➜ systemctl status printer-streamer

3D Printer Streamer

Unattended live streaming and timelapses for a Prusa CORE One, driven by print state.

It watches the printer. When a print starts, it opens the camera, composites a live telemetry panel over the video, and pushes the result to YouTube. When the print ends, it stops on its own and uploads a timelapse. Nobody has to be at the machine for any of it.

How it fits together

The control panel

The camera was never degrading

For five weeks the stream would collapse from a healthy 22 fps to somewhere between 2 and 8, in bursts split by multi-second freezes. The camera was suspected, then the wifi, then the bitrate. All three were wrong. The pipeline was strangling its own input.

ffmpeg's split=2 only advances when both branches accept a frame. One branch fed the encoder; the other wrote preview.jpg to disk every three seconds. Every slow write stalled the split — and a stalled graph stops draining the RTSP socket, so TCP backpressures the camera, and the camera drops the frames it can no longer buffer. The corrupt frames in the logs were holes we were punching ourselves.

Bisected against the live camera, five minutes per arm, one variable at a time:

Writing the JPEG through ffmpeg's fifo muxer decouples the branch from the graph and the symptom disappears. A 7h29m soak afterwards held 22.73 fps with zero gaps. It has run at 1080p and 5000 kbps ever since.

That result also decided the timelapse design. The obvious approach — branch the filter graph for timelapse frames — is the same construct that broke the stream twice. So the timelapse samples the preview.jpg that already exists on disk, which costs the pipeline nothing.

Other decisions worth the time

• RTSP, not WebRTC

  WebRTC is the mobile app's transport and is undocumented, and it would still need decoding and re-encoding to reach YouTube. RTSP is documented, stable, and costs the same round trip.

• The core references nothing

  The session state machine depends on no I/O, no clock and no ffmpeg. That one constraint is what makes all thirty of its transitions testable without a printer plugged in.

• A finished print is not just FINISHED

  PrusaLink can drop the job object on completion, so a vanished or changed job id across two consecutive polls counts as an ending too. The failure being guarded against is a stream that never stops.

• An unreachable printer does not stop the broadcast

  Losing telemetry is not losing the print. The overlay greys out and shows stale while the video keeps going. A dropped stream, by contrast, reconnects with backoff capped at 30s so YouTube keeps one continuous video.

• A stall timeout, because of what failure looks like at 3am

  Without one, an overnight failure broadcasts a motionless print bed until morning. When it fires, the app returns to armed, so clearing the fault produces a fresh stream on its own.

• DPAPI at machine scope, deliberately

  Secrets are encrypted at LocalMachine rather than CurrentUser scope. User scope would break the actual workflow — configure interactively, then install as a service running as LocalSystem — by silently failing to decrypt after the switch. The trade is that any local process on that box can decrypt the file, which is acceptable on a single-owner machine and would not be on a shared one.

• It can be tested with no hardware at all

  A synthetic source swaps the camera for an ffmpeg test pattern, so arming produces a real recording and a live preview before the camera is even plugged in. The smoke test runs against the real ffmpeg binary — it is the only thing that proves the filter graph is valid.

What it does when things go wrong

Settings

How it is built

.NET 9 with a Blazor front end, SkiaSharp for the overlay, and ffmpeg doing the video work. It publishes self-contained, so the always-on box it runs on needs no .NET installed.

Watch a print

Every finished print is also uploaded as a timelapse, sped up 180× — an eight-hour print becomes about two and a half minutes. The channel is past a hundred of them now, every one posted without anybody being asked to do anything.

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