Behind the scenes of Episode 2 part 2

Carrying on from yesterdays post.

So the problem we now have is that the part that we’ve painted over only fits on one frame.  Once you move to another frame it gets left behind.

This is where a technique comes in called tracking. Here’s an image of what tracking in a software package called After Effects looks like. In each of the squares are smaller squares. Within these squares are points.  Basically what you have to do is find a part of the image that has good contrast to it’s surrounding. So for example a hook on a white door would be a good item to track.  You then place these tracking boxes over the high contrast item or spot. The boxes around the inner point tell the software how far to look for the particular contrasty object as it moves in each frame.

tracking points

tracking points

In the example above I’ve added four tracking points. Each of these points knows where the other tracking points are in relation to itself. This means that After effects can work out not only where about an object is in 2D space but can also tell if it’s coming closer or further away and at what angle, although this shouldn’t be confused with 3D trackers, see below.

There are many types of tracking software. Some track in 2D space, like the above example.  These can use a pixel to track or some packages use whole planes to track, so for example the side of a car or some furniture.

Others can track in 3D space. This means they use special algorithms that solve where the camera would of been in space. This allows us to create a virtual camera that mimics exactly what the real camera would have done on set, which then gives us the ability to add 3D objects to our scene, for example a giant robot or dinosaur.

However for this task we’re dealing with 2D objects (things with no depth or no perceived depth) so we don’t need to use a 3D tracker.

By feeding the tracking information we have to the part of the image we painted over the top we can have it move exactly the same way as the background footage as seen below.


Also note the slight join where I’ve placed the red arrow.  This is because although the tracking was good the perspective had changed slightly and so the original image that I’d used in the wider part of the shot now didn’t quite match despite being scaled and rotated correctly according to the track points.

This is where I should of used tracking that takes perspective into account.  However I decided that it was so subtle that hopefully no one would notice…apart from people that have read this.

The next issue was to deal with the electrical plug sockets and various other modern looking items on the left hand wall.

Although the core techniques for this were pretty much the same, it did introduce a whole other collection of problems.  One of which is the fact that the boy runs across where we need to paint out our objects.  We also have a rather annoying problem that his shadow also passes this area.

So tomorrow I’ll complete this mini making of  with techniques I had to employ to solve this.

See you then.


One thought on “Behind the scenes of Episode 2 part 2

  1. Pingback: It’s all in the eyes…the zombie eyes | 22 Days Later!

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