adding more nodes

docs/nodes/Rank Filter.md

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+# Rank Filter
+
+Apply a general rank-order (morphological) filter to a DATA_FIELD. Selects the k-th smallest value within a local window, encompassing erosion, dilation, median, and arbitrary percentile operations. Equivalent to Gwyddion's `rank-filter.c` module.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| filtered | DATA_FIELD | Rank-filtered field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| radius | INT | 3 | Radius of the circular filter window in pixels (1–50) |
+| operation | dropdown | median | Filter operation: erosion (local minimum), dilation (local maximum), median (50th percentile), or percentile (custom rank) |
+| percentile | FLOAT | 50.0 | Custom percentile rank, used only when operation is percentile (0.0–100.0) |
+
+## Notes
+
+- Erosion (local min) shrinks bright features and expands dark ones. Dilation (local max) does the opposite.
+- Median is equivalent to percentile = 50 and provides edge-preserving smoothing.
+- The percentile parameter is ignored unless operation is set to percentile. A percentile of 0 is equivalent to erosion, and 100 is equivalent to dilation.
+- The filter uses a circular (disc-shaped) kernel; the actual window diameter is 2*radius + 1 pixels.
+- Combining erosion followed by dilation (opening) or dilation followed by erosion (closing) can be achieved by chaining two Rank Filter nodes.