AVAssetReader / AVAssetWriter 加入不同分辨率的mp4文件

Question

我正在编写一个 iPad 应用程序，我需要在其中加入不同分辨率的 mp4 文件。为此，我使用 AVAssetReader 的组合来读取 mp4 源文件，并使用 AVAssetWriter 将这些源文件写入单个 mp4 输出文件中。

我尝试使用 AVAssetExportSession 但我遇到的问题是不同的连接文件之间存在黑框。

我现在面临的问题是，一切看起来都正常，但 AVAssetWriter 的完成处理程序从未被调用。

这是我的选择器，将 mp4 文件 URL 列表、单个输出文件 URL 和完成处理程序作为输入。

- (void)resizeAndJoinVideosAtURLs:(NSArray *)videoURLs toOutputURL:(NSURL *)outputURL withHandler:(void(^)(NSURL *fileURL))handler
{
    /*
     First step: create the writer and writer input
     */
    NSError *error = nil;
    self.videoAssetWriter = [[AVAssetWriter alloc] initWithURL:outputURL fileType:AVFileTypeMPEG4 error:&error];

    NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:AVVideoCodecH264, AVVideoCodecKey,[NSNumber numberWithInt:640], AVVideoWidthKey,[NSNumber numberWithInt:480], AVVideoHeightKey,nil];

    AVAssetWriterInput* videoWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
    videoWriterInput.expectsMediaDataInRealTime = NO;

    if([self.videoAssetWriter canAddInput:videoWriterInput])
    {
        [self.videoAssetWriter addInput:videoWriterInput];
        [self.videoAssetWriter startWriting];
        [self.videoAssetWriter startSessionAtSourceTime:kCMTimeZero];

        /*
         Second step: for each video URL given create a reader and an reader input
         */

        for(NSURL *videoURL in videoURLs)
        {
            NSLog(@"Processing file: %@",videoURL);
            AVAsset *videoAsset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];
            AVAssetReader *videoAssetReader = [[AVAssetReader alloc] initWithAsset:videoAsset error:&error];
            AVAssetTrack *videoAssetTrack = [videoAsset tracksWithMediaType:AVMediaTypeVideo].firstObject;
            NSDictionary *videoOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] forKey:(id)kCVPixelBufferPixelFormatTypeKey];

            AVAssetReaderTrackOutput *videoAssetTrackOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:videoAssetTrack outputSettings:videoOptions];
            videoAssetTrackOutput.alwaysCopiesSampleData = NO;

            if([videoAssetReader canAddOutput:videoAssetTrackOutput])
            {
                [videoAssetReader addOutput:videoAssetTrackOutput];
                [videoAssetReader startReading];

                /*
                 Step three: copy the buffers from the reader to the writer
                 */
                while ([videoAssetReader status] == AVAssetReaderStatusReading)
                {
                    if(![videoWriterInput isReadyForMoreMediaData]) continue;

                    CMSampleBufferRef buffer = [videoAssetTrackOutput copyNextSampleBuffer];
                    if(buffer)
                    {
                        [videoWriterInput appendSampleBuffer:buffer];
                        CFRelease(buffer);
                    }
                }


            } else NSLog(@"ERROR: %@",error);
        }

       [videoWriterInput markAsFinished];

    } else NSLog(@"ERROR: %@",error);

    __weak ClipBuilder *weakself = self;
    [self.videoAssetWriter finishWritingWithCompletionHandler:^{
        handler(outputURL);
        weakself.videoAssetWriter = nil;
    }];
}

我的输出文件存在并且 AVAssetWriter 存在，因为它是一个属性，但仍然没有调用完成处理程序。这能用什么解释呢？

感谢您的帮助。

这能解释什么吗？

Answer 1

这是我最终实现的解决方案，通过 AVAssetReader / AVAssetWriter 的组合来加入不同分辨率的 mp4 文件。

- (void)reencodeComposition:(AVComposition *)composition toMP4File:(NSURL *)mp4FileURL withCompletionHandler:(void (^)(void))handler
{
    self.status = EncoderStatusEncoding;

    /*
     Create the asset writer to write the file on disk
     */

    NSError *error = nil;
    if([[NSFileManager defaultManager] fileExistsAtPath:mp4FileURL.path isDirectory:nil])
    {
        if(![[NSFileManager defaultManager] removeItemAtPath:mp4FileURL.path error:&error])
        {
            [self failWithError:error withCompletionHandler:handler];
            return;
        }
    }

    self.assetWriter = [[AVAssetWriter alloc] initWithURL:mp4FileURL fileType:AVFileTypeMPEG4 error:&error];

    if(self.assetWriter)
    {
        /*
         Get the audio and video track of the composition
         */
        AVAssetTrack *videoAssetTrack = [composition tracksWithMediaType:AVMediaTypeVideo].firstObject;
        AVAssetTrack *audioAssetTrack = [composition tracksWithMediaType:AVMediaTypeAudio].firstObject;

        NSDictionary *videoSettings = @{AVVideoCodecKey:AVVideoCodecH264, AVVideoWidthKey:@(self.imageWidth), AVVideoHeightKey:@(self.imageHeight)};

        /*
         Add an input to be able to write the video in the file
         */
        AVAssetWriterInput* videoWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
        videoWriterInput.expectsMediaDataInRealTime = YES;

        if([self.assetWriter canAddInput:videoWriterInput])
        {
            [self.assetWriter addInput:videoWriterInput];

