add acf and psdf nodes

backend/nodes/acf.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+from backend.node_registry import register_node
+from backend.data_types import DataField
+from backend.nodes.spectral_common import acf_field_from_data, preprocess_spectral_data
+
+
+@register_node(display_name="ACF")
+class ACF:
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "field": ("DATA_FIELD",),
+                "level": (["mean", "plane", "none"], {"default": "mean"}),
+            }
+        }
+
+    RETURN_TYPES = ("DATA_FIELD",)
+    RETURN_NAMES = ("acf",)
+    FUNCTION = "process"
+
+    DESCRIPTION = (
+        "Compute the two-dimensional autocorrelation function with Gwyddion-style "
+        "mean or plane levelling before correlation. The output is centered on zero shift "
+        "and uses the default half-range extents from acf2d."
+    )
+
+    def process(self, field: DataField, level: str) -> tuple:
+        data = preprocess_spectral_data(field, level=level, windowing="none")
+        return (acf_field_from_data(field, data),)