Merge pull request #6469 from dotCooCoo/fix/3d-expand-keeps-curves

3D: expanded solids keep the art's curves in caps and the walls' cap edges

docs/target-app-parity.md

@@ -1062,6 +1062,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - Extrude & Bevel, Inflate and Rotate apply to type: it makes a solid of its glyph outlines, painted with its own fills and strokes or, when it has none, its characters' colour (the first run's), and stays type, so editing the text reshapes the solid (#4715).
   - Extrude & Bevel, Inflate and Rotate (Effect › 3D and Materials): live effects in the `three-d` crate with depth, twist, taper, solid or hollow caps, five original bevel shapes (width, height, repeat, spacing, inside, both sides), Inflate's volume, both sides, twist, taper and cap, Rotate's turned fills and strokes, Position presets, the rotation and light gizmos, SVG/PDF vector output and Expand Appearance; one 3D effect per object; covered by geometry, effect, canvas-versus-export, command and dialog tests ([three-d.md](three-d.md)).
   - Extrude & Bevel and Inflate build detailed shapes: each solid takes up to 65,536 outline points and 163,840 faces (VectorCraft's own limits, was 2,048 points and 32,768 faces), counted before any point is sampled; when the faces do not fit, every curve takes fewer pieces, down to 4. A closed path of 2,000 curves extrudes with a bevel and inflates, and paths of 5,000 and 10,000 curves build at the default settings (`crates/three-d/src/solid/tests_limits.rs`).
+  - Extrude & Bevel (both kinds) and Rotate keep the art's curves in the faces they draw, export and expand: caps and the edges of walls and bevels where they meet a cap are cubics with smooth anchors where the art's are, fitted within 0.001 pt in perspective and within the sampling tolerance on a bevel's inset. An edge of a twisted wall that is cut into pieces, a piece of curve that does not fit after 8 halvings, and the curves past 1,048,576 for one solid stay straight (VectorCraft's own values; [three-d.md](three-d.md#what-differs-from-the-public-documentation)). Tests `solid::tests::a_cap_keeps_the_curves_of_the_art`, `a_cap_in_perspective_fits_the_projected_curves`, `a_beveled_cap_and_its_walls_share_their_curves`, `twisted_beveled_tiny_and_degenerate_caps_keep_their_curves`.
   - Export 3D Object (Asset Export's menu): an object with Extrude & Bevel, Revolve, Inflate or Rotate becomes a 3D asset whose Format is USDA (the default), USDZ, glTF or OBJ, and Export writes each of its solids as a mesh with a material in its paint's color (`assets.add3D`, `assets.set3DFormat`, `assets.export3D`; [export 3D vector artwork](https://helpx.adobe.com/illustrator/desktop/special-effects-styles/create-3d-graphics/export-3d-vector-artwork.html)).
   - Initial live Revolve (#605, #846): a portable geometry and shading crate (`three-d`), editable path and compound profiles, compact options with artboard rotation and light gizmos, SVG/PDF vector output and Expand Appearance that can trim covered faces. Open ends have continuous caps by default, shading is smooth with one to ten colored lights, and intersecting surfaces' visibility is approximate ([revolve.md](revolve.md)).
   - Revolve takes type, as its glyph outlines, and groups or layers, whose members sweep about one axis as [the help page](https://helpx.adobe.com/illustrator/desktop/special-effects-styles/create-3d-graphics/create-3d-objects.html) describes; a surface can have up to 16,384 profile points, and curves take fewer samples when a surface would pass 163,840 faces (#3617).

docs/three-d.md

@@ -277,6 +277,16 @@ out, perspective). No Adobe software, assets, presets or bevel shapes were used.
   builds them, against the same 163,840. The point limit is counted on the shape as drawn and, when its contours
   meet, again on the merged outline, which can have more points (bars crossing in a grid trace every hole between
   them); past it, the shape is too complex for the effect.
+- The faces drawn, exported and made by **Object › Expand Appearance** keep the shape's curves: the caps of
+  Extrude & Bevel, the flat face of Rotate, and the edges of the walls and bevels where they meet a cap are cubic
+  curves, with smooth anchors where the art's are smooth, while visibility and depth are worked out on the
+  sampled points. Without perspective and bevel each curve maps exactly. With perspective, a curve is split into
+  cubics until each lies within 0.001 pt of the projected outline. A face inset or outset by a bevel follows its
+  sampled outline within the sampling tolerance above, one curve for each sampled edge through its two sampled
+  points, so the cap and the walls next to it draw the edge they share as the same curves. The edges between two bevel steps or two wall slices, a
+  piece of curve that does not fit after 8 halvings, an edge of a twisted wall cut into pieces, and the curves
+  past 1,048,576 for one solid stay straight. The tolerances and limits are VectorCraft's own values. Inflate's
+  faces are polygons.
 - Inflate also limits the work of filling its grid, which grows with the outline's points times the grid cells its
   outline crosses. An outline of tens of thousands of points that crosses nearly every cell (a comb of 4,000 teeth
   across the shape) is refused because the 3D surface is too complex; a closed path of 65,536 points round a