iOS视频重编码
移动端编码无外乎两种：
- 软编码,利用CPU来对视频做编码和解码的，但效率不高.（大部分移动端通过FFMpeg来实现软编码。）
- 硬编码,利用GPU或者专用处理器来对视频做编码和解码。iOS 8.0之后开放的Video ToolBox框架就是来实现硬件的编码和解码的.
一. VideoToolbox基本数据结构
Video Toolbox视频编解码前后需要应用的数据结构进行说明
（1）CVPixelBuffer：编码前和解码后的图像数据结构。
（2）CMTime、CMClock和CMTimebase：时间戳相关。时间以64-bit/32-bit的形式出现。
（3）CMBlockBuffer：编码后，结果图像的数据结构。
（4）CMVideoFormatDescription：图像存储方式，编解码器等格式描述。
（5）CMSampleBuffer：存放编解码前后的视频图像的容器数据结构。
二. 硬解码和硬编码使用方法
-
创建AVAssetReader，将H264码流转换成解码前的CMSampleBuffer。
（1）提取sps和pps生成format description。
（2）提取视频图像数据生成CMBlockBuffer。
（3）根据需要，生成CMTime信息。
创建AVAssetWriter，设置输出及压缩属性。
下面的代码简要示例了使用asset reader 和 writer 对一个asset中的第一个video 和 audio track 进行重新编码并将结果数据写入到一个新文件中.（可以改变视频的尺寸，帧率码率，文件格式等）
初始化设置
在创建和配置asset reader 和 writer 之前, 需要进行一些初始化设置. 首先需要为读写过程创建三个串行队列.
NSString *serializationQueueDescription = [NSString stringWithFormat:@"%@ serialization queue", self];
// Create the main serialization queue.
self.mainSerializationQueue = dispatch_queue_create([serializationQueueDescription UTF8String], NULL);
NSString *rwAudioSerializationQueueDescription = [NSString stringWithFormat:@"%@ rw audio serialization queue", self];
// Create the serialization queue to use for reading and writing the audio data.
self.rwAudioSerializationQueue = dispatch_queue_create([rwAudioSerializationQueueDescription UTF8String], NULL);
NSString *rwVideoSerializationQueueDescription = [NSString stringWithFormat:@"%@ rw video serialization queue", self];
// Create the serialization queue to use for reading and writing the video data.
self.rwVideoSerializationQueue = dispatch_queue_create