National Instruments IMAQTM - Manual (Page 91)

Chapter 6 Calibrating Images
IMAQ Vision for Visual Basic User Manual 6-6 ni.com
Specifying Scaling Factors
Scaling factors are the real-world distances between the dots
in the calibration grid in the x and y directions and the units in
which the distances are measured. Use
CWIMAQCalibrationGridOptions.GridDescriptor to
specify the scaling factors.
Choosing a Region of Interest
Define a learning ROI during the learning process to define a region of the
calibration grid you want to learn. The software ignores dot centers outside
this region when it estimates the transformation. Creating a user-defined
ROI is an effective way to increase correction speeds depending on the
other calibration options selected. Pass a CWIMAQRegions collection
representing the region you want to learn to the
Regions parameter of
CWIMAQVision.LearnCalibrationGrid or
CWIMAQVision.LearnCalibrationPoints .
Note The user-defined ROI represents the area in which you are interested. The learning
ROI is separate from the calibration ROI that is generated by the calibration algorithm.
Refer to Figure 6-6 for an illustration of calibration ROIs.
Choosing a Learning Algorithm
Select a method in which to learn the calibration information: perspective
projection or nonlinear. Figure 6-5 illustrates the types of errors the image
can exhibit. Figure 6-5a shows an image of a calibration grid with no
errors. Figure 6-5b shows an image of a calibration grid with perspective
projection. Figure 6-5c shows an image of a calibration grid with nonlinear
distortion.
Figure 6-5. Types of Image Distortion
a. c. b.