VFX 101: Masking

(Herman Wang) #1

This is the fourth 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.

I’ve touched on masking in previous columns as one of the steps needed to do special effects such as this one or for greenscreening. But masking by itself can also be a powerful tool if used creatively.

Erasing Things
Here’s a simple clip from an upcoming episode of our show:

Pretty basic, right? What you might not have noticed is that this clip already has a masking effect in it. Here’s the raw clip:

During editing, I felt that the white thermostat on the wall stood out in a distracting way. So I got rid of it - with masking. Here are the things I assessed to make sure masking would work here:

  • The thermostat never comes close to Maria-Crystal’s body. If it got too close to her, masking would leave visual artifacts on her shoulder. As it was, I took advantage of the fact that the viewer’s attention would not be on the wall.
  • The wall is a fairly uniform colour and texture. You’ll see why this helps shortly.
  • The thermostat is relatively small in the scene’s framing. So again, masking artifacts would be small enough to avoid drawing the viewer’s eye.

The first step was to go through the entire clip and draw a mask border around the thermostat. The camera is moving quite actively in this take, so that meant the mask had to be adjusted as the camera moved. Most advanced editing programs will let you do this by adding something called “keyframes” at different points along the clip’s timeline. (This picture is from Vegas Pro, but other editors will have something similar.)

Here’s an important point - the mask I created here was a “negative” mask, which meant that I wanted to keep everything except what was inside the mask box. So when I played the clip back, the thermostat was gone! Except there was a noticeable pure-black hole in its place, which unfortunately didn’t match up with the black of the wall. It would have been worse if the wall were some other colour like blue.

So how did I fix this? I put an unmasked copy of the exact same clip underneath the current one (which meant that the thermostat reappeared in the hole). But here’s the simple trick: I took that unmasked lower copy and moved it down a distance about the same height as the thermostat. So now instead of the thermostat filling that hole, it was filled with the part of the wall just above the thermostat - this is why it helped that the wall was fairly uniform. And because the clip is a copy of the one above it, the camera movements match exactly, so that part of the wall was always in the right spot for this to work.

Masking to erase is much easier, of course, when the camera is tripod-mounted so the framing doesn’t move around. I used a masked part of this clip without my body in front of the shed to erase when my body is in front:

You can also use this technique to erase random members of the public who were in the background of your exterior shoots, which comes in handy when you can’t afford exterior location permits.

Twinning Actors
Another simple masking trick is used to create a scene with twins of a single actor, or triplets/quadruplets/etc if you’re feeling “Orphan Black” ambitious.

  1. First, you must mount the camera on a tripod (and maybe tape the tripod down to prevent accidentally bumping it). If the camera doesn’t remain still throughout the entire shoot this effect won’t work.
  2. Decide where the two twins are going to be. For this example, I’ll do a simple left/right split. It’s markedly easier if the two bodies don’t overlap.
  3. Shoot the two separate performances.
  4. Mask the two clips and stitch them together.

You’ll have to rehearse beforehand to get the timing of conversations between the two twins right, but once you get to editing, it should be a simple matter of masking to get the result you want.

Is there a topic you’d like covered for a future column? Please let me know!

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(James Peniata) #2

I WISH this was out when I first started learning how to do fx work :frowning:
It would have saved me soooo much time! hahaha
Thank the stars above for this forum though! Future filmmakers coming up are in good hands!

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(Herman Wang) #3

I’m glad you think it’s helpful. I’m figuring out what the next column should be :slight_smile:

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