you can
change Image Tests and Sequence Tests either to Quick, which performs less tests, or None, which
does not perform any tests at all. If the defects are too severe e.g. if the calibration marks or the even
the calibration plate are not found, the Calibration button will be grayed out, making it impossible to
calibrate unless all images of such poor quality are deleted from the list.
The test results referring to the calibration plate’s tilting may be ignored if later measurements are always
conducted in exactly the same plane. In this case, however, the values for the Focal Length and Z are
not correct each for itself but only in their combination. The reason for this is that neither of these values
can be determined for itself which leads to the result that if you get, for example, a Focal Length that
is double the value that it should be, Z will be half as high and vice versa. Besides, the further you place
239
Assistants
7.2.2 How to Calibrate with the Calibration Assistant