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docs/nodes/Grain Mark.md

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+# Grain Mark
+
+Mark grains by thresholding height, slope magnitude, or curvature. Thresholds are relative (0–1) to the data range. Small regions below min_size pixels are removed. Equivalent to Gwyddion's grain_mark.c module.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input surface |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| mask | IMAGE | Binary mask of marked grains |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| criterion | dropdown | height | What to threshold: height, slope, or curvature |
+| threshold_low | FLOAT | 0.3 | Lower bound of the normalized threshold range (0–1) |
+| threshold_high | FLOAT | 1.0 | Upper bound of the normalized threshold range (0–1) |
+| min_size | INT | 10 | Minimum grain size in pixels; smaller regions are removed (1–100000) |
+| inverted | BOOLEAN | False | Invert the mask to mark valleys instead of peaks |
+
+## Notes
+
+- Thresholds are relative to the data range: 0.0 = minimum value, 1.0 = maximum value.
+- "slope" uses Sobel gradient magnitude — useful for marking edges and steep features.
+- "curvature" uses the Laplacian — useful for marking bumps or pits regardless of absolute height.
+- Use inverted=True to mark valleys, pores, or depressions instead of raised features.
+- For Otsu-based automatic thresholding, use the Threshold Mask node instead.