HATCH and LWPOLYLINE in a tilted plane are read as seen from above (#1886)

Their boundary and vertices are in the object's plane (OCS). Only a
mirrored plane (0,0,-1) was handled, by flipping x; a tilted plane whose
normal points down was taken for a mirror too, and one pointing up kept
its plane coordinates, so each face landed turned about the origin by
its own angle. A tilted one is now mapped to the WCS and drawn as seen
from above at its mean height: bulged edges are flattened (they are
elliptical there), and a hatch's pattern direction and origin follow.
Its extrusion becomes Z, since its points are plan coordinates now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/file-format-parity.md

@@ -73,7 +73,7 @@ on read with a note, so such files no longer exceed the DXF limit (#1482). **DWG
 
 | Content | Read | Write | Notes |
 |---|---|---|---|
-| LINE, POINT, CIRCLE, ARC, ELLIPSE, LWPOLYLINE (widths, bulges, elevation), SPLINE, RAY, XLINE | yes | yes | round-trip tests |
+| LINE, POINT, CIRCLE, ARC, ELLIPSE, LWPOLYLINE (widths, bulges, elevation), SPLINE, RAY, XLINE | yes | yes; a LWPOLYLINE in a tilted plane is read as seen from above (arcs flattened, #1886) | round-trip tests |
 | POLYLINE 2D/3D | yes | yes (as LWPOLYLINE / 3D POLYLINE) | polyface and polygon meshes flattened: lossy |
 | TEXT, MTEXT, ATTDEF, INSERT + ATTRIB | yes; multiline attributes (embedded MTEXT) read and drawn | yes; multiline attributes written as embedded MTEXT (DXF 2007 and later; older versions: one line); changed attribute values refresh justified baselines, including small width changes | ATTDEF prompt kept (#137); unchanged values retain imported-font placement; a justified TEXT's insertion point (group 10, which AutoCAD takes from a DWG as it is) is computed as AutoCAD computes it: Middle (M) centred on the string's own extents, bottom justifications at a TrueType face's descent (#2229) |
 | DIMENSION (all 7 kinds) | yes | yes, with anonymous `*D` blocks | a reopened dimension's `*D` block is regenerated under its name, unreferenced ones dropped (#495); DIMASSOC objects written for every associated kind — linear/aligned, angular, radial/diametric, ordinate and arc length — and read back without CADCraft's xdata (#1651) |
@@ -81,7 +81,7 @@ on read with a note, so such files no longer exceed the DXF limit (#1482). **DWG
 | DIMENSION (all 7 kinds) | yes | yes, with anonymous `*D` blocks; their text keeps the diameter sign as `%%c`, so SHX styles plot it (#1645) | a reopened dimension's `*D` block is regenerated under its name, unreferenced ones dropped (#495); DIMASSOC objects written |
 | LEADER | yes | yes | |
 | MULTILEADER | yes (#459) | yes, native MULTILEADER with MLEADERSTYLE (#459) | several landings per multileader are folded into one; per-object overrides and annotative scale contexts not kept |
-| HATCH pattern / solid | yes | yes; associative hatches keep their boundary objects in blocks too, and the boundary carries the hatch reactor (stretched dynamic blocks drag the fill in AutoCAD) | |
+| HATCH pattern / solid | yes | yes; associative hatches keep their boundary objects in blocks too, and the boundary carries the hatch reactor (stretched dynamic blocks drag the fill in AutoCAD); a hatch in a tilted plane is read as seen from above (#1886) | |
 | HATCH gradient | yes | yes (#162, fixes #80) | |
 | SOLID, TRACE, 3DFACE | yes | yes | |
 | ACAD_TABLE | yes | yes (entity + block) | |
@@ -140,6 +140,7 @@ our output in AutoCAD black-box.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-31 | minor | HATCH and LWPOLYLINE in a tilted plane (extrusion off the Z axis) are read as seen from above at their mean height, instead of keeping plane coordinates that turned them about the origin (#1886) |
 | 2026-10-29 | minor | R2000 DXF: CLASS records written without group 91 (instance count), which came with the 2004 format; AutoCAD LT 2026 stopped responding on every R2000 DXF CADCraft saved (checked black-box in AutoCAD LT 2026) |
 | 2026-10-26 | minor | CC-046: ATTEDIT and -ATTEDIT value changes invalidate derived justified baselines before DXF/DWG export; preserve alignment anchors, unchanged values and Aligned/Fit endpoints. Review follow-up: baseline recomputation preserves insertion elevation independently of alignment elevation. The shared writer correction also applies to justified TEXT. Independent of attribute editor UI; reverting the follow-up restores elevation loss on recomputation, while reverting the value setter restores stale XY baselines. |
 | 2026-10-25 | minor | LAYOUT SAveas (`layout.saveas`) writes a layout as a drawing template: its page setup and paper space objects, no model space objects or other layouts, and only the named objects they use |