Creating a Spatial Calibration (Advanced)
To create a new spatial calibration, you must either know the number of pixels per unit to specify for the image, or there must be a feature pictured in the image that has a known length value in terms of the units of interest (see Pixels per Unit for more information).
If your calibration must also correct for aspect ratio, you must either know the value l/h (length divided by height) for a perfect square as represented in the image, or there must be a feature pictured in the image that is known to be a perfect square (see Aspect Ratio for more information).
To create a spatial calibration:
On
the
Click the Calibration Options tool.
The Spatial Calibration dialog box appears in the panel on the right.
Click New Calibration on the toolbar at the top of the Spatial Calibration dialog box.
The dialog box controls are enabled. The default name “Spatial Cal <n>” is applied to the new calibration.
Type over the default name "Spatial Cal" with a name that is more meaningful.
In the Unit area, pull down the Name list box and select the units to which you want the pixel-level measurements converted.
For example, if the size of objects in the image is best expressed in terms of millimeters, select “Millimeter” from the list.
In the Abbr. text box, enter the text you want to use as the abbreviation for the selected units.
For example, a typical abbreviation for ‘Millimeters’ is “mm.” The specified abbreviation will appear on measurement reports.
In the Pixel/Unit group, do one of the following:
If you know the number of pixels per unit to specify:
In the X spin box, type the number of horizontal pixels it takes to represent one unit of length. For example, if you are converting to millimeters and 4 pixels equals one millimeter, type “4.”
Ignore the Y spin box.
If there is a feature pictured in the image that has a known length value:
Click the From Image button in the Pixel/Unit area.
A dialog box appears in the Image-Pro workspace, and a horizontal annotation line appears in the image.
Click and drag the line over the reference object, and then adjust the line’s control points so that the beginning and ending of the line correspond to the beginning and ending of the reference object.
In the Reference length (units) spin box, type the known length of the pictured object.
Click OK.
The Spatial Calibration dialog box is redisplayed and the calculated pixels/unit value appears in the X and Y spin boxes.
To correct for a skewed aspect ratio:
Click the Unlock tool at the bottom of the Spatial Calibration dialog.
Click Advanced Options to display the conversion and aspect ratio controls.
In the Apect Ratio area, do one of the following:
If you know the aspect ratio value to specify, or if there is no aspect ratio problem to correct for:
In the Aspect Ratio spin box, type the aspect ratio value to be used.
For example, if there is no aspect ratio problem to correct for, enter “1.”
Image-Pro automatically adjusts the value for the X spin box to match the entered Aspect Ratio value.
If there is a feature pictured in the image that represents a perfect square:
Click the From Image tool to the right of the Aspect Ratio spin box.
A dialog box appears in the Details panel, and a diagonal annotation line appears in the image.
Click and drag the line over the reference object, and then adjust the line’s control points so that the beginning of the line corresponds with the bottom-left corner of the reference object and the end of the line corresponds with the top-right corner of the reference object.
Click OK.
The Spatial Calibration dialog box is redisplayed and the calculated aspect ratio value appears in the Aspect Ratio spin box. Additionally, the X and Y spin box values are automatically adjusted to account for the aspect ratio.
Click on the Apply Calibration tool.
The spatial calibration is applied and saved for the session. To make it available across sessions, you must save it to a file (see Saving Calibrations).