Chapter 6
Calibrating Images
©
National Instruments Corporation
6-3
IMAQ Vision for LabWindows/CVI User Manual
Defining a Reference Coordinate System
To express measurements in real-world units, you need to define a
coordinate system in the image of the grid. Use the
CoordinateSystem
structure to define a coordinate system by its origin, angle, and axis
direction.
The origin, expressed in pixels, defines the center of your coordinate
system. The angle specifies the orientation of your coordinate system
with respect to the angle of the topmost row of dots in the grid image.
The calibration procedure automatically determines the direction of the
horizontal axis in the real world. The vertical axis direction can either be
indirect, as shown in Figure 6-2a, or direct, as shown in Figure 6-2b.
Figure 6-2.
Axis Direction in the Image Plane
If you do not specify a coordinate system, the calibration process defines a
default coordinate system. If you specify a grid for the calibration process,
the software defines the following default coordinate system, as shown in
Figure 6-3:
1.
The origin is placed at the center of the left, topmost dot in the
calibration grid.
2.
The angle is set to 0
°
. This aligns the x-axis with the first row of dots
in the grid, as shown in Figure 6-3b.
3.
The axis direction is set to indirect. This aligns the y-axis to the first
column of the dots in the grid, as shown in Figure 6-3b.
X
Y
X
Y
a.
b.
