National Instruments IMAQ Vision for LabWindows TM /CVI - Manual (Page 91)

Chapter 6 Calibrating Images
IMAQ Vision for LabWindows/CVI 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 the
GridDescriptor structure 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. Depending on the other
calibration options selected, this is an effective way to increase correction
speeds. Set the user-defined ROI using the roi parameter of either
imaqLearnCalibrationGrid() or
imaqLearnCalibrationPoints() .
Note The user-defined ROI represents the area in which you are interested. The learning
ROI is different from the calibration ROI 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 your 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.