video-output

A small live-video-to-display appliance for Fedora. It receives live video and puts it fullscreen on one chosen monitor. That is all it does.

Two jobs:

Command	Path
video-output camera	ATEM Mini Pro over USB → V4L2 → MJPEG decode → GPU texture → fullscreen HDMI
video-output ndi	ProPresenter → NDI → GPU texture → fullscreen HDMI → ATEM chroma key

No MPV, no GStreamer, no OBS, no browser, no MQTT, no audio, no recording, no playback controls. One process that a launcher starts and kills.

Status: the camera path is complete and verified on real hardware. The NDI backend is stubbed and reports that it is not implemented yet.

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Why Rust, SDL3, libjpeg-turbo

• Rust — the toolchain was already present, cargo test gives the test suite with no extra machinery, and a long-running unattended process that parses frames off a USB bus benefits from bounds checking. The whole dependency tree is two crates: sdl3-sys and libc. V4L2, TurboJPEG and (later) NDI are hand-written FFI, so there is no binding crate to rot.
• SDL3 — the one portable way to get a borderless fullscreen GPU window on a chosen Wayland output without writing raw xdg-shell code. Used through sdl3-sys (raw FFI) rather than a safe wrapper, for direct control over display selection and YUV texture formats.
• libjpeg-turbo (TurboJPEG API) — SIMD MJPEG decode, and its tj3DecompressToYUVPlanes8 hands back raw YUV planes so the CPU never does a colour conversion.

The pixel path

The ATEM emits baseline 4:2:2 JPEG. Rather than decode to RGBA (4 bytes per pixel, CPU colour conversion), the planes are interleaved into packed YUY2 (2 bytes per pixel) and uploaded to an SDL YUY2 texture, letting the GPU sampler convert. Half the upload bandwidth and no CPU matrix multiply.

MJPEG → TurboJPEG → Y/U/V planes → interleave to YUY2 → SDL texture → GPU

4:2:0 sources skip even the interleave (planes go straight in as I420). Anything else (4:4:4, 4:4:0, 4:1:1, grayscale) falls back to packed BGRX.

Latency

Two places drop stale data, both on purpose:

1. Driver queue — each wakeup drains every queued V4L2 buffer and decodes only the newest, returning the rest unread.
2. Frame slot — a one-deep mailbox between the input thread and the renderer. Publishing a second frame throws the first away.

A backlog can never form. If decoding falls behind, frames are dropped, not queued. Latency stays bounded by one capture interval plus one present interval.

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Fedora dependencies

sudo dnf install -y SDL3-devel turbojpeg-devel gcc

Package	Why
SDL3-devel	headers and the libSDL3.so link symlink (the SDL3 runtime package alone is not enough to build)
turbojpeg-devel	libturbojpeg.so for the MJPEG decoder
gcc	the linker driver cargo invokes

Rust, if not present:

sudo dnf install -y cargo rust      # or: https://rustup.rs

You must be in the video group to open capture devices:

groups | grep -q video || sudo usermod -aG video "$USER"   # then log out and back in

NDI (phase 2, not yet wired up)

Download the NDI SDK for Linux from ndi.video, then either install its libndi.so.6 into /usr/local/lib and run sudo ldconfig, or point LD_LIBRARY_PATH at it. The SDK is not redistributable, so it is not vendored here. The receiver will load it at runtime via dlopen, so the camera path keeps working on machines that do not have it.

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Build

cargo build --release
sudo install -m755 target/release/video-output /usr/local/bin/

The release binary is about 560 KB.

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Discovering devices and displays

Capture devices

video-output devices

/dev/video0
    card    : ATEM Mini Pro: Blackmagic Desig
    driver  : uvcvideo
    bus     : usb-0000:67:00.3-1
    usable  : yes, video capture + streaming
    [0] MJPG (Motion-JPEG)  [needs JPEG decode]
          1920x1080  @ 29.97 fps

/dev/video1
    card    : ATEM Mini Pro: Blackmagic Desig
    (not a video capture node -- nothing to stream from here)

This is the same information as v4l2-ctl --list-formats-ext, without needing v4l-utils installed.

On the ATEM Mini Pro specifically: /dev/video1 is a metadata node, not a second video node. It exposes no formats at all, so there is no uncompressed (YUYV/NV12) alternative on this hardware — MJPEG is the only option. The code is structured so an uncompressed device would only need a new branch in pixel_format_for_fourcc, not a restructure.

Displays

video-output displays

Video driver: wayland
2 display(s):
  [0] Sceptre Tech Inc Sceptre F27 1920x1080+0+1080 @ 110.00 Hz  (primary)
  [1] HXA BMD HDMI   1920x1080+0+0 @ 29.97 Hz

Important, and probably not what you expect: on KDE Plasma / Wayland, SDL reports the monitor's model name, not the connector name. You will see HXA BMD HDMI, not HDMI-A-2. Always run video-output displays and use a name from that list.

--display accepts, in order of precedence:

1. an exact name (case-insensitive) — --display "HXA BMD HDMI"
2. a unique prefix — --display Sceptre
3. a unique substring — --display BMD
4. an index — --display 1 or --display #1
5. primary, the default

An ambiguous fragment is an error, never a guess: picking the wrong screen mid-service is the failure this is designed to avoid. Names are strongly preferred over indices, which shift when an output is hotplugged.

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Running it

Simplest possible form:

video-output camera --device /dev/video0 --display BMD
video-output ndi    --source "ProPresenter Lower Thirds" --display HDMI

Full options: video-output --help.

