Search code examples
swiftaugmented-realityarkitrealitykit

ARKit 4, RealityKit - Converting local transform to world transform

In ARKit 4 with RealityKit, one could find, for example, left hand transform relatively to skeleton base (hip). With lefthand transform (relative to hip) and hip transform (relative to world), how to calculate the lefthand transform relative to world? Where is the API? It seems that when I figured out the math formula, I could use SIMD api. But I guess there should be a simple API do this kind of math? Thanks.

EDIT: Adding some code to make it clear ..

guard let bodyAnchor = anchor as? ARBodyAnchor else { continue }
let skeleton = bodyArchor.skeleton
let leftHandModelTransform = skeleton.modelTransform(for: .leftHand)

// at this point. I have access to:
// hip transform (bodyArchor.tranform) which is in the world coordinate system;
// lefthand transform which is relative to hip of the skeleton model. 
// what to do next? any api? or do I have to figureout math and then use simd apis ? 
//

EDIT: also, could someone recommend a book / web page describing how to use SIMD in different 3D computation? I found this post(Augmented Reality 911 — Transform Matrix 4x4) really useful. But it did not touch upon conversion between two vector spaces.

Solution

  • Ok. After reviewing all the math about matrix, 3D transformation, and quaternion, I finally found a solution! It is actually buried in RealityKit API:

    func convert(normal: SIMD3<Float>, from referenceEntity: Entity?) -> SIMD3<Float>

    I had to create two entities to use this API. But at least it works ...