            /*
             Add an input to be able to write the audio in the file
             */
// Use this only if you know the format
//            CMFormatDescriptionRef audio_fmt_desc_ = nil;
//
//            AudioStreamBasicDescription audioFormat;
//            bzero(&audioFormat, sizeof(audioFormat));
//            audioFormat.mSampleRate = 44100;
//            audioFormat.mFormatID   = kAudioFormatMPEG4AAC;
//            audioFormat.mFramesPerPacket = 1024;
//            audioFormat.mChannelsPerFrame = 2;
//            int bytes_per_sample = sizeof(float);
//            audioFormat.mFormatFlags = kAudioFormatFlagIsFloat | kAudioFormatFlagIsPacked;
//            
//            audioFormat.mBitsPerChannel = bytes_per_sample * 8;
//            audioFormat.mBytesPerPacket = bytes_per_sample * 2;
//            audioFormat.mBytesPerFrame = bytes_per_sample * 2;
//            
//            CMAudioFormatDescriptionCreate(kCFAllocatorDefault,&audioFormat,0,NULL,0,NULL,NULL,&audio_fmt_desc_);
//            
//             AVAssetWriterInput* audioWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:nil sourceFormatHint:audio_fmt_desc_];
//            
//            AVAssetWriterInput* audioWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:nil sourceFormatHint:((__bridge CMAudioFormatDescriptionRef)audioAssetTrack.formatDescriptions.firstObject)];

            audioWriterInput.expectsMediaDataInRealTime = YES;

            if([self.assetWriter canAddInput:audioWriterInput])
            {
                [self.assetWriter addInput:audioWriterInput];
                [self.assetWriter startWriting];
                [self.assetWriter startSessionAtSourceTime:kCMTimeZero];

                /*
                 Create the asset reader to read the mp4 files on the disk
                 */
                AVAssetReader *assetReader = [[AVAssetReader alloc] initWithAsset:composition error:&error];
                NSDictionary *videoOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] forKey:(id)kCVPixelBufferPixelFormatTypeKey];

                /*
                 Add an output to be able to retrieve the video in the files
                 */
                AVAssetReaderTrackOutput *videoAssetTrackOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:videoAssetTrack outputSettings:videoOptions];
                videoAssetTrackOutput.alwaysCopiesSampleData = NO;

                if([assetReader canAddOutput:videoAssetTrackOutput])
                {
                    [assetReader addOutput:videoAssetTrackOutput];
                    /*
                     Add an output to be able to retrieve the video in the files
                     */
                    AVAssetReaderTrackOutput *audioAssetTrackOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:audioAssetTrack outputSettings:nil];
                    videoAssetTrackOutput.alwaysCopiesSampleData = NO;

                    if([assetReader canAddOutput:audioAssetTrackOutput])
                    {
                        [assetReader addOutput:audioAssetTrackOutput];

                        [assetReader startReading];

                        /*
                         Read the mp4 files until the end and copy them in the output file
                         */
                        dispatch_group_t encodingGroup = dispatch_group_create();

                        dispatch_group_enter(encodingGroup);
                        [audioWriterInput requestMediaDataWhenReadyOnQueue:self.encodingQueue usingBlock:^{
                            while ([audioWriterInput isReadyForMoreMediaData])
                            {
                                CMSampleBufferRef nextSampleBuffer = [audioAssetTrackOutput copyNextSampleBuffer];

                                if (nextSampleBuffer)
                                {
                                    [audioWriterInput appendSampleBuffer:nextSampleBuffer];
                                    CFRelease(nextSampleBuffer);
                                }
                                else
                                {
                                    [audioWriterInput markAsFinished];
                                    dispatch_group_leave(encodingGroup);
                                    break;
                                }
                            }
                        }];

                        dispatch_group_enter(encodingGroup);
                        [videoWriterInput requestMediaDataWhenReadyOnQueue:self.encodingQueue usingBlock:^{
                            while ([videoWriterInput isReadyForMoreMediaData])
                            {
                                CMSampleBufferRef nextSampleBuffer = [videoAssetTrackOutput copyNextSampleBuffer];

                                if (nextSampleBuffer)
                                {
                                    [videoWriterInput appendSampleBuffer:nextSampleBuffer];
                                    CFRelease(nextSampleBuffer);
                                }
                                else
                                {
                                    [videoWriterInput markAsFinished];
                                    dispatch_group_leave(encodingGroup);
                                    break;
                                }
                            }
                        }];

                        dispatch_group_wait(encodingGroup, DISPATCH_TIME_FOREVER);

                    } else [self failWithError:error withCompletionHandler:handler];
                } else [self failWithError:error withCompletionHandler:handler];
            } else [self failWithError:error withCompletionHandler:handler];
        } else [self failWithError:error withCompletionHandler:handler];

        __weak Encoder *weakself = self;
        [self.assetWriter finishWritingWithCompletionHandler:^{
            self.status = EncoderStatusCompleted;
            handler();
            weakself.assetWriter = nil;
            self.encodingQueue = nil;
        }];
    }
    else [self failWithError:error withCompletionHandler:handler];
}

和

- (dispatch_queue_t)encodingQueue
{
    if(!_encodingQueue)
    {
        _encodingQueue = dispatch_queue_create("com.myProject.encoding", NULL);
    }
    return _encodingQueue;
}

这个实现是针对我的项目TS2MP4的，但我最终不需要它。