Metric Calibration Tool

AQSENSE's Metric Calibration Tool permits the reconstruction of the COP with metric units (cm, mm, etc.) removing the perspective distortion introduced by the scene projection onto the image sensor. In laser triangulation systems, there are several ways to reconstruct a scene: linear scanning, using rotating tables, using robot arms, etc.

Metric input data

The Metric tool accepts range maps obtained from several laser triangulation layouts:

The SAL3D Metric Calibration is also working for special configurations, when having more than one camera-laser system. Please, take a look to the section Special Metric Layouts.

This calibration tool is adjustable for working in different scenarios, one specific for linear scanning (Linear Metric Calibration) and a second one (Static Metric Calibration).

In case of using the Linear Metric Calibration, the linear displacement is characterized by a pulse (e.g. encoder), therefore it is possible to accumulate all consecutive profiles and to obtain the COP from a single RangeMap.

By using the the Static Metric Calibration, the 3D coordinates are obtained from a single profile. Then, The information of consecutive profiles must be merged to get a full COP. For example, in case of using a rotating table, each profile should be rotated in consonance with the rotating axis.

What you must know about the metric calibration

The effects of Perspective Distortion

For a human observer, closer objects seem to be bigger than the same objects in a farther position (related to the observer).

The observer has the impression that a long pipe, as looked at from one of its extremes, seems to be decreasing in diameter as the observer looks farther along the pipe.

This is known as Perspective Effect (see following figure) or in terms of 3D acquisition, Perspective Distortion.

An example of Perspective Distortion
The following pictures show an example of the reconstruction of a tilted cylinder using proportional reconstruction and using SAL3D Metric reconstruction. The proportional reconstruction assumes a constant ratio between pixels and metric units, but, due to perspective distortion, this assumption is not true.

Proportional cylinder reconstruction. a) Range Map; b) COP; c) ZMap
SAL3D Metric cylinder reconstruction. a) Range Map; b) COP; c) ZMap

Observing the cloud of points and also the ZMap obtained in both situations, it is clear that the proportional reconstruction distortes the object, while the SAL3D Metric reconstruction preserves the original shape.

Calibration of the acquisition system

The Metric Calibration Tool requires the scanning of a calibration pattern to determine the parameters which relate Profiles or RangeMaps coordinates with metric 3D coordinates. Two kind of calibrations are supported: linear and static. For angular calibrations please refer to the Angular Metric Calibration Tool

Linear Metric Calibration

This approach involves the acquisition of the RangeMap of a specially designed object (see Linear calibration pattern). The procedure return a Metric Configuration (MetricConfig) suitable for mapping RangeMap coordinates to 3D metric coordinates.

All the functionality is provided by a single function of the sal3d::Metric namespace, overloaded for the multiple input parameters:

Static Metric Calibration

This approach only requires a single profile to determine the relation between camera pixels and metric units. This profile should correspond to the projection of the laser plane over one of the static calibration patterns.

Choosing the kind of static pattern

There are some different reasons to choose what static pattern to use. Several factors play in the game: accuracy, cost, weight, material, region of interest, etc.

The Static calibration pattern in general is more adecuate when working with FOV with similar height and width.

In case where the FOV width is quite big compare to the FOV height, we recommend the use of the Zigzag calibration pattern. This pattern can be also used to calibrate big FOV covering it with multiple profiles of this pattern.

In addition, the Zigzag calibration pattern should be easier to manufacturer with higher accuracy compared to other patterns.

In case where the FOV height is bigger than the FOV width, or in case of big FOV, we recommend the use of Chessboard Metric Calibration.

Metric Calibration using Chessboard grid

This approach only requires a single 2D frame of a chessboard pattern aligned with the laser plane. More information in Section Chessboard Metric Calibration.

The Metric Calibration Tool does not correct Lens Distortion unless it is combined with the Lens Distortion Tool.
Metric calibration tool considers the camera region of interest, therefore, the acquisition system must be recalibrated in case of modifying the camera ROI.