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Puppet Kaos - where Kelvin Kao plays with puppets and tell random stories

How to do a split-screen effect with a matte: the theory

This post is about how to do a split screen effect, where there’s a background image across both parts of the screen, but you film the left and right halves seperately to be put together later. Here’s some ideas on how it works. For a more detailed step-by-step tutorial on how to actually accomplish this effect in an editing software, come back in a few days!

Let’s say you want to film you sitting on one end of the couch talking to someone on the other end of the couch, who is also played by yourself… something like this:
matte_theory_composite.gif

You’ll need three items to accomplish this effect:

1. The left half: Film yourself on the left half of the screen. We’ll call this Clip 1:
matte_theory_left.gif

2. The right half: Film yourself on the right half of the screen. We’ll call this Clip 2:
matte_theory_right.gif

3. Make a matte that defines how to composite these two halves together:
matte.gif
(Note: The outside red frame isn’t part of the matte. I only put it there in case you are reading this post in an RSS reader with white background.)

Now, what’s a matte and how does it work? A matte is something that you apply to a layer of video to decide how opaque or transparent the final result is going to be. Here we are using a grayscale picture, which is a picture with different shades of gray, with full black being the darkest gray, and full white being the lightest gray. When you apply this matte to a layer of video, the part that gets the white will be entirely opaque, and the part that gets the black will be entirely transparent (meaning, you won’t see anything). The gray parts are the semi-transparent parts. How opaque or how transparent the gray part is depends on how far it is from full black and full white.

For those that want to know why it’s that way (which is not essential for using this effect but might help your understanding), think about your monitor or TV when it’s off. When it’s off, it looks black. This is because your monitor displays colors by adding three beams of light (red, green and blue) together to form one pixel. When there’s no light, you get black. When they are all on full power, you get the sum, which is white. Different mixes of light beam / color strengths would add up to different color. So in a matte, white means that you want all colors to pass through, making that part fully opaque. On the other hand, black means the absence of colors, or not letting lights through. In that case, you get an image that’s transparent. A mathematical way to look at it is, for white, you are multiplying every pixel value by 1 and for black, you are multiplying every pixel value by 0. For gray, it’d be something in between like 0.2, 0.5, 0.8, hence making the pixel semi-transparent.

Now you want to put the two images together. First you want to put Clip 1 in the bottom layer (background) and Clip 2 in the top layer (foreground), and then you apply the matte to Clip 2. What this is going to do is:

1. The black part of the matte, which is the left half, will make the left half of Clip 2 transparent. When that part is transparent, it will show what’s in the background. This is how the left half of Clip 1 shows through.
2. The white part of the matte, which is the right half, will make the right half of Clip 2 fully opaque. So what we are seeing in the final composite is the left half of Clip 1 and the right half of Clip 2.
3. The gray gradient part of the matte, which is down the middle line, will make the middle region of Clip 2 semi-transparent. This is to accomplish a natural blending between the two videos. You don’t want to see a harsh dividing edge between the two halves. That’s why you want the two videos to blend down in the middle.

And then feed these three elements into your editing software (I’ll show you how in the next post), and you shall get the final result:
matte_theory_composite.gif

So that’s how mattes work. A detailed tutorial with actual video footages in a real editing software will be posted up in a few days. Stay tuned. :-)

PS. This effect was first experimented in Episode 3: Crazy Names, but you can see it in pretty much every single episode after that.

Related posts:
Editing Trick: How to have two puppets on screen at the same time with only one puppeteer
How to do a green screen effect: the theory

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