Profiles

So the launcher only needs a stable word:

sudo mkdir -p /etc/video-output
sudo cp examples/multiview.toml /etc/video-output/
video-output multiview

A profile is a flat key = value file using the same names as the long options, plus mode = "camera" or mode = "ndi". Searched in order:

./<name>.toml
$XDG_CONFIG_HOME/video-output/<name>.toml
~/.config/video-output/<name>.toml
/etc/video-output/<name>.toml

Flags given after a profile name override it:

video-output multiview --display Sceptre

Unknown keys are rejected at startup rather than ignored — a typo should fail before the service, not during it.

From a launcher

It is an ordinary foreground process:

• start it, and it opens fullscreen on the chosen output
• SIGTERM (or SIGINT/SIGHUP) makes it exit cleanly
• it never exits on its own because of a missing or broken source

video-output multiview &
LAUNCHER_PID=$!
# ...
kill -TERM "$LAUNCHER_PID"

Nothing needs to be restarted because the camera was unplugged or ProPresenter stopped sending; it reconnects on its own. An optional systemd template is in packaging/[email protected].

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Display behaviour

Borderless, fullscreen, no title bar, no controls, no OSD, cursor hidden, screensaver disabled. Fullscreen is fullscreen-desktop: it adopts the output's current resolution rather than forcing a mode change, so the HDMI link is never renegotiated on a live output.

Aspect ratio is preserved by SDL's letterbox logical presentation, so a 16:9 source fills a 16:9 output exactly and is never stretched.

Wayland gives a client no way to place a window, only to request fullscreen on a particular wl_output. The window is therefore created hidden, positioned on the target display, set fullscreen, and only then shown — and afterwards the program re-reads which display it actually landed on. If the compositor overrode the request you get a warning naming both displays, rather than a silently wrong monitor.

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Failure behaviour

Situation	What happens
Device missing at startup	one clear error, then quiet retries with backoff (250 ms → 5 s); the window stays up and black
Device busy	says so explicitly, keeps retrying
Permission denied	tells you to join the video group
Device unplugged while running	signal lost logged once, black output, reopens automatically when it returns
Malformed frame	dropped, counted, stream continues
Fatal decoder error	the JPEG handle is rebuilt so one bad frame cannot poison the ones after it
Backend panics	caught, logged, backend restarted; the display never goes down with it
No valid frame	black

Logging is edge-triggered — a camera unplugged for twenty minutes produces one signal lost line, not one per retry — plus one throughput line per minute (--stats <seconds>, 0 disables). Set VIDEO_OUTPUT_LOG=debug or --log debug for per-frame diagnostics. Under systemd, journald severity prefixes are emitted automatically so journalctl -p warning filters correctly.

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Testing

cargo test

87 tests covering CLI and profile parsing, invalid configuration, display selection (including the ambiguity rules), the frame slot's drop-stale guarantee, backoff and signal-transition state, the V4L2 struct ABI against the kernel headers, and real JPEG decoding of all four subsampling paths — plus truncation and bit-flip fuzzing to prove a corrupt frame cannot panic or wedge the decoder.

Regenerate the JPEG fixtures with python3 tests/data/generate.py (needs Pillow). They are solid green on purpose: green is what the ATEM keys on, so a wrong YUV matrix or range shows up in a test rather than on stage.

Manual validation on Fedora

Hardware paths that cannot be unit tested:

# 1. Devices and displays are discovered
video-output devices
video-output displays

# 2. Windowed, so it does not take over a screen; expect ~29.97 fps and
#    "0 undecodable"
video-output camera --device /dev/video0 --display <name> --windowed --stats 3

# 3. Fullscreen on the intended output. Confirm the "Opened display:" line
#    names the monitor you meant, with no placement warning after it
video-output camera --device /dev/video0 --display <name> --stats 5

# 4. Recovery: unplug the ATEM's USB cable while it runs.
#    Expect "signal lost", black output, then "signal restored" on replug,
#    with no restart and no repeated log lines
# 5. Clean shutdown
kill -TERM <pid>       # exits promptly, no error

Verified on this machine: Fedora 44, KDE Plasma on Wayland, ATEM Mini Pro on USB, 1920x1080 MJPEG @ 29.97 — sustained 30.0 fps decoded, 0 undecodable frames, 0 stale buffers, 0 frames dropped before display, ~33% of one CPU core and ~98 MB RSS.

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Troubleshooting

cannot find -lSDL3 or cannot find -lturbojpeg when building SDL3-devel / turbojpeg-devel are not installed. The runtime packages alone do not provide the link symlinks.

cannot open /dev/video0: Permission denied Join the video group and log out and back in.

device is busy Something else is capturing — often another copy of this program, or OBS.

no display matches "..." Run video-output displays. Remember the names are monitor models, not connector names, on KDE Wayland.

display "HDMI" is ambiguous Two outputs match that fragment. Use a longer one, or the exact name.

Warning that the compositor placed the window on the wrong display KWin overrode the fullscreen output request. Check for a KWin window rule matching app id video-output.

Low frame rate, many "stale buffers skipped" You are running a debug build. Use cargo build --release; the YUY2 interleave is several times slower without optimisation.

Log line says "decoded despite a libjpeg warning" Normal for the ATEM. It pads frames with a few bytes before the JPEG end-of-image marker, which makes libjpeg warn on an otherwise perfect image. These frames are complete and are kept. Truncated frames are caught separately by a structural check and dropped.