This is the ninth of a semi-regular series of columns I’ll be presenting on how to do some basic special effects, for people who have little to no experience. We use these effects quite frequently in our Harry Potter-based web series The Spell Tutor.
A few times during shooting of The Spell Tutor, I needed to do motion tracking during post. I used a plugin from Boris FX for Vegas Pro, but many different packages exist. In this column, I’ll be talking less about how to use such a plugin (just follow their instructions) and more about how to prepare your shoot so that the footage will end up being workable.
What Is Motion Tracking?
When superimposing a CGI object onto your footage, you want to make sure its placement synchronizes with the camera movement. Motion tracking is just the computer doing all the locational keyframing work that you would otherwise have to do by hand. It uses picture recognition algorithms to accomplish this and when it works correctly, it gives excellent results.
When Do I Use Motion Tracking?
Ideally, never, especially if you’re working with an indie budget. Motion tracking is finicky even in the best of conditions. If you can find another way to accomplish what you want to do, then do that instead.
- If you want to have a handheld-style shot that would require motion tracking, try shooting the scene with a tripod-mounted camera, do insert VFX on the static footage and then add “handheld” motion to the frame with a plugin during post. It’ll be much easier and cause less of a headache.
- If you’re thinking of adding VFX to an actor’s eyes using motion tracking, don’t - I speak from experience here. As soon as the actor blinks, the motion tracking software will lose track of their eyes. Use FX contact lenses instead.
That being said, there are times when motion tracking is unavoidable, typically when they involve humans, who are incapable of holding perfectly still. In these cases, it’s a good idea to keep it simple by following a few basic rules:
- Keep the motion slow. Fast motion will blur in the footage, which throws off motion tracking software.
- Keep the motion small. A large range of motion will make it more difficult to get realistic-looking results.
- Put something in the shot for motion tracking to follow. You can use either a special texture (more on that below) or something simple like a brightly coloured dot.
In our episode “The Mirror of Mnemosyne”, the girls look into a magic mirror to see the past. Motion tracking was necessary because of the slight movement of “Emma’s” hands. Here’s the raw shot:
You’ll notice that we put a special texture into the mirror frame to simplify motion tracking. The important characteristics of this texture are:
- It’s black and white (thus high-contrast) and has sharp edges and corners. This will help the picture recognition do its work.
- There’s a mixture of smaller and larger shapes, spread out over the surface I want to track. This allows you to use whichever size and location best suits the plugin during post.
- The shot is set up in such a way so that the girls never block it. If the texture is obscured, even for an instant, the picture recognition will lose track of where it is in your footage.
I did make one mistake in this shot - I left the glass plate in the frame. Consequently, reflections off the glass obscured a lot of the texture underneath, which caused a few problems for the plugin. Fortunately, I was able to compensate, but it’s best to prevent this problem in the first place during the shoot by making sure the texture isn’t covered.
After the plugin did its processing of the raw footage, we simply added the VFX mirror content to the shot. Because of the plugin’s work, this added effect locked position with the mirror frame as intended. You can see the final result here:
The key to good motion tracking is to plan your shots carefully and use a proper texture, all of which will make life much easier during post. Good luck!