Which format file for 3d model SceneKit/ARKit better to use

Question

I read several tutorials how to place 3d objects in SceneKit/ARKit applications and all of them uses .scn format files for the objects.

But I found there is no any issues if I use original .dae format and do not convert it to .scn format.

I don't really see any difference between .dae and .scn formats.

Actually result seems to me the same but can you explain what the difference between them and what I should use in what cases?

Thank you!

Answer 1

The difference between .dae and .scn formats is significant. Collada .dae (which stands for data asset exchange) is a cross-platform XML-based schema file format used for graphics and visual effects in 3D authoring apps, like Maya or Blender. SceneKit's .scn is Xcode's native file format, which works in iOS and macOS apps. Unfortunately, scene's space, speed and hierarchical structure aren’t the .dae and .scn formats’ strong side. So, the most robust and effective today 3D file format that you can use in SceneKit/ARKit and RealityKit is a USDZ format.

USD family – Universal Scene Description, developed by Pixar. It is a cross-platform Python-based schema file format has the ability to create a 3D scenes by composing many sources files together into successively larger aggregations.

SceneKit formats

SceneKit supports many popular 3D formats. Geometry kept in those formats is visible thru ARSCNView.

• Collada's Digital Asset Exchange .dae

• Pixar's Zipped Universal Scene Description .usdz

• Pixar's ASCII Universal Scene Description .usda

• Pixar's Binary Universal Scene Description .usd, .usdc

• Wavefront Object .obj along with its material description .mtl

• Alembic interchange file format .abc

• Polygon file format .ply

• Stereolithography file format .stl

• Apple proprietary SceneKit Scene format .scn

• To convert .fbx and .glTF files into .usdz use command line.

RealityKit formats

Since 2019, the RealityKit framework has been gaining popularity. It works with the following three file formats:

• Pixar's Zipped Universal Scene Description .usdz

• Apple Reality Composer multi-scene hierarchy .rcproject

• Apple proprietary file format (may include multiple models) .reality

Read about Reality Composer formats .rcproject and .reality here.

Answer 2

DAE (Digital Asset Exchange, aka Collada) is a vendor-neutral format for 3D assets. It supports a wide range of features that exist in multiple 3D authoring and presentation tools, but not every possible feature in SceneKit. Historically, it was the only asset format for early versions of SceneKit.

SCN format is a serialization of the SceneKit object graph. (There are convenience methods for reading/writing it on SCNScene, but really it's the same thing you get by passing an SCNScene to NSKeyedArchiver/NSKeyedUnarchiver.) Thus, it by definition supports all features of SceneKit, including physics, constraints, actions, physically based cameras, and shader modifiers.

If you're using DAE assets, deploying to iOS (or tvOS or watchOS), and not seeing any difference vs using SCN assets, there are two possible reasons:

• Your assets use only those SceneKit features that are available in DAE format.

• When deploying to iOS/tvOS/watchOS, Xcode (via scntool) automatically converts all 3D asset resources to SCN format. (And applies other transformations, like interleaving geometry buffers, for optimal rendering performance on iOS/tvOS/watchOS devices.) The filename in the built app's Resources directory still has a .dae extension, but the file contents are the same as SCN format.

  (SceneKit running in iOS/tvOS/watchOS actually can't read DAE, so it relies on this preprocessing by Xcode.)