Converting SCNMatrix4 from swift to Objective-c - swift

Question

I can not for the life of me figure out how to create a SCNMatrix4 from a transform in objective-c.
The swift code I'm trying to use in objective-c:
let affineTransform = frame.displayTransform(for: .portrait, viewportSize: sceneView.bounds.size)
let transform = SCNMatrix4(affineTransform)
faceGeometry.setValue(SCNMatrix4Invert(transform), forKey: "displayTransform")
I got the first and third line but I can't find anyway to create this SCNMatrix4 from the CGAffineTransform.
CGAffineTransform affine = [self.sceneView.session.currentFrame displayTransformForOrientation:UIInterfaceOrientationPortrait viewportSize:self.sceneView.bounds.size];
SCNMatrix4 trans = ??
[f setValue:SCNMatrix4Invert(trans) forKey:#"displayTransform"];
There is no SCNMatrix4Make, I tried simd_matrix4x4 but that didn't seem to work either.
Thank you
edit:
The swift code is from Apples Example project "ARKitFaceExample", this is the full code:
/*
See LICENSE folder for this sample’s licensing information.
Abstract:
Demonstrates using video imagery to texture and modify the face mesh.
*/
import ARKit
import SceneKit
/// - Tag: VideoTexturedFace
class VideoTexturedFace: TexturedFace {
override func renderer(_ renderer: SCNSceneRenderer, nodeFor anchor: ARAnchor) -> SCNNode? {
guard let sceneView = renderer as? ARSCNView,
let frame = sceneView.session.currentFrame,
anchor is ARFaceAnchor
else { return nil }
#if targetEnvironment(simulator)
#error("ARKit is not supported in iOS Simulator. Connect a physical iOS device and select it as your Xcode run destination, or select Generic iOS Device as a build-only destination.")
#else
// Show video texture as the diffuse material and disable lighting.
let faceGeometry = ARSCNFaceGeometry(device: sceneView.device!, fillMesh: true)!
let material = faceGeometry.firstMaterial!
material.diffuse.contents = sceneView.scene.background.contents
material.lightingModel = .constant
guard let shaderURL = Bundle.main.url(forResource: "VideoTexturedFace", withExtension: "shader"),
let modifier = try? String(contentsOf: shaderURL)
else { fatalError("Can't load shader modifier from bundle.") }
faceGeometry.shaderModifiers = [ .geometry: modifier]
// Pass view-appropriate image transform to the shader modifier so
// that the mapped video lines up correctly with the background video.
let affineTransform = frame.displayTransform(for: .portrait, viewportSize: sceneView.bounds.size)
let transform = SCNMatrix4(affineTransform)
faceGeometry.setValue(SCNMatrix4Invert(transform), forKey: "displayTransform")
contentNode = SCNNode(geometry: faceGeometry)
#endif
return contentNode
}
}
In case anyone ever needs this, here is the extension I was missing
extension SCNMatrix4 {
/**
Create a 4x4 matrix from CGAffineTransform, which represents a 3x3 matrix
but stores only the 6 elements needed for 2D affine transformations.
[ a b 0 ] [ a b 0 0 ]
[ c d 0 ] -> [ c d 0 0 ]
[ tx ty 1 ] [ 0 0 1 0 ]
. [ tx ty 0 1 ]
Used for transforming texture coordinates in the shader modifier.
(Needs to be SCNMatrix4, not SIMD float4x4, for passing to shader modifier via KVC.)
*/
init(_ affineTransform: CGAffineTransform) {
self.init()
m11 = Float(affineTransform.a)
m12 = Float(affineTransform.b)
m21 = Float(affineTransform.c)
m22 = Float(affineTransform.d)
m41 = Float(affineTransform.tx)
m42 = Float(affineTransform.ty)
m33 = 1
m44 = 1
}
}

Answer 1

To replicate the Swift extension for creating an SCNMatrix4 from a CGAffineTransform you can implement the following function:
Some .h file:
extern SCNMatrix4 SCNMatrix4FromTransform(CGAffineTransform transform);
Some .m file:
SCNMatrix4 SCNMatrix4FromTransform(CGAffineTransform transform) {
SCNMatrix4 matrix;
matrix.m11 = transform.a;
matrix.m12 = transform.b;
matrix.m21 = transform.c;
matrix.m22 = transform.d;
matrix.m41 = transform.tx;
matrix.m42 = transform.ty;
matrix.m33 = 1;
matrix.m44 = 1;
return matrix;
}
Then your code becomes:
CGAffineTransform affineTransform = [self.sceneView.session.currentFrame displayTransformForOrientation:UIInterfaceOrientationPortrait viewportSize:self.sceneView.bounds.size];
SCNMatrix4 transform = SCNMatrixFromTransform(affineTransform);
[f setValue:[NSValue valueWithSCNMatrix4:SCNMatrix4Invert(transform)] forKey:#"displayTransform"];
Note the use of NSValue valueWithSCNMatrix4:. This is needed to convert the struct to an object and should satisfy the use of KVC for setting the displayTransform property.