screengen 
A library for generating screenshots from video files, and a command-line tool for generating thumbnail grids.
Install the package
go get -u gitlab.com/opennota/screengen
Install the command-line tool
go get -u gitlab.com/opennota/screengen/cmd/screengen
Also
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There is a similar program by that name.
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For a more feature-rich command-line thumbnail grid generator (which also doesn't require ImageMagick) see mutschler/mt.