From RMIT Visual Effects
Jump to: navigation, search

Nuke's 2D Tracker is a very powerful tool. It allows for an unlimited amount of points to be tracked, and to output the average of these tracks.

A common use for the 2D tracker is to pass on tracked information to the CornerPin node.

This is probably the most important tab. Here new trackers may be added, and the information required of them may be defined (translate, rotate and scale). The most commonly used info is translate. Trackers need to be first selected here before the track can be started. In the image below, the a few trackers have been selected prior tracking (only selected trackers will return a track). Also, all of them have been set to return only 'translate' information.
The tracker tab of the Tracker node
The Tracker can pass on its tracked info to a linked Transform node. However, the tracker can also apply the tracked motion itself using this tab. Unless you want do do this then leave it alone.
Here the sensitivity of the tracker can be adjusted. Its best left at default. If you tracks are not working, then review #Tracking tips.
Tracker toolbar
The Tracker is one of the few nodes that has a presence in the Viewer. Using the controls here, the tracks may be defined and edited.
The Tracker toolbar at the top of the Viewer

A simple use case

The following describes how the Tracker might be used in a simple scenario. The task is to track a simple camera pan and add an element to it, using basic translate information (i.s. left/right and up/down movement).

  1. Inspect your footage. Determine where the track should begin and end, and what features it will track.
  2. Add a Tracker node underneath your Read node. Make sure that its properties are visible. This will add a new tracker toolbar to the top of the Viewer.
  3. In the timeline, navigate to the first frame.
  4. In the Viewer, press the 'add tracks' button (this is in the far left of the tracker toolbar). By clicking on a feature, a new track will be added. Add as many tracks as you need.
  5. In the Properties of the Tracker, text the 'T' value of each tracker. This will specify that you wish to get the translate info from the trackers.
  6. Then shift select all the tracks (they will be highlighted in orange when they are selected).
  7. Press the 'track fwd range' button, in the top bar and input your beginning and end frame values. Wait for it to do its thing.
  8. Some tracks will have failed. These will show up as pale blue (no key frames) in the 'Tracker' tab, against the dark blue of the successfully key framed tracks. These may be deleted, by selecting and pressing the 'Delete Tracks' button.
  9. From the 'Export' drop-down menu, select 'Transform (match-move)'. Then press 'create'.
  10. This will create a Transform node, the animation of which has been derived from the tracked data. This can be placed under the foreground elements and merged over the footage.

With a bit of luck, your track will work. If not, then check out the tips below:

Tracking tips

  • Always check the footage before you start tracking. Look for what region is to be tracked, the frame range and any problems which might arise (e.g. a sudden camera movement).
  • The tracker can go wrong in one of two ways: it will either start giving you bad information (e.g. start tracking from the wrong place) or stop tracking altogether.
  • If it goes wrong, backtrack to where the trouble started and start tracking again\.
  • It is sometimes the case that the features aren not clear enough for the Tracker to 'see'. In this case, channels should be inspected one by one, and whichever has the clearest info should be duplicated into the other two channels. After that, a Grade node might be added to exaggerate further the features.
  • If the points that you are tracking go off-screen then you will need to Offset the tracking window by Command dragging on it.
  • Tracking on poor quality or low-rez footage is hell. Avoid.
  • If you are having trouble tracking, you might wish to change the size of the tracker box in the Viewer. The inner box defines the area to be tracked, the other box defines the region in which nuke will look for this region. The trick is to make these box suite their needs e.g. fast, panned motion will require a long horizontal box.

Linking output

Sometimes you may wish to attach a Roto to the output of a Tracker. One way to do this is through linking. The process is this:

  1. Make your Roto shape and place it where you need it to be in the scene.
  2. With the 'Transform' tab open, select the root shape in the roto list.
  3. Right click In the 'x' and 'y' 'translate' values and select 'tracker / tracker1 / translate' (tracker1 is the name of the tracker, yours might be named differently).
  4. Perhaps your shape might zoom off somewhere unexpected. If this happens, go to the 'Transform' tab of your tracker and set the 'reference frame' to whatever frame you are currently in.
The tracker box in the Viewer

External links

Start here to learn the basics

Another intro (longer)

How to track around occlusions (an occluded region is one that is hidden from view by something, eg tracking a man walking behind some bushes)

How to combine many tracks