The static metric calibrate tools require a calibration pattern to get the parameters that relates camera pixels with metric units. Several considerations are required during the manufacturing and acquisition of the calibration pattern to get accurate results.
This calibration locates the coordinate system aligned with the laser plane, so, x-coordinates are always zero.
Static calibration pattern and its coordinate axis
Using the static calibration for linear scanning applications
In case of linear scanning applications, instead of using the linear calibration, also the static calibration can be use. Although in general, the linear calibration is preferred for linear scanning, in some applications the static calibration is recommended. For example, in case where vibrations or other mechanic problems do not guarantee a constant motion of the pattern.
In that case, the user should introduce the encoder step (metric distance between acquisitions), so then, it is possible to directly transform a RangeMap to COP.
Config sal3d::Metric::calibrate (const Profile &profile, const PatternSPPoints &points, float xStep)
Advantages of static calibration
- No scanning is required.
- Possible mechanical vibrations do not affect the calibration
limitations of static calibration
- Laser plane MUST be aligned with the calibration pattern.
- In case of using the static calibration for linear scanning applications, the motion MUST be perpendicular to the laser plane and the calibration pattern.
Description of the Calibration Points
Points corresponding to the vertexes of the calibration pattern are used to describe its shape and to identify points on the RangeMap with their corresponding 3D metric points on the real object. The 3D coordinates of these metric points can be set by the user. In addition to the calibration points shown in the following figure, the base area of the pattern (the calibration pattern's lower flat area) is also used during the calibration procedure. Hence, the base of the pattern should also be captured in the Profile.
Description of the calibration points
The relation of the calibration points within range map to 3D pattern is not arbitrarily. The order depends on the laser stripe type and the camera orientation. The term laser stripe type refers to orientation of the laser stripe on the camera frame when this is projected over the calibration pattern. The camera orientation refers how the camera observes the pattern, in correct orientation or with the inverted orientation.
| Laser Stripe Orientation | Camera Orientation | 3D points |
|---|---|---|
| Vertical Laser Stripe | 
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| Vertical Laser Stripe | 
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| Horizontal Laser Stripe | 
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| Horizontal Laser Stripe | 
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Build Your own Calibration Pattern
In order to calibrate your acquisition system, you should use a calibration target according to your field-of-view requirements. However, you should follow the following advices:
- Do not change the shape of the object: parallel lines must remain parallel, object should remain symmetric, fixed relations between distances, etc.
- Base plane contains enough area in both sides (similar to the top plane).
- Be aware of the plane slopes (between points 1 and 2 and 3 and 4). If they are too high, it will be difficult to acquire.
The following figure presents an example of a good pattern acquisition. Analysing the acquired profile, user can observed that proportions between each segment are more or less one to one. Pattern is centered on the image and it covers all the profile. Acquisition noise is not important and occlusion area is not significant. When profile contains points not belonging to the calibration pattern, user can add a region of interest on the calibration to avoid any influence of that parts.
Pattern example: Profile of the calibration pattern
Following figures present some examples of bad designs. Left figure can produce inaccurate results, or in some case will not calibrate, due to acquisition noise, center figure has problems due to not enough points in the base plane. Right figure will not calibrate due to points not belonging to the calibration pattern are also acquired, in addition it is preferred that the pattern's profile covers all the image. This case can be solved by adding to the calibrate function the starting and ending pattern points:
Config sal3d::Metric::calibrate (const Profile &profile, const PatternSPPoints &points, int left, int right, float xStep=0)
Bad design examples: They can produce inaccurate results or return an error during the calibration.
Material Considerations
In addition to the dimensions, the material is also important for the calibration. As laser is projected over the pattern, specular surfaces are not recommended. In case of metal surfaces, a white anodized or sand blasting can be performed to minimize reflections.
Depending on the application, expansion must also be considered. In case of important temperatures variations, specially temperatures far from the standard (20ºC), low thermal expansion coefficient materials are preferred to decrease errors due to pattern expansion.
