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docs/nodes/Median Background.md

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+# Median Background
+
+Extract background using a local median filter and subtract it. The radius controls the filter window — larger values capture broader background variations. More robust than polynomial leveling for surfaces with sparse tall features. Equivalent to Gwyddion's median-bg.c module.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| result | DATA_FIELD | Background-subtracted field or extracted background |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| radius | INT | 20 | Half-size of the median filter window in pixels; the full window is (2×radius+1)² (2–500) |
+| output | dropdown | subtracted | Output mode: subtracted (original minus background) or background (extracted background) |
+
+## Notes
+
+- The median filter is robust to outliers — tall features (particles, grains) do not bias the background estimate.
+- Choose a radius larger than your largest feature but smaller than the background curvature scale.
+- For polynomial background removal, use Plane Level or Polynomial Level instead.
+- Processing time increases with radius; for very large fields, consider downsampling first.