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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.