如何在Objective-C中从CGAffineTransform创建SCNMatrix4？

我无法在Objective-C中实现从CGAffineTransform创建SCNMatrix4的功能，以下是想要迁移到Objective-C的Swift代码：

let affineTransform = frame.displayTransform(for: .portrait, viewportSize: sceneView.bounds.size)
let transform = SCNMatrix4(affineTransform)
faceGeometry.setValue(SCNMatrix4Invert(transform), forKey: "displayTransform")

我已经实现了第一行和第三行代码，但找不到在Objective-C中从CGAffineTransform创建SCNMatrix4的方法：

CGAffineTransform affine = [self.sceneView.session.currentFrame displayTransformForOrientation:UIInterfaceOrientationPortrait viewportSize:self.sceneView.bounds.size];
SCNMatrix4 trans = ??
[f setValue:SCNMatrix4Invert(trans) forKey:@"displayTransform"];

不存在SCNMatrix4Make方法，尝试过simd_matrix4x4也无效。

背景说明 #

上述Swift代码来自苹果示例项目「ARKitFaceExample」，完整代码如下：

/*
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
    }
    
}

转换逻辑说明 #

Swift中通过扩展SCNMatrix4实现了转换逻辑，代码如下：

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
    }
}

Objective-C实现方案 #

对应的Objective-C实现可通过自定义函数完成：

SCNMatrix4 SCNMatrix4FromCGAffineTransform(CGAffineTransform affineTransform) {
    SCNMatrix4 matrix = SCNMatrix4Identity;
    matrix.m11 = (float)affineTransform.a;
    matrix.m12 = (float)affineTransform.b;
    matrix.m21 = (float)affineTransform.c;
    matrix.m22 = (float)affineTransform.d;
    matrix.m41 = (float)affineTransform.tx;
    matrix.m42 = (float)affineTransform.ty;
    return matrix;
}

使用时直接调用该函数即可：

CGAffineTransform affine = [self.sceneView.session.currentFrame displayTransformForOrientation:UIInterfaceOrientationPortrait viewportSize:self.sceneView.bounds.size];
SCNMatrix4 trans = SCNMatrix4FromCGAffineTransform(affine);
[f setValue:SCNMatrix4Invert(trans) forKey:@"displayTransform"];

内容的提问来源于stack exchange，提问作者IamRob