Video scrubbing previews

Design the video-scrubbing-preview (storyboard / sprite-sheet) and poster-frame system for a VOD platform. When a user hovers/drags the seek bar, the player must instantly show a thumbnail of the frame at that position across the whole timeline, for videos from 30s clips to 6-hour streams, on bandwidth-constrained clients. The platform also wants a good auto-selected poster frame per video. Design how previews are generated, packaged, and delivered, and how you keep client bandwidth and request count low.