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Timeline scrubbing & previews

When you drag along the timeline to find a moment in recorded footage, Crumb shows a small preview image that updates as you move, so you can spot the right frame without playing through everything. It works the same in the desktop app, the mobile apps, and the web console.

It just works, and gets faster as you go​

You don't have to turn anything on. The first time you drag to a spot, Crumb grabs a preview frame from the recording; after that, that spot is remembered, so dragging back and forth over a region you've already looked at is instant. Scrubbing a whole wall of cameras at the same moment works the same way.

In practice, revisiting a spot you have already scrubbed redraws in about a millisecond (a cached image read) rather than the couple hundred milliseconds it takes to decode a fresh frame on a 4K camera.

This smoothness is the reason Crumb records its own footage instead of reading a detector's recordings; if you run Frigate, see why Crumb records its own footage.

Two optional settings can make it even better on larger systems. Most people never need either one.

Make the first drag instant too (background previews)​

By default, the very first time you scrub to a brand-new spot, Crumb builds that preview on the spot, a brief moment of work before the image appears. If you'd rather have previews ready in advance so even the first drag is instant, turn on background pre-generation. Crumb then quietly builds previews for recent footage as it records.

The trade-off is honest: it uses some ongoing CPU and a little disk space whether or not you scrub much, which is why it's off by default (a small box shouldn't do work nobody asked for). Turn it on if you review footage often and want it to feel instant everywhere.

The easiest way to turn it on is the admin console: Server settings → Scrub previews. Flip "Pre-generate scrub previews" and it starts within a few seconds, no restart, no container access needed. The same panel lets you adjust how far back it backfills, how often it wakes up, and how much cache space and time it's allowed, all of it applies live. Turning it back off also takes effect within a few seconds, even in the middle of a large backfill.

Prefer the .env file instead (for example, to bake the setting into your compose file for a scripted deploy)? It works the same way:

THUMB_PREGEN_ENABLED=true

Then apply it:

docker compose up -d api

Whichever knob you set last wins: once you change a setting from the admin console, that value takes over from the .env default for good. The environment reference lists every knob (the fine-tuning ones and the console-editable ones) with its default and whether it's console-editable.

Keep it snappy on a busy hard-drive system (put previews on an SSD)​

Previews are tiny images, and on most setups it makes no difference where they live. But if you run a lot of cameras for weeks on a regular spinning hard drive, reading all those small scattered files can get slow, exactly when you're trying to scrub. If you have a spare SSD or NVMe drive, point the preview cache at it so scrubbing stays snappy while your footage stays on the big, cheap drive.

This is completely safe. Previews are just cached copies of frames Crumb can always rebuild. If that SSD fills up, fails, or you wipe it, no footage is lost: Crumb regenerates previews from the recordings as needed.

  1. Mount the fast drive into the api container (a compose volume, for example at /mnt/thumbs).

  2. In your .env:

    THUMB_CACHE_DIR=/mnt/thumbs

  3. Apply it:

    docker compose up -d api

You don't have to manage the cache​

Crumb keeps the preview cache from growing forever on its own: once it passes a size or age limit, the oldest previews are dropped, and any preview that's missing is simply rebuilt the next time it's needed. If you ever want to change those limits, they're in the environment reference.