A calibration point pair track is similar to an offset origin except that it defines two points with fixed assigned world coordinates. World coordinates may consist of an x-component, y-component or both. For a given coordinate system, these world coordinates uniquely determine the scale, origin, and/or angle at each step. When either of the calibration points is dragged in the main video view, the properties of the coordinate system are modified in order to maintain the assigned world coordinates. Calibration points are the easiest way to set coordinate system properties when two features with known world coordinates are visible in all video frames.

Note: Calibration points are very powerful. It is strongly recommended to "play" with some calibration points while both the axes and tape measure are visible to see how they work together to control the coordinate system.

1. Marking calibration points

After creating a calibration points track, click on a feature in the video image that has known world coordinates. Then click on a second feature in the same image. Marking a single frame is all that's required. The calibration points are initially assigned the current world coordinates of the clicked image features.

2. Axis options

The "Axes" spinner determines whether the calibration points control both x- and y-axes or only a single axis. When a single axis is selected (here "X only"), moving a calibration point along that axis changes the scale and translates the origin without changing the angle. Moving a calibration point perpendicular to the selected axis has no effect. This is very useful when one axis is unknown or unimportant--for example, with vertical motion studies or optical spectrum analysis using a line profile track.

3. Changing the world coordinates of a calibration point

The world coordinates of either calibration point are displayed on the toolbar when it is selected. A point that is "X Only" will display only the x-component.

Note: Changing the world coordinates changes the coordinate system scale, origin and/or angle so that the image positions of both points remain unchanged.

4. Moving a calibration point

Select and drag or nudge either calibration point to a desired location in the main video view.

Note: Moving a calibration point changes the coordinate system scale, origin and/or angle so that the world coordinates of both points and the image position of the unselected point remain unchanged.

5. Locking the calibration points

Locking the calibration points track prevents it from making any changes to the coordinate system. Lock the calibration points track by turning on the Locked property in its track menu.