Pre-generate ONE trickplay sprite (montage JPEG of frames sampled every
library.trickplay.interval, default 10s) + a JSON manifest per file during the
scan/auto-scan prewarm, cached in .unarr next to the media. The web scrubber
shows tiles from it instead of extracting frames live — removing the ffmpeg
contention with the active stream that broke seekbar previews (the original
'no thumbnail' report was the auto-scan prewarm decoding the same file the HLS
transcode was reading, not a seek-index fault).
- config: [library.trickplay] enabled/interval/width (default on, 10s, 240px),
editable + a toggle; IntervalSeconds() with a 10s fallback.
- mediainfo: GenerateTrickplay (one ffmpeg fps=1/interval,scale,tile pass; idle
I/O priority; ceil() frame count so no black trailing tile; a 16.7M-px cap
coarsens the interval for long media so a single sprite stays decodable on
iOS/Safari) + sprite/manifest sidecar cache helpers.
- engine: /trickplay endpoint (manifest JSON, ?kind=sprite JPEG); the agent owns
the tile width so the web requests by path only; thumb:<sha256> token reused.
- prewarm: a trickplay job per item, gated; scan.go + daemon.go wire the config.
Tests: parseDims; synthetic 3x2 / exact-multiple / 1x1; real-file e2e smoke
(S02E08 → 143 tiles, 662KB sprite). Non-breaking: the existing 5-frame panel
prewarm + on-demand /thumbnail stay until the web migrates to the sprite.
