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MagicWand

QGIS plugin to make polygon easily and automatically by analyzing MapCanvas.

Supports QGIS 3.44 or later, including QGIS 4.x (Qt6).

Screen.Recording.2026-07-20.at.1.30.25.mov

Japan GSI Seamlessphoto

Usage

  • Left-click mapcanvas where you want to make polygon: the polygon is created and saved immediately, like the magic wand tool of image editors.
  • Or right-click to preview first: a tentative polygon is shown semi-transparently. Keep right-clicking to add seed points into the same selection — useful when one visual region spans colors beyond the threshold (e.g. a shaded part), instead of loosening the threshold. All seed colors are combined into one color model.
  • While previewing, adjust the Color Threshold slider to update the tentative polygon live. Left-click to confirm and save it; press Backspace to undo the latest seed point, or Escape to discard the whole preview.
  • Polygons are added as layer edits (one undo step per click): press Ctrl+Z to undo a creation, and save the layer edits to make them permanent.
  • The polygon is traced from the area connected to the clicked point (flood fill). Colors are compared perceptually (CIELAB delta-E), and the selection follows smooth gradients (region growing) while sharp color edges stop it.

Processing algorithm

The same magic-wand selection is available as a processing algorithm magicwand:polygonizebyseeds ("Polygonize by seed points"): it takes an 8-bit RGB raster and a seed point layer, and outputs one multipolygon feature per seed feature (with a seed_id attribute). One seed feature is one selection — all points of a multipoint feature contribute to the same selection, like clicking multiple points in the interactive preview. To run it against styled map layers, render them first with the built-in "Convert map to raster" algorithm. Works in the model designer, batch mode, and qgis_process.

Development

Setup

# https://docs.astral.sh/uv/
uv sync

Lint / Format

uv run ruff check .
uv run ruff format .

Test

Tests run inside the official QGIS Docker image (same as CI):

docker run --rm -v "$(pwd)":/plugin -w /plugin qgis/qgis:3.44 sh -c "
  pip3 install --break-system-packages pytest pytest-qgis &&
  xvfb-run -s '+extension GLX -screen 0 1024x768x24' python3 -m pytest tests/ -v
"