dora-plot

Dora node that overlays bounding boxes, labels and text on an image input and displays it in an OpenCV window. A display sink for debugging a vision pipeline: it produces no outputs.

Based on opencv-plot from dora-hub, brought up to the dora 1.0 node contract and packaged with uv.

Usage

Add to your dataflow:

nodes:
  - id: plot
    git: ssh://git@gitea.liria.cl:5001/liria/dora-plot.git
    tag: v0.1.0
    build: pip install .
    path: opencv-plot
    inputs:
      image: camera/image
      bbox: detector/bbox
      text: detector/text

Run the dataflow with --uv (dora run dataflow.yml --uv) so the node gets its own managed environment.

Runnable example

dataflow.yml wires a webcam and a fixed caption from dora-hub into the viewer:

dora run dataflow.yml --uv

It builds the node from this checkout (path: + build:) rather than from a tag, which is what lets dora validate pick up the manifest described below.

bbox is left unwired there, because every bbox source is a detector and pulling one in would put a model download in front of a webcam preview. To see boxes, add dora-yolo between the camera and the viewer:

  - id: detector
    git: https://github.com/dora-rs/dora-hub.git
    build: pip install ./node-hub/dora-yolo
    path: dora-yolo
    inputs:
      image: camera/image
    outputs:
      - bbox

  - id: plot
    path: opencv-plot
    build: pip install .
    inputs:
      image: camera/image
      bbox: detector/bbox

To display a dora-zed camera instead, wire image: zed-camera/image_bgr — that node publishes bgr8 with the width, height and encoding metadata this one expects.

Node manifest

dora-node.yml declares the node's inputs with type URNs. No source package is shared between node repositories, so it is the only contract a consumer has, and it is what makes a wiring mistake fail at build time instead of at runtime:

$ dora validate dataflow.yml
  node "plot": injected 2 contract type(s) from .../dora-node.yml
All type annotations OK.

image is std/core/v1/UInt8 and text is std/core/v1/String; the payloads are raw buffers described by their metadata, so the URN names the element type rather than a struct. bbox is deliberately left untyped — senders disagree on whether its numeric fields are 32- or 64-bit, and the node reads either.

Behavior

The node keeps a current frame, bbox set and text string. Each input updates its slice of that state:

• An image event decodes the frame and immediately redraws: it draws every stored bbox (green rectangle plus label, conf text), draws the stored text at the top left, resizes to PLOT_WIDTHxPLOT_HEIGHT if both are set, and shows it in a window titled Dora Node: opencv-plot. Press q in the window to quit.
• A bbox or text event only updates stored state; it is drawn on the next image redraw.

Setting CI=true suppresses the display: the node still decodes every input but opens no window. That is what makes it safe to run in a pipeline test.

Inputs

Input	Payload	Metadata
image	Flat UInt8 array	width, height, encoding
bbox	Arrow struct array, element [0] with fields bbox, conf, labels	format
text	Arrow array of size 1	—

Supported encoding values: bgr8, rgb8, jpeg/jpg/jpe/bmp/webp/png (encoded byte stream), yuv420, avif. width and height are only needed by the raw encodings — the rest carry their own dimensions. An unsupported encoding raises.

Supported format values: xyxy and xywh (centre-based, converted to xyxy).

node.send_output("image", pa.array(frame.ravel()),
                 {"width": 640, "height": 480, "encoding": "bgr8"})

node.send_output("bbox", pa.array([{
    "bbox": np.array([...]),    # flattened, four values per box
    "conf": np.array([...]),
    "labels": np.array([...]),
}]), {"format": "xyxy"})

node.send_output("text", pa.array(["hello"]))

Outputs

None — it is a display sink.

Environment variables

Variable	Default	Meaning
PLOT_WIDTH	image width	Resize the displayed frame to this width, in pixels.
PLOT_HEIGHT	image height	Resize the displayed frame to this height, in pixels.

Resizing only happens when both are set. Both are also available as --plot-width and --plot-height; the environment variable wins when both are given.

CI=true suppresses the display window; 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 on any of them — including AVIF support, which comes from pillow-avif-plugin rather than OpenCV.

Requirements: dora 1.0+ (verified with dora-cli 1.0.1), Python 3.11+, uv, and a display to show the window on (or CI=true to run without one).

Test

uv run pytest

The tests cover decoding, the bbox format conversion and the headless draw path, so they need neither a camera, a dataflow nor a display.