add notes about inverted clipping paths

notes/pages/XCLIP.md

@@ -13,4 +13,15 @@
 		- 0: The frame is not visible and it is not plotted. The frame temporarily reappears during selection preview or object selection.
 		- 1: The clipped xref frame is displayed and plotted
 		- 2: The clipped xref frame is displayed but not plotted
-	- Init value of `XCLIPFRAME` is 2.
+	- Init value of `XCLIPFRAME` is 2.
+-
+- ## Inverted Clipping Path
+	- The inverted clipping path is stored in a [[XRECORD]] entity linked to the [[SPATIAL_FILTER]] entity via  the [[Extension Dictionary]] by the key `ACAD_XREC_ROUNDTRIP`
+	- The structure looks like this: ![image.png](../assets/image_1708519308154_0.png)
+	- The sub section `ACAD_INVERTEDCLIP_ROUNDTRIP` contains the previous original clipping path
+	- The sub section `ACAD_INVERTEDCLIP_ROUNDTRIP_COMPARE` contains the inverted clipping path which also stored in the [[SPATIAL_FILTER]] entity as new clipping path
+	- The inverted clipping path __must__ follow some rules otherwise [[AutoCAD]] doesn't open the DXF file with the error message "Xdata wasn't read -- Invalid or incomplete DXF input -- drawing discarded."
+	- I don't understand these rules for creating inverted clipping paths yet.
+		- ... but I am happy to know what the problem is
+		- [[BricsCAD]] accepts the inverted clipping paths created by `ezdxf`
+	-

src/ezdxf/entities/blockrecord.py

@@ -69,6 +69,15 @@
 )
 acdb_blockrec_group_codes = group_code_mapping(acdb_blockrec)
 
+# optional handles to existing block references in DXF2000+
+# 2: name
+# 340: explode
+# 102: "{BLKREFS"
+# 331: handle to INSERT
+# ...
+# 102: "}"
+
+
 # optional XDATA for all DXF versions
 # 1000: "ACAD"
 # 1001: "DesignCenter Data" (optional)

src/ezdxf/xclip.py

@@ -359,6 +359,8 @@ def _rect_path(vertices: Iterable[Vec2]) -> Sequence[Vec2]:
 def _get_inverted_clip_compare_vertices(
     bbox: BoundingBox2d, hole: Sequence[Vec2]
 ) -> Sequence[Vec2]:
+    # AutoCAD does not accept this paths and from further tests it's clear that the 
+    # geometry of the inverted clipping path is the problem not the DXF structure!
     from ezdxf.math.clipping import make_inverted_clipping_polygon
 
     assert (bbox.extmax is not None) and (bbox.extmin is not None)