Step export. U3D and PRC import (these are from 3D PDFs). Fixed error in duplicating solids for SolidAssembly.

Author: massonib

STEPImportExport/FacetedStepWriter.cs

@@ -1,11 +1,27 @@
 # TVGL.STEPImportExport
 
-TVGL.STEPImportExport provides STEP (`.step`, `.stp`) file import capabilities for TVGL (Tessellation and Voxelization Geometry Library).
+TVGL.STEPImportExport provides STEP (`.step`, `.stp`) import and export capabilities for TVGL (Tessellation and Voxelization Geometry Library).
 
 ## Features
 - **Accurate B-Rep Meshing**: Discretizes Boundary Representation solids into high-quality watertight triangle meshes powered by Open CASCADE Technology (OCCT).
 - **Analytic Primitive Recovery**: Automatically extracts and maps analytic geometry surfaces (`Plane`, `Cylinder`, `Cone`, `Sphere`, `Torus`) with their exact mathematical parameters.
 - **Full Doubly Linked Primitives**: Constituent `TriangleFace` objects and `PrimitiveSurface` objects are fully doubly linked (`face.BelongsToPrimitive` and `primitive.Faces`).
 - **Freeform Surface Support**: Freeform surfaces (NURBS, B-Spline, Bezier, Offset) are represented as `UnknownRegion` primitives with their descriptive surface type preserved.
 - **Multi-Body & Assembly Support**: Reconstructs multi-body STEP files into individual `TessellatedSolid`s and `SolidAssembly` hierarchies.
+- **Hybrid Export**: `STEP.Save(solid, filePath)` and `STEP.SaveHybrid(solid, filePath)` preserve reliable TVGL planes, circularly bounded cylinders and cones, closed spheres, and closed ring tori as analytic STEP surfaces. Other regions use triangle faces.
+- **Faceted Export**: `STEP.SaveFaceted(solid, filePath)` writes every triangle as a bounded planar STEP face. Watertight meshes become faceted B-reps; open meshes become surface models.
+- **Assembly Export**: `STEP.Save(assembly, filePath)` writes transformed assembly instances as distinct STEP bodies.
+- **Stream Symmetry**: STEP solids and assemblies can be opened from or saved to caller-owned streams as well as files.
+- **Unit Preservation**: Export converts supported TVGL units to the millimeter representation declared in the STEP context, including analytic dimensions.
+
+## Export fidelity
+
+Faceted export preserves the TVGL triangle geometry and winding. It does not
+recreate analytic surfaces from primitive metadata. Hybrid export reconstructs
+the validated analytic cases above. Trimmed spheres and tori, cylinders or cones
+with non-circular trim curves, spindle tori, and NURBS regions remain faceted.
+Closed spheres are written as eight exact spherical octants and closed tori as
+sixteen exact analytic patches so their periodic topology is explicit. The
+writer uses a temporary file and replaces the requested destination only after
+the STEP write succeeds.
 

STEPImportExport/README.md

@@ -102,7 +102,12 @@ private static void ReconstructPrimitives(
                 var faceSlice = new List<TriangleFace>(end - start);
                 for (int t = start; t < end; t++)
                 {
-                    faceSlice.Add(solid.Faces[t]);
+                    var face = solid.Faces[t];
+                    // Integrity repair can invalidate negligible triangles
+                    // created by OCCT tessellation. Do not pass those dead
+                    // faces into analytic primitive constructors.
+                    if (face?.A != null && face.B != null && face.C != null)
+                        faceSlice.Add(face);
                 }
 
                 if (faceSlice.Count == 0)

STEPImportExport/STEP.cs

@@ -380,11 +380,22 @@ public enum FileType
         /// </summary>
         TVGLz,
 
+        /// <summary>
+        /// Universal 3D, including the Right Hemisphere Adobe mesh extension.
+        /// </summary>
+        U3D = 19,
+
+        /// <summary>
+        /// PRC (ISO 14739-1), the 3D format embedded in 3D PDFs. Its display tessellation and
+        /// product structure are read.
+        /// </summary>
+        PRC = 20,
+
         // now the 2D formats
         /// <summary>
         /// Simply a set of 2D coordinates, no additional information
         /// </summary>
-        CSV,
+        CSV = 13,
 
         /// <summary>
         /// S Vector Graphics

STEPImportExport/STEPReconstructor.cs

@@ -145,6 +145,12 @@ private static bool Open<T>(Stream s, string filename, out T solid, TessellatedS
                 case FileType.OBJ:
                     solidInner = GetMostSignificantSolid(OBJFileData.OpenSolids(s, filename, tsBuildOptions), out _);
                     break;
+                case FileType.U3D:
+                    solidInner = GetMostSignificantSolid(U3DFileData.OpenSolids(s, filename, tsBuildOptions), out _);
+                    break;
+                case FileType.PRC:
+                    solidInner = GetMostSignificantSolid(PRCFileData.OpenSolidAssembly(s, filename, tsBuildOptions), out _);
+                    break;
                 case FileType.OFF:
                     solidInner = OFFFileData.OpenSolid(s, filename, tsBuildOptions);
                     break;
@@ -310,6 +316,12 @@ public static bool Open(Stream s, string filename, out TessellatedSolid[] tessel
                     case FileType.OBJ:
                         tessellatedSolids = OBJFileData.OpenSolids(s, filename, tsBuildOptions);
                         break;
+                    case FileType.U3D:
+                        tessellatedSolids = U3DFileData.OpenSolids(s, filename, tsBuildOptions);
+                        break;
+                    case FileType.PRC:
+                        tessellatedSolids = PRCFileData.OpenSolids(s, filename, tsBuildOptions);
+                        break;
                     case FileType.OFF:
                     case FileType.PLY_ASCII:
                     case FileType.PLY_Binary:
@@ -409,6 +421,12 @@ public static bool Open(Stream s, string filename, out SolidAssembly solidAssemb
                     case FileType.OBJ:
                         solidAssembly = new SolidAssembly(OBJFileData.OpenSolids(s, filename, tsBuildOptions));
                         break;
+                    case FileType.U3D:
+                        solidAssembly = new SolidAssembly(U3DFileData.OpenSolids(s, filename, tsBuildOptions));
+                        break;
+                    case FileType.PRC:
+                        solidAssembly = PRCFileData.OpenSolidAssembly(s, filename, tsBuildOptions);
+                        break;
 
                     case FileType.OFF:
                     case FileType.PLY_ASCII:
@@ -694,6 +712,8 @@ public static FileType GetFileTypeFromExtension(string extension)
                 case "model": return FileType.Model3MF;
                 case "amf": return FileType.AMF;
                 case "obj": return FileType.OBJ;
+                case "u3d": return FileType.U3D;
+                case "prc": return FileType.PRC;
                 case "off": return FileType.OFF;
                 case "ply": return FileType.PLY_ASCII;
                 case "shell": return FileType.SHELL;
@@ -724,6 +744,8 @@ public static string GetExtensionFromFileType(FileType fileType)
                 case FileType.Model3MF: return "model";
                 case FileType.AMF: return "amf";
                 case FileType.OBJ: return "obj";
+                case FileType.U3D: return "u3d";
+                case FileType.PRC: return "prc";
                 case FileType.OFF: return "off";
                 case FileType.PLY_ASCII:
                 case FileType.PLY_Binary: return "ply";
@@ -1662,4 +1684,4 @@ protected static string TvglDateMarkText
         #endregion Save/Write
         #endregion
     }
-}
+}