Post Production Techniques – Green Screen and Tracking
Thursday 10th February 2011
Green Screen – Tracking with After Effects CS5
In reference to the previous Blog on green screen video, Fridays Tutorial included a run through of Tracking using After Effects. Basically this allows you to track the movements of an object or part of an object and then associate this movement to another object. Let me expand upon that statement for example I’ve tracked the movement of the subjects left hand in the video below and I’ve attached the movement of the hand to the image of the football.
Tracking can be found under the Animation menu heading. Selecting Track Motion brings up the Tracker TAB with all the options associated with Tracking. So to get started a motion source is selected from the drop down menu. The Track type in this case is Transform and the box for position has been ticked. Next select a Target from the edit target (this can be done later) and we are ready to analyse.
Now position the Target Symbol in the Viewer that’s the two boxes with the cross inside and position this onto the object you want to track which in this case is the hand. Click the universal symbol for play and this should automatically position the boxes onto the hand and track the hand as it moves around the screen. Unfortunately in my experience it doesn’t and quickly loses it’s position.
Stop the automatic analysis and then using the move forward or back single frame buttons re-position the boxes over the hand and so editing the tracking path. I had to do this manually for the whole sequence because the subjects hand flipped several times during the movement changing the light intensity and so the tracking could not follow it automatically.
With the Tracking analysis done I imported an image of a football which I set to enter from off screen and bounce towards the subject to the point where I had set my tracking path to start. I then went back to the tracking menus and edited the target to be the Football.
When I run the completed composition the ball enters from the left connects with the tracking path and then follows the hand movements and then finally exits the screen to the right at the end.