National Instruments IMAQ Vision for Measurement Studio - Manual (Page 81)

Chapter 6 Calibration
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 region of interest (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. Set the
user-defined ROI using the ROI parameter of either
imaqLearnCalibrationGrid() or
imaqLearnCalibrationPoints() .
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
Choose the perspective projection algorithm when your system exhibits
perspective errors only. A perspective projection calibration has an accurate
transformation even in areas not covered by the calibration grid, as shown
in Figure 6-6. Set the
mode element of the options parameter to
a. c. b.