Behind the Green Screen Part III

In the first two parts of this series, we walked you through how we set up a makeshift green screen studio and do simple keying and editing. In our prior Dear Zombies releases we were able to substitute various silly background images. We decide to be more ambitious and film a sequence where we’re swinging light sabers around at one another. Here’s what we found:

–          A single person sitting in front of our green screen. No problem.

–          Two people sitting in front of our green screen. No problem.

–          One person standing in front of our green screen. No problem.

–          Two people standing in front of our green screen. No problem as along as we made doubly sure the camera did not pick up any background other than the green screen.

–          One person moving around in front of the green screen. Same as above.

–          Two people moving around in front of the green screen. Problem.

The 10×20’ background sounds like it would be enough, but we ran into issues both with keeping everyone and everything in front of the screen and lighting. The light kit we illustrated in Part II of this series is sufficient for all but the two-person motion scenario. Both issues can be resolved with some judicious post-editing. Of course, our goal is to minimize the amount of post-editing. Some effects can only be done in post, like the flames in our Cocktails for Zombies video. Others, we’d rather avoid, like the custom masking and erasing we had to do for our light saber scene. Nonetheless, mistakes are made in filming. We’re not going to reshoot that scene. That can be a real issue in full scale productions. Actors and locations are available for a limited time and, often, reshooting simply isn’t an option. You need to deal with what you have and make the best of it.

The steps that follow illustrate with I did to get around leaving the green screen area. They might not be the most effective or most appropriate for every situation. I do know it worked. If there is a better way to remove unwanted background artifacts, I’d love to hear them.

Here the original clip complete with sound and background.

ou’ll see that at about six seconds my right elbow, light saber hilt and left hand leave the green screen area on the right side of the screen. That’s bad. Ideally, I should have remained in frame. Well, we’re not going to reshoot.

The first thing we want to do is add a mask that eliminates the area to the left and right of the green screen. Only the green screen area should be visible. Select Layer -> Mask -> New Mask and draw a rectangle which goes to the edges of the green screen. At six seconds into the clip my right elbow and left hand comes out of the mask area and disappear. That’s okay. We’ll deal with it in a moment. The light saber also comes out of the mask area, but we don’t care about that.  It will be replaced with a rotoscoping lightsaber effect.

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