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.

You’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.

At this point, we’ll get rid of the green screen. This step was covered in the last post.

Next, we’ll animate the mask shape to allow for my elbow to remain visible even after it leaves the green screen. I’ll apply additional vertices, using the Pen tool, on the right side of the mask and manipulate them individually.  I’ll move the vertexes to manipulate the mask around my elbow and hand. Some of the background will remain in the clip.

Select the Mask Shape and click the stop watch with the time indicator positioned at the start of the clip. This will give us a clean shape at the beginning.

After Effects Mask Selection

Select the Pen tool and add 5-10 vertices on the right side of the image.  We’ll manipulate them in the frames where I leave the green screen.

After Effects Green Screen

Use the Page Down key to advance a single frame forward. Each vertex is moved around my elbow, hand and light saber handle as I come out of the green screen. The next step will be to use the eraser tool frame by frame.  The less there is to erase, the better.

After Effects Green Screen Gone

Changing the view to alpha in the composition shows everything that is in our alpha channel. Dark artifacts that would blend in with the background are highlighted and made apparent. The drop down below appears in the bottom of the composition and layer window.

After Effects Alpha Channel Selection

Here’s the intermediate video after the mask has been animated.  You’ll still see some artifacts on the right side.

Now comes the tedious frame-by-frame erasing. Select the Erase tool in the toolbar. Change the Erase option in your Paint toolbox to Layer Source & Paint and select Single Frame for the Duration.

After Effects Select Eraser

Position the current time indicator at the first location where the background comes into view and start erasing. Hit the Page Down for the next frame and keep going. It’s tedious, but it gets the job done.

After Effects Erase Image

After you think all the artifacts are erased, switch to the alpha view and review the clip in entirety. There are probably some small problem spots that were missed the first time around. I know I missed a few and this helped me true it up. Once the clip has been cleaned up, a video or background image can be dropped in a layer below. Here we are fighting it out on Tatooine.

On a side note, the chapters on masking and painting in the book, Creating Motion Graphics, were particularly helpful.

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