Coordinate System

When you mark a point in Tracker's main video view, you are defining its image position. Image positions are measured in pixel units relative to the top left corner of the video image. In a 320 x 240 pixel image the upper left corner is at image position (0.0, 0.0) and the lower right is at (320.0, 240.0).

Since a video image is a camera view of the real world, a physical object within that image also has a world coordinates. World positions are measured in scaled world units (e.g., meters) relative to a specified reference frame. The origin of the reference frame may be anywhere on or off the image.

The coordinate system is a set of transformations used to convert image positions into world coordinates. The coordinate system defines for each frame of the video:

- scale (image units per world unit)
- origin (image position of the reference frame origin)
- angle (counterclockwise angle from the image x-axis to the world x-axis.

1. Setting Coordinate System Properties

By default, the scale, origin and angle of the coordinate system are fixed--that is, they do not vary from frame to frame. However, any or all of these may be allowed to vary by turning off the appropriate Fixed property in the Coords menu on the menu bar. When a property varies, it must be set for every step in the video clip; when it is fixed, setting it once is sufficient.

The positions and properties of the origin that you set are those of the default reference frame. The Coords/Reference Frame menu enables you to select other reference frames in which the origin moves along with a point mass or center of mass track. Center of mass reference frames are particularly useful when studying collisions.

2. Setting the Scale (Calibrating the Image)

Set the scale using either the tape measure or calibration points.

3. Setting the Origin

Set the position of the origin using the axes, an offset origin or calibration points.

4. Setting the Angle

Set the angle of the x-axis using the axes or calibration points.

5. Locking the Coordinate System

Locking the coordinate system prevents any changes to the scale, origin and angle. Lock it by turning on the Locked property in the Coords menu.