v0.1.0-preview.2

First published preview of the scene contract. Names and members may still change before 0.1.0.

Added

• The scene model: SceneDocument with models, elements, instances, geometries, materials, textures, property sets,
  levels, grids, viewpoints, schedules and clash tests. Metres, right-handed, Z up, one coordinate space per
  document; every cross-reference is by id.
• Geometry with a topology (triangles, lines, points), indexed or not, placed by instances with an affine transform.
• Typed property values (SceneValue) with a display string, a number in coherent SI and a unit, and SceneUnits
  with the unit grammar.
• Identity members: SceneModel.Key, SceneElement.SourceId and SceneElement.IfcGuid.
• SceneDocument.Validate() with stable diagnostic codes, ComputeBounds() and the AddMesh quick-start helper.
• SceneJson: scene JSON of kind ioscene, format 1, with stream and file methods. Math values are number arrays,
  vertex lists are flat, textures are base64. Files of another kind or of a newer format are rejected; unknown
  members are ignored. SceneJson.CreateOptions() gives the serializer rules of scene JSON to a document of another
  kind that holds a scene; the parents-first order of elements and the geometry size check of Write are not part
  of them.
• Math types Vector2d, Vector3d, BoundingBox3d, Transform3d and RgbaColor.
• docs/contract.md (the specification) and docs/keys.md (the registry of property key namespaces).

Changed since 0.1.0-preview.1, which was not published

• SceneClashResult.Distance is double?: null means not reported, below 0 is the penetration depth and from 0 up
  the clearance, in metres. A missing Distance in a file reads as null.
• Validate() reports id-range for an element id outside -(2^53 - 1) to 2^53 - 1, also where an element,
  instance, schedule task or clash names one, and checks a clash distance only when it is there.
• SceneTaskType and SceneProjection are open enums: a consumer treats an unknown value as Construct and as
  Perspective.
• The contract fixes colour (sRGB, straight alpha, colour times texture), texture coordinates, Models[0], ParentId
  by Kind, the property lookup by key, the meaning of a null Unit, SceneTask.Progress from 0 to 1, grid line
  elevation and the frozen IfcGuid derivation. A per-instance index range is never added. Large vertex coordinates
  are allowed; a consumer rebases them before it narrows them to 32-bit floats.
• docs/keys.md: keys are compared whole and never parsed back, a lookup by key takes the first set, and
  SourceFormat is the id of the content format (ifc covers .ifc and .ifczip).