Tracks

A track represents a video feature that evolves over time. All interactive elements in Tracker, including the axes and tape measure, are tracks.

The position or shape of the feature in a single video frame is known as a Step; thus, a track is a series of steps. Each step can be selected and manipulated with the mouse or keyboard. Some steps, like those for point mass tracks, have only a single moveable point, but others, like vector steps, have two end points plus a center handle point.

There are nine different types of user-defined tracks. For detailed information on a specific type, see its corresponding help topic: point mass, center of mass, vector, vector sum, offset origin, calibration point pair, line profile, rgb region, analytic particle model or dynamic particle model.

1. Track control

The track control lets you easily create new tracks and control most display properties of existing tracks. Click the Track Control button on the toolbar to show the track control.

The track control includes (from left to right):

- New button lets you create new tracks.
- Trails button shows and hides all trails.
- Labels button shows and hides all labels.
- Positions button shows and hides all point mass positions.
- Velocities button shows and hides all point mass velocity vectors.
- Accelerations button shows and hides all point mass acceleration vectors.
- Stretch button stretches all vectors.
- Dynamics button multiplies all motion vectors by mass.

2. Creating a track

Create a new track by selecting the desired track type from the New button menu on the track control or the Track|New menu on the menu bar. A newly created track is automatically selected for marking.

3. Selecting a track

Tracks can be marked or edited only when they are selected. The name, color and footprint of the currently selected track are displayed on the toolbar.

To select a track, click its button on the track control or select its name from the dropdown list on the toolbar. Double-click the background (video) or choose "no selection" to deselect the currently selected track.

4. Marking a track

Marking a track refers to the process of defining its position on each frame in the video clip using the crosshair cursor (shown above at twice actual size). To mark, hold down the shift key and click the mouse on the feature of interest as the video automatically steps through the video clip. Don't skip frames--if you do, velocities and accelerations cannot be determined.

Marking is done in the main video view. For more accurate marking, magnify the image up to 8x using the zoom tool or by right-clicking on the video and choosing the desired zoom level.

There is some variability in marking requirements. Vector and line profile tracks require dragging rather than clicking. Point mass and vector tracks expect every step to be marked, but offset origin, calibration point pair, line profile and rgb region tracks require marking only a single frame. Center of mass, vector sum and particle model tracks are marked automatically.

In rare circumstances you may wish to control the marking process in more detail. You can mark a track without holding down the shift key by checking the Mark By Default option in the tracks menu. And if you prefer that the video not automatically step forward while marking you can uncheck the Autostep option.

5. Duplicating or importing a track

Make a duplicate copy of an existing track by selecting the track and copying it to the clipboard using the Edit|Copy menu item, then pasting using the Edit|Paste item. You can paste into the same tab or a different one. When pasting into the same tab, you will be prompted to rename the track since tracks must have unique names.

Since the tape measure and axes are tracks, they can also be copied. When pasted, they replace the existing tape or axes in the tab. When no track is selected, the entire tracker panel (video clip, coordinate system and tracks) is copied.

Tracks can also be imported directly from saved tracker files into an open tab using the File|Import menu item. For more information see tracker files.

6. Track menus

Every track has a menu with items for setting its properties. Track menus can be accessed either from the Tracks menu on the menu bar, by right-clicking the main video view, or by clicking the track's button on the track control.

7. Customizing and documenting a track

Every track is identified by its name, color, footprint (visible shape) and description. Newly created tracks are assigned default values for the first three properties that depend on the type of track. For example, a point mass might initially be named ”mass A” and be drawn as a red diamond.

A track's name, footprint and color are displayed on the toolbar when the track is selected. To change the default values, select the track and enter a new name in the editable name field or click the footprint button and choose a new footprint or color. These properties can also be set from the track menu.

To enter a description for a track, open the description window by clicking the button at the right end of the toolbar. As you enter the description, the field turns yellow to indicate the changes have not been saved. To save the description, click the Save button.

8. Controlling visibility

Hide a track by turning off the Visible property in the track's menu. Or use the trails, labels, positions, velocities and accelerations buttons on the track control to toggle the visibility of these features on all tracks.

9. Selecting and identifying points

To select a point, move the mouse cursor over it in the main video view. The cursor will become a hand pointer and the point will be identified in the bottom right message box. Click to select it. Note: To select a point while marking, release the shift key.

When a point is selected, its track is selected, its editable properties (position, etc) are displayed on the toolbar, and it is identified with a square selection icon.

10. Editing a step

To edit a step, select and drag one of its points. A selected point can also be nudged one pixel at a time with the arrow keys on the keyboard. Holding down the shift key increases the nudge distance.

Many tracks also provide input fields on the toolbar for setting positions or other properties of the selected step.

11. Deleting

Delete a single step by selecting it and hitting the delete key on the keyboard. Delete an entire track by name or clear all tracks in the Edit menu.

A track can also be deleted by choosing the Delete item in its track menu.

12. Locking a track

Locking a track prevents any changes to its steps. Lock a track by turning on the Locked property in the track's menu.