Swift 3:如何访问48字节matrix_float3x3中的CFData值？

Question

我试图访问这个回答之后的内在矩阵。

通过运行下面的推荐，我获得了一个48字节的AnyObject，并进一步将其转换为CFData。

let camData = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, nil)

但是，我检查了sampleBuffer在CMSampleBuffer.h中的输出。

/*! @constant   kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix
     @abstract  Indicates the 3x3 camera intrinsic matrix applied to the current sample buffer.
     @discussion Camera intrinsic matrix is a CFData containing a matrix_float3x3, which is column-major.
        ....
 */
CM_EXPORT const CFStringRef kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix  // CFData (matrix_float3x3) __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_11_0);

如何从matrix_float3x3访问CFData中的值

Answer 1

这应该是可行的：

• 使用从CFData到NSData到Data的桥接，以及
• 获取指向数据字节的所需类型指针的withUnsafeBytes方法，
• .pointee以取消指针的引用。

示例：

if let camData = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, nil) as? Data {
    let matrix: matrix_float3x3 = camData.withUnsafeBytes { $0.pointee }

    // ...

}

从上下文推断指针类型(闭包中的$0)为UnsafePointer<matrix_float3x3>。

Answer 2

以下是马丁·R( Martin )的回答的更新，因为withUnsafeBytes已经被否决了：

if let camData = CMGetAttachment(sampleBuffer, 
                                 key: kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, 
                                 attachmentModeOut: nil) as? Data {
    let matrix = camData.withUnsafeBytes { $0.load(as: matrix_float3x3.self) }
    ...
}

Answer 3

这是Martin R对目标-c的回答的版本：

CFTypeRef cameraIntrinsicData = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, nil);
if(cameraIntrinsicData != nil){
    CFDataRef cfdr = (CFDataRef)(cameraIntrinsicData);
    matrix_float3x3 camMatrix = (matrix_float3x3 *)(CFDataGetBytePtr(cfdr));
}