The effect was created by first extracting the foreground in Flowblade video editor [the FFmpeg version of this process was demonstraited previously¹]
In the animation, the Flowblade window is shown. The blur amount of a filter on the mask is being altered, which demonstraits the mask area growing and shrinking, and the edges of the mask get fuzzier.
Each cut-out frame was then sequentially merged, and overlayed on a background frame, creating the final animation. However, this was not the original goal of the process. If it were, it may have been easier to create using ImageMagick's stacking functionality²
[ Demonstrating the full procedure involved in foreground extraction using Flowblade may appear in a future blog post ]
¹ see also: https://oioiiooixiii.blogspot.com/2016/09/ffmpeg-extract-foreground-moving.html
² see also: https://oioiiooixiii.blogspot.com/2017/01/long-exposure-photography-compared-to.html
source video: https://www.youtube.com/watch?v=S6bQibFNs2E
original upload: (Twitter) August 5, 2016
