Merge a149d1f854 into 196cb02d10

docs/extending/custom_image_filters.md

@@ -36,3 +36,17 @@ Use the filter in a template, like so:
 
 {% image page.photo width-400 blur-7 %}
 ```
+
+If your custom image filter depends on fields within the ``Image``, for instance those defining the focal point, add a ``vary_fields`` property listing those field names to the subclassed ``FilterOperation``. This is so that a new image rendition is created when any of these fields are changed:
+
+```python
+class BlurOutsideFocusPointOperation(FilterOperation):
+    vary_fields = (
+        "focal_point_width",
+        "focal_point_height",
+        "focal_point_x",
+        "focal_point_y",
+    )
+
+    ...
+```