Fix glyphs without triangles for self-overlapping font contours (Bahnschrift Light "e")

[dsn27]

Problem

With TriangulateText == true some glyphs are missing in rendered text, observed with "Bahnschrift Light" and the letter "e": FontCache.Get produced an empty display list, CloseList returned null, and because null was not treated as a cached result the glyph was triangulated again on every repaint.

Cause

Variable fonts (Bahnschrift, Roboto Flex, Source Sans 3, Recursive, Inter, …) keep overlapping contours. Glyphs like "e", "6", "9", "P", "&" are drawn as a single contour that overlaps itself: bowl and bar are one closed path, the eye of the "e" is not a contour of its own but only the region enclosed by the overlap. GetOutline2D delivers this contour correctly as one closed path, SimpleShape/Face.MakeFace accept it, but Face.Triangulate cannot triangulate a self-intersecting border and returns no triangles. Nothing in FontCache.Get noticed this.

Reproduced on Linux with the glyph outlines of Roboto Flex Light, Source Sans 3 Light and Recursive Light (extracted with fontTools, fed through the same path conversion as GDI+ data): "e", "6", "9" (Roboto Flex, Source Sans 3), "B", "D", "e" (Recursive) and "P" (Inter) had 0 triangles with the old code. Bahnschrift itself is not available outside Windows; its "e" uses the same construction (the glyph has one contour with an overlap flag), so this is the failure you traced.

A second, previously ignored case: separate contours that overlap each other (e.g. "K", "A", "R" of Roboto Flex, the "raumTalk" case mentioned in the code). The old code only subtracted contained contours and ignored intersecting ones.

Fix

• New GlyphShapeBuilder (internal, GDI-free): converts the GDI+ path data to Path2D and builds the glyph area with the non-zero winding rule of fonts:
  • a self-intersecting contour is split at its intersections into a planar graph; the faces with non-zero winding number are the filled area, adjacent faces are united over their shared edges (so the result is one simple shape with holes, not a pile of touching faces);
  • separate overlapping contours are united when they have the same orientation as the outer contour, subtracted when opposite;
  • nested contours are handled exactly as before (containment only, orientation is ignored because some fonts are inconsistent). Verified with 14 font files × 94 glyphs: for every glyph without overlaps the resulting shape is identical to the old algorithm.
• FontCache.Get(…, IPaintTo3D) safety net (logic of 0002-fontcache-outline-fallback-for-glyphs-without-face.patch from LogiCal, extended): the triangulation of every part is checked with Face.GetTriangulation; a part without triangles is painted by its outline, a glyph without any face by its paths; exceptions of a single glyph are caught; the display list is closed in a finally and the result is cached even when the list is null (DictVal.listCreated), so a failure is not repeated every frame.
• The three copies of the shape construction in FontCache (Get, Get with IPaintTo3D, GetCenterLines) now share one implementation. As a side effect the IPaintTo3D variant no longer loses islands inside holes (like the "c" of "©"), which the GetCenterLines variant had already fixed.

Tests

• GlyphShapeBuilderTests: synthetic contours (hole, island in hole, overlapping "+", opposite orientation, keyhole hole, figure eight) and the real "e" of Roboto Flex Light (OFL) at all three font precisions. Run on Linux, all pass.
• FontCacheGlyphTests: sends 0x20–0x7E of Arial, Calibri, Bahnschrift, Bahnschrift Light, Segoe UI Variable, Segoe UI through FontCache.Get with a recording IPaintTo3D and expects triangles for every glyph with a closed outline, plus the cache check; Inconclusive when GDI or the font is not available. An ignored survey test reports all installed fonts. These need a Windows machine with the fonts, they could not be executed here.
• CADability.Tests is signed with CADabilityKey.snk and gets InternalsVisibleTo in CADability.

Font check (item 4 of the task)

Outside Windows only substitutes were possible. Result with the new code, all 94 printable ASCII glyphs each: Barlow Light, DejaVu Sans, FreeSans, Liberation Sans, Noto Sans Light, Open Sans Light/Bold, Oswald Light, Inter Light/Regular, Recursive Sans Linear Light, Roboto Flex Light/Regular, Source Sans 3 Light: 0 glyphs without triangles, 0 area mismatches against skia-pathops as reference. Before the change: Roboto Flex "69e", Source Sans 3 "69e", Recursive "BDe", Inter "P" without triangles. Bahnschrift, Segoe UI Variable, Calibri and Arial must be checked with FontCacheGlyphTests on Windows.

The version is bumped to 1.1.10 in a separate commit on this branch.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QL5zfiEqxsCepsdBixLDS1

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Generated by Claude Code