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, 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 identification and marking.
3. 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 document (tab) or a different one. When pasting into the same document, you will be prompted to rename it 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 document. 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 data files into an open document using the File|Import menu item. For more information see XML Data Files.
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 called ”mass A” and be drawn as a blue 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.
5. Selecting a Track
Select a track by clicking its button on the track control or by selecting its name from the dropdown list on the toolbar. Double-click the background (video) or choose "no selection" to deselect the currently selected track.
6. Track Menus
Every track has a menu with items for setting its properties. Track menus can be accessed either from the Track menu on the menu bar, by right-clicking the main video view, or by clicking the track's button on the track control.
7. Marking a Track
Marking a track refers to the process of identifying 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 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 not just clicking but also dragging when marking. Point mass and Vector tracks expect every step to be marked, but Offset Origin, Calibration Point pair and Line Profile 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 track menu. And if you prefer that the video not automatically step forward while marking you can uncheck the Autostep option.
8. Controlling Visibility
Hide a single track by turning off the Visible property in the track's menu. The track control lets you toggle the visibility of all trails, labels, positions, and velocity and acceleration vectors.
9. Selecting a Point
To select a point, start by moving the mouse cursor over it in the main video view. When the cursor becomes a hand pointer, click it. The selected point shape becomes a small square.
Note: To select a point while marking, release the shift key.
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.
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.
