The technique; essentially recording a subject in front of a uniformly colored screen, often green, then taking that recording figuratively making all the green background transparent and finally overlaying on another video. The result, the subject filmed in from of the green screen appearing in the context of the second video. Easiest and safest way to film your kids in a shark tank.
Before we get started, the majority of my FFMpeg posts use filters and capabilities that have existed in FFMpeg for quite some time (e.g. v2.7.2), but the chroma key filter is fairly new, I used v3.2.4, but I suspect the capability was introduced some time earlier.
There are a good deal of green screen videos available on YouTube for playing around. Let's demonstrate how to generate a crude green screen.
Let's start with the green screen subject.
Then, let's look at a video we wish to introduce our subject into.
Sprinkle in a little FFMpeg magic;
Prior to overlaying the two videos, they need to be similar in size and framerate.
$ ffmpeg -n -i foreground.mp4 -ss 20 -t 120 -strict -2 -s hd720 -r 30 foreground-720p.mp4
$ ffmpeg -n -i background.mp4 -ss 20 -t 120 -strict -2 -s hd720 -r 30 background-720p.mp4
Then, specify the two videos, the color of the green screen and some blending factors.
$ ffmpeg -y -i background-720p.mp4 -i foreground-720p.mp4 -filter_complex '[1:v]colorkey=0x008000\:0.3\:0.2[ckout];[0:v][ckout]overlay[out]' -map '[out]' -t 10 greenScreen.mp4
Now, take your new skills and your own creativity and show the world your new-found powers!