Tape Measure

The tape measure is used to measure distances and angles, to calibrate the video scale, and to correct for tilt in the video image. To display the tape, clicking the tape measure button on the toolbar.

Since a tape measure is a track, it has a menu that is accessible in the Tracks menu on the menu bar or by right-clicking the main video view.

1. Measuring distances and angles

You can use the tape to measure both the distance between any two points (tape length) and the angle relative to the +x-axis of a line connecting the points (tape angle) by dragging the ends of the tape. The length is displayed on the tape readout and both the length and angle are displayed on the toolbar.

To move the tape without changing its length or angle, drag the middle.

2. Measuring angular arcs

You can use the tape to measure angular arcs using a jackknife-like arm. To select the arm, move the cursor along the tape to a point about midway between the center and the end until a small circle is displayed. Click and drag the circle to open the arm. The tape readout and the angle field on the toolbar will display the angular arc between the tape and arm. Set the arc to a specific angle by clicking either the angle field on the toolbar or the tape readout itself and entering the desired angle in degrees.

To close the arm, drag the circle toward the hinge end of the tape until it disappears.

3. Setting the scale (calibrating the video)

To calibrate a video image, drag the ends of the tape to positions that are a known world distance apart (for example, the ends of a meter stick or other object with known dimensions in the video image). Then click either the length field on the toolbar or the tape readout itself and enter the known distance. For example, in the figure below the scale is set in cm units using a white meter stick with black stripes every 10 cm.

4. Setting the angle (correcting tilt in the video image)

The tape angle can be used to correct for camera tilt when shooting a video. Drag the ends of the tape so that it is parallel to a video feature that is horizontal or at some other known angle. Then enter the known angle into the tape angle field on the toolbar. This will rotate the coordinate system so the +x-axis is correctly oriented to horizontal in the video image. Note that it does NOT rotate the tape itself.

5. Unfixing the tape

By default, the tape measure is fixed--that is, its end positions do not vary from frame to frame. Uncheck the Fixed checkbox in its track menu to allow the end positions to vary.

Note that fixing the tape measure is not the same as fixing the scale of the coordinate system!

6. Locking the tape measure

Locking the tape measure prevents it from making any changes to the scale. Lock the tape by turning on the Locked property in its track menu.