Chapter 6
Calibrating Images
IMAQ Vision for LabWindows/CVI User Manual
6-2
ni.com
Refer to Chapter 5,
Performing Machine Vision Tasks
, for more
information about applying calibration information before making
measurements.
Defining a Calibration Template
You can define a calibration template by supplying an image of a grid or
providing a list of pixel coordinates and their corresponding real-world
coordinates. This section discusses the grid method in detail.
A calibration template is a user-defined grid of circular dots. As shown in
Figure 6-1, the grid has constant spacings in the x and y directions. You can
use any grid, but follow these guidelines for best results:
•
The displacement in the x and y directions should be equal (
dx
=
dy
).
•
The dots should cover the entire desired working area.
•
The radius of the dots in the acquired image should be 6–10 pixels.
•
The center-to-center distance between dots in the acquired image
should range from 18 to 32 pixels, as shown in Figure 6-1.
•
The minimum distance between the edges of the dots in the acquired
image should be 6 pixels, as shown in Figure 6-1.
Figure 6-1.
Defining a Calibration Grid
Note
Click
Start»Programs»National Instruments»Vision»Documentation»
Calibration Grid
to use the calibration grid installed with IMAQ Vision. The dots have
radii of 2 mm and center-to-center distances of 1 cm. Depending on your printer, these
measurements may change by a fraction of a millimeter. You can purchase highly accurate
calibration grids from optics suppliers, such as Edmund Industrial Optics.
1
Center-to-Center Distance
2
Center of Grid Dots
3
Distance Between Dot Edges
dy
dx
1
3
2
