iphone - Avfoundation 通过录制创建迷你剪辑

Question

我想知道如何在不停止录制的情况下每隔一定时间从录制中创建迷你视频？我试图为视频寻找与 AvAssetImageGenerator 等效的示例，这样会很好。

Answer 1

最简单的方法是使用两个AVAssetWriters并在当前记录时设置下一个写入器，然后在 x 次后停止并交换写入器。您应该能够在不丢失任何帧的情况下交换编写器。

编辑：

如何进行AVAssetWriter“杂耍”

第 1 步：为编写器和像素缓冲区适配器创建实例对象（您还需要这些文件的文件名）

AVAssetWriter*                        mWriter[2];
AVAssetWriterInputPixelBufferAdaptor* mPBAdaptor[2];
NSString*                             mOutFile[2];
int                                   mCurrentWriter, mFrameCount, mTargetFrameCount;

第 2 步：创建一个设置 writer 的方法（因为您将一遍又一遍地这样做）

-(int) setupWriter: (int) writer
{
NSAutoreleasePool* p = [[NSAutoreleasePool alloc] init];

NSDictionary* writerSettings = [NSDictionary dictionaryWithObjectsAndKeys: AVVideoCodecH264, AVVideoCodecKey, [NSNumber numberWithInt: mVideoWidth], AVVideoWidthKey, [NSNumber numberWithInt: mVideoHeight], AVVideoHeightKey, nil];
NSDictionary* pbSettings = [NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:mVivdeoWidth],kCVPixelBufferWidthKey,
                        [NSNumber numberWithInt:mVideoHeight], kCVPixelBufferHeightKey,
                        [NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsLeftKey,
                        [NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsRightKey,
                        [NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsTopKey,
                        [NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsBottomKey,
                        [NSNumber numberWithInt:mVideoWidth],kCVPixelBufferBytesPerRowAlignmentKey,
                        [NSNumber numberWithUnsignedInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange], kCVPixelBufferPixelFormatTypeKey, nil];

AVAssetWriterInput* writerInput = [AVAssetWriterInput assetWriterWithMediaType: AVMediaTypeVideo outputSettings: writerSettings];
// Create an audio input here if you want...
mWriter[writer] = [[AVAssetWriter alloc] initWithURL: [NSURL fileURLWithPath:mOutfile[writer]] fileType: AVFileTypeMPEG4 error:nil];

mPBAdaptor[writer] = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput: writerInput sourcePixelBufferAttributes: pbSettings];

[mWriter[writer] addInput: writerInput]; 
// Add your audio input here if you want it
[p release];
}

第 3 步：必须拆掉这些东西！

- (void) tearDownWriter: (int) writer 
{
if(mWriter[writer]) {

   if(mWriter[writer].status == 1) [mWriter[writer] finishWriting]; // This will complete the movie.
   [mWriter[writer] release]; mWriter[writer] = nil;
   [mPBAdaptor[writer] release]; mPBAdaptor[writer] = nil;
}

}

第4步：交换！拆除当前的 writer 并在另一个 writer 正在写入时异步重新创建它。

- (void) swapWriters
{
 NSAutoreleasePool * p = [[NSAutoreleasePool alloc] init];
 if(++mFrameCount > mCurrentTargetFrameCount) 
 { 
    mFrameCount = 0;
    int c, n;
    c = mCurrentWriter^1;
    n = mCurrentWriter;  // swap.

    [self tearDownWriter:n];
    __block VideoCaptureClass* bSelf = self;
    dispatch_async(dispatch_get_global_queue(0,0), ^{
      [bSelf setupWriter:n];
              CMTime time;
    time.value = 0;
    time.timescale = 15; // or whatever the correct timescale for your movie is
    time.epoch = 0;
    time.flags = kCMTimeFlags_Valid;
    [bSelf->mWriter[n] startWriting];
    [bSelf->mWriter[n] startSessionAtSourceTime:time];
    });
    mCurrentWriter = c;
 } 

[p release];
}

注意：启动时，您必须创建并启动两个编写器。

第 5 步：捕获输出

   - (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
   // This method will only work with video; you'll have to check for audio if you're using that.
   CMTime time = CMSampleBufferGetPresentationTimeStamp(sampleBuffer); // Note: you may have to create your own PTS.

   CVImageBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

   [mPBAdaptor[mCurrentWriter] appendPixelBuffer:pixelBuffer withPresentationTime: time];

   [self swapBuffers];
  }

如果不需要，您可以跳过像素缓冲区适配器。这应该让您大致了解如何做您想做的事情。 mTargetFrameCount表示您希望当前视频的长度为多少帧。音频可能需要额外考虑，如果您使用音频，您可能希望基于音频流而不是视频流来确定长度。