Swift -从CMSampleBuffer获取kCMSampleBufferAttachmentKey_DroppedFrameReason

Question

我想知道为什么我的AVCaptureOutput会丢帧。在captureOutput(_ output: AVCaptureOutput, didDrop sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection)委托方法中，我得到了一个CMSampleBuffer，它应该包含一个附件，解释帧被丢弃的原因(doc)

原因预计是其中之一的CFString：

kCMSampleBufferDroppedFrameReason_FrameWasLate // "FrameWasLate"
kCMSampleBufferDroppedFrameReason_OutOfBuffers // "OutOfBuffers"
kCMSampleBufferDroppedFrameReason_Discontinuity // "Discontinuity"

从文档中看，真的不清楚如何获得这个值。我尝试过使用CMGetAttachment，但这返回了一个CMAttachmentMode，也就是UInt32

func captureOutput(_ output: AVCaptureOutput, didDrop sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
  var reason: CMAttachmentMode = 0
  CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_DroppedFrameReason, &reason)
  print("reason \(reason)") // 1
}

我真的不知道如何将这个UInt32与CFString常量相匹配

Answer 1

我愚蠢地没有看到正确的输出：

var mode: CMAttachmentMode = 0
let reason = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_DroppedFrameReason, &mode)
print("reason \(String(describing: reason))") // Optional(OutOfBuffers)