dora-webcam

Dora node that captures frames from a webcam and publishes them as std/media/v1/Image. A source for a vision pipeline: it takes a tick and produces frames.

Based on opencv-video-capture from dora-hub, brought up to the dora 1.0 node contract, narrowed to Linux camera selection, and changed to publish the typed Arrow image struct rather than a flat buffer plus metadata.

Usage

Add to your dataflow:

nodes:
  - id: camera
    git: ssh://git@gitea.liria.cl:5001/liria/dora-webcam.git
    tag: v0.1.0
    build: uv pip install .
    path: dora-webcam
    inputs:
      tick: dora/timer/millis/33
    outputs:
      - image

Run the dataflow with --uv (dora run dataflow.yml --uv): the build command is uv pip install ., so the node gets its own managed environment and never installs into an ambient one.

Runnable dataflow

dataflow.yml is the camera on its own, and is what CI runs:

dora run dataflow.yml --uv

Nothing consumes the frames, so to actually see them set PREVIEW:

PREVIEW=true dora run dataflow.yml --uv

A window opens showing the frames as they are published. Press q to stop. That is enough to check a camera, an encoding or a flip without wiring a second node — which is why there is one dataflow here rather than one per consumer.

For a real pipeline, wire image into any node that takes std/media/v1/Image, from your own dataflow. This repository names no consumer: it is a source, and depending on something downstream of it only to demonstrate it would be a cycle.

Output

Output	Payload	Metadata
image	std/media/v1/Image struct	none — the struct describes itself

The struct carries width: UInt32, height: UInt32, encoding: Utf8 and data: LargeBinary, so a receiver needs nothing from the message metadata. For a compressed encoding, width and height are the image's dimensions, not the length of data.

Inputs

Input	Description
tick	Publishes one frame. Wire to dora/timer/millis/N; the node stops when it closes.

Configuring it

Every setting below is an environment variable, which is convenient for one and awkward for several. A dataflow can carry them instead, per node, which keeps the configuration with the wiring it belongs to and out of whatever shell happens to start it:

nodes:
  - id: camera
    git: ssh://git@gitea.liria.cl:5001/liria/dora-webcam.git
    tag: v0.1.0
    build: uv pip install .
    path: dora-webcam
    env:
      CAMERA_ID: usb-Acme_Webcam
      IMAGE_WIDTH: 1280
      IMAGE_HEIGHT: 720
      ENCODING: jpeg
      JPEG_QUALITY: 85
      FLIP: HORIZONTAL
    inputs:
      tick: dora/timer/millis/33
    outputs:
      - image

Values may be strings, numbers or booleans, and dora applies them to both the build and the run.

A node's env: wins over the environment the dataflow was started in. A variable the node does not set still passes through, so PREVIEW=true dora run ... works alongside the block above — but a variable pinned there cannot be overridden from the shell, which is worth remembering before pinning something a test needs to vary.

Environment variables

Variable	Default	Meaning
CAMERA_ID	unset	Substring of a /dev/v4l/by-id/ entry. Wins over CAPTURE_PATH.
CAPTURE_PATH	auto	Device path (/dev/video1) or index. A numeric string is an index.
IMAGE_WIDTH	native	Requested capture width; frames are also resized to it.
IMAGE_HEIGHT	native	Requested capture height; frames are also resized to it.
ENCODING	bgr8	One of bgr8, rgb8, yuv420, jpeg/jpg/jpe, png, bmp, webp.
JPEG_QUALITY	95	Quality 0-100, used only by the JPEG encodings.
FLIP	unset	VERTICAL, HORIZONTAL or BOTH.
PREVIEW	unset	true opens a window of the published frames; q stops the node.
CAPTURE_FRAMES	0	Stop after this many frames. 0 runs until the tick closes.

Resizing only happens when both IMAGE_WIDTH and IMAGE_HEIGHT are set.

With neither CAMERA_ID nor CAPTURE_PATH set, the node opens the first /dev/video* that actually delivers a frame. Merely opening is not enough to go on: a UVC camera exposes a second node for its metadata stream — /dev/video1 beside /dev/video0 on a one-camera machine — which opens happily and never yields an image, so the lowest device number is wrong about as often as it is right.

CAMERA_ID is worth preferring on a robot anyway: a by-id name is stable across reboots and across which USB port the camera is in, and neither an index nor discovery order is.

CI=true makes a failed read publish a labelled black frame instead of raising, which is what lets the dataflow run on a machine with no camera. It is not a configuration knob.

Build

uv sync

Every dependency ships prebuilt wheels for linux-x86_64, linux-aarch64 and macos-arm64, so the node installs with no system packages and no compiler.

Requirements: dora 1.0+ (verified with dora-cli 1.0.1), Python 3.11+, uv, and a V4L2 camera. PREVIEW also needs a display. On a bare Linux image opencv-python needs the libGL shared libraries (libgl1, libglib2.0-0), which it links at import time.

Test

uv run pytest

Unit tests over device resolution, encoding and the image struct; they need neither a camera nor a dataflow. The headless dataflow.yml run covers the wiring and the event loop.

Camera selection on other platforms

Selecting by CAMERA_ID reads /dev/v4l/by-id/, so it is Linux only. Upstream also enumerates cameras through macOS system_profiler and Windows PowerShell; that is not carried here. CAPTURE_PATH works everywhere.