我正在使用AVCaptureSession从设备摄像头捕获视频，然后使用AVAssetWriterInput和AVAssetTrack压缩/调整视频大小，然后再将其上传到服务器。最终视频将通过html5视频元素在网上观看。
我遇到了多个问题，试图让视频的方向正确。我的应用程序仅支持横向，所有捕获的视频都应横向显示。但是，我想允许用户将设备保持在横向方向（即左侧或右侧的主页按钮）。
我能够使用以下代码行以正确的方向显示视频预览
_previewLayer.connection.videoOrientation = UIDevice.currentDevice.orientation;
通过AVAssetWriterInput和朋友处理视频时问题就出现了。结果似乎并未考虑视频捕获的左右横向模式.IOW，有时视频会颠倒过来。经过一些谷歌搜索后，我发现很多人建议以下代码行解决这个问题
writerInput.transform = videoTrack.preferredTransform;
......但这似乎不起作用。经过一些调试后，我发现videoTrack.preferredTransform始终是相同的值，无论视频的捕获方向如何。
我尝试手动跟踪视频的捕获方向，并根据需要将writerInput.transform设置为CGAffineTransformMakeRotation(M_PI)。这解决了问题!!!
...几分
当我在设备上查看结果时，此解决方案按预期工作。录制时，无论左右方向如何，视频都是正面向上的。不幸的是，当我在另一个浏览器中观看完全相同的视频时（Mac书上的chrome），它们都是颠倒的！？！？！？
我做错了什么？
修改
这是一些代码，如果它有用......
-(void)compressFile:(NSURL*)inUrl;
{
NSString* fileName = [@"compressed." stringByAppendingString:inUrl.lastPathComponent];
NSError* error;
NSURL* outUrl = [PlatformHelper getFilePath:fileName error:&error];
NSDictionary* compressionSettings = @{ AVVideoProfileLevelKey: AVVideoProfileLevelH264Main31,
AVVideoAverageBitRateKey: [NSNumber numberWithInt:2500000],
AVVideoMaxKeyFrameIntervalKey: [NSNumber numberWithInt: 30] };
NSDictionary* videoSettings = @{ AVVideoCodecKey: AVVideoCodecH264,
AVVideoWidthKey: [NSNumber numberWithInt:1280],
AVVideoHeightKey: [NSNumber numberWithInt:720],
AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
AVVideoCompressionPropertiesKey: compressionSettings };
NSDictionary* videoOptions = @{ (id)kCVPixelBufferPixelFormatTypeKey: [NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange] };
AVAssetWriterInput* writerInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
writerInput.expectsMediaDataInRealTime = YES;
AVAssetWriter* assetWriter = [AVAssetWriter assetWriterWithURL:outUrl fileType:AVFileTypeMPEG4 error:&error];
assetWriter.shouldOptimizeForNetworkUse = YES;
[assetWriter addInput:writerInput];
AVURLAsset* asset = [AVURLAsset URLAssetWithURL:inUrl options:nil];
AVAssetTrack* videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
// !!! this line does not work as expected and causes all sorts of issues (videos display sideways in some cases) !!!
//writerInput.transform = videoTrack.preferredTransform;
AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoTrack outputSettings:videoOptions];
AVAssetReader* assetReader = [AVAssetReader assetReaderWithAsset:asset error:&error];
[assetReader addOutput:readerOutput];
[assetWriter startWriting];
[assetWriter startSessionAtSourceTime:kCMTimeZero];
[assetReader startReading];
[writerInput requestMediaDataWhenReadyOnQueue:_processingQueue usingBlock:
^{
/* snip */
}];
}
答案 0 :(得分：8)
问题是修改writerInput.transform属性只会在视频文件元数据中添加一个标记，指示视频播放器在播放期间旋转文件。这就是为什么视频在你的设备上以正确的方向播放的原因（我猜他们也可以在Quicktime播放器中正确播放）。
相机捕获的像素缓冲区仍然以捕获它们的方向布局。许多视频播放器不会检查首选方向元数据标签，只会以原始像素方向播放文件。
如果您希望用户能够以横向模式录制持有手机的视频，则需要在压缩前通过对每个视频帧的CVPixelBuffer执行变换，在AVCaptureSession级别对此进行纠正。 Apple Q＆amp; A涵盖了它（请查看AVCaptureVideoOutput文档）：
https://developer.apple.com/library/ios/qa/qa1744/_index.html
调查上述链接是解决问题的正确方法。另一种解决同样问题的 fast n'dirty 方法是将应用程序的录制UI锁定为只有一个横向，然后使用ffmpeg旋转所有视频服务器端。
答案 1 :(得分：1)
为了压缩/调整视频大小，我们可以使用AVAssetExportSession。
{
-(void)trimVideoWithURL:(NSURL *)inputURL{
NSString *path1 = [inputURL path];
NSData *data = [[NSFileManager defaultManager] contentsAtPath:path1];
NSLog(@"size before compress video is %lu",(unsigned long)data.length);
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:inputURL options:nil];
AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPreset640x480];
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *outputURL = paths[0];
NSFileManager *manager = [NSFileManager defaultManager];
[manager createDirectoryAtPath:outputURL withIntermediateDirectories:YES attributes:nil error:nil];
outputURL = [outputURL stringByAppendingPathComponent:@"output.mp4"];
fullPath = [NSURL URLWithString:outputURL];
// Remove Existing File
[manager removeItemAtPath:outputURL error:nil];
exportSession.outputURL = [NSURL fileURLWithPath:outputURL];
exportSession.shouldOptimizeForNetworkUse = YES;
exportSession.outputFileType = AVFileTypeQuickTimeMovie;
CMTime start = CMTimeMakeWithSeconds(1.0, 600);
CMTime duration = CMTimeMakeWithSeconds(1.0, 600);
CMTimeRange range = CMTimeRangeMake(start, duration);
exportSession.timeRange = range;
[exportSession exportAsynchronouslyWithCompletionHandler:^(void)
{
switch (exportSession.status) {
case AVAssetExportSessionStatusCompleted:{
NSString *path = [fullPath path];
NSData *data = [[NSFileManager defaultManager] contentsAtPath:path];
NSLog(@"size after compress video is %lu",(unsigned long)data.length);
NSLog(@"Export Complete %d %@", exportSession.status, exportSession.error);
/*
Do your neccessay stuff here after compression
*/
}
break;
case AVAssetExportSessionStatusFailed:
NSLog(@"Failed:%@",exportSession.error);
break;
case AVAssetExportSessionStatusCancelled:
NSLog(@"Canceled:%@",exportSession.error);
break;
default:
break;
}
}];}
答案 2 :(得分：0)
如果它对任何人都有帮助，这里是我最终得到的代码。我最终不得不对视频进行处理，而不是作为后期处理步骤。这是一个管理捕获的辅助类。
接口
#import <Foundation/Foundation.h>
#import <AVFoundation/AVFoundation.h>
@interface VideoCaptureManager : NSObject<AVCaptureVideoDataOutputSampleBufferDelegate>
{
AVCaptureSession* _captureSession;
AVCaptureVideoPreviewLayer* _previewLayer;
AVCaptureVideoDataOutput* _videoOut;
AVCaptureDevice* _videoDevice;
AVCaptureDeviceInput* _videoIn;
dispatch_queue_t _videoProcessingQueue;
AVAssetWriter* _assetWriter;
AVAssetWriterInput* _writerInput;
BOOL _isCapturing;
NSString* _gameId;
NSString* _authToken;
}
-(void)setSettings:(NSString*)gameId authToken:(NSString*)authToken;
-(void)setOrientation:(AVCaptureVideoOrientation)orientation;
-(AVCaptureVideoPreviewLayer*)getPreviewLayer;
-(void)startPreview;
-(void)stopPreview;
-(void)startCapture;
-(void)stopCapture;
@end
实施（带一点编辑和一些小TODO）
@implementation VideoCaptureManager
-(id)init;
{
self = [super init];
if (self) {
NSError* error;
_videoProcessingQueue = dispatch_queue_create("VideoQueue", DISPATCH_QUEUE_SERIAL);
_captureSession = [AVCaptureSession new];
_videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
_previewLayer = [AVCaptureVideoPreviewLayer layerWithSession:_captureSession];
[_previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
_videoOut = [AVCaptureVideoDataOutput new];
_videoOut.videoSettings = @{ (id)kCVPixelBufferPixelFormatTypeKey: [NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange] };
_videoOut.alwaysDiscardsLateVideoFrames = YES;
_videoIn = [AVCaptureDeviceInput deviceInputWithDevice:_videoDevice error:&error];
// handle errors here
[_captureSession addInput:_videoIn];
[_captureSession addOutput:_videoOut];
}
return self;
}
-(void)setOrientation:(AVCaptureVideoOrientation)orientation;
{
_previewLayer.connection.videoOrientation = orientation;
for (AVCaptureConnection* item in _videoOut.connections) {
item.videoOrientation = orientation;
}
}
-(AVCaptureVideoPreviewLayer*)getPreviewLayer;
{
return _previewLayer;
}
-(void)startPreview;
{
[_captureSession startRunning];
}
-(void)stopPreview;
{
[_captureSession stopRunning];
}
-(void)startCapture;
{
if (_isCapturing) return;
NSURL* url = put code here to create your output url
NSDictionary* compressionSettings = @{ AVVideoProfileLevelKey: AVVideoProfileLevelH264Main31,
AVVideoAverageBitRateKey: [NSNumber numberWithInt:2500000],
AVVideoMaxKeyFrameIntervalKey: [NSNumber numberWithInt: 1],
};
NSDictionary* videoSettings = @{ AVVideoCodecKey: AVVideoCodecH264,
AVVideoWidthKey: [NSNumber numberWithInt:1280],
AVVideoHeightKey: [NSNumber numberWithInt:720],
AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
AVVideoCompressionPropertiesKey: compressionSettings
};
_writerInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
_writerInput.expectsMediaDataInRealTime = YES;
NSError* error;
_assetWriter = [AVAssetWriter assetWriterWithURL:url fileType:AVFileTypeMPEG4 error:&error];
// handle errors
_assetWriter.shouldOptimizeForNetworkUse = YES;
[_assetWriter addInput:_writerInput];
[_videoOut setSampleBufferDelegate:self queue:_videoProcessingQueue];
_isCapturing = YES;
}
-(void)stopCapture;
{
if (!_isCapturing) return;
[_videoOut setSampleBufferDelegate:nil queue:nil]; // TODO: seems like there could be a race condition between this line and the next (could end up trying to write a buffer after calling writingFinished
dispatch_async(_videoProcessingQueue, ^{
[_assetWriter finishWritingWithCompletionHandler:^{
[self writingFinished];
}];
});
}
-(void)writingFinished;
{
// TODO: need to check _assetWriter.status to make sure everything completed successfully
// do whatever post processing you need here
}
-(void)captureOutput:(AVCaptureOutput*)captureOutput didDropSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection*)connection;
{
NSLog(@"Video frame was dropped.");
}
-(void)captureOutput:(AVCaptureOutput*)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
if(_assetWriter.status != AVAssetWriterStatusWriting) {
CMTime lastSampleTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
[_assetWriter startWriting]; // TODO: need to check the return value (a bool)
[_assetWriter startSessionAtSourceTime:lastSampleTime];
}
if (!_writerInput.readyForMoreMediaData || ![_writerInput appendSampleBuffer:sampleBuffer]) {
NSLog(@"Failed to write video buffer to output.");
}
}
@end