spritesheet are used for thumbnail seeking in the player. It is used along with webvtt file, defined as sparial-dimension of media. https://www.w3.org/TR/media-frags/#naming-space
This plugin only generate the output buffer with rows and columns of tiles(thumbnails). It sends a signal on every tile on spritesheert. Signal has all the infomation to generate entry in the webvtt file.
Attached is the example python script to generate spritesheet and webvtt. spritesheet-demo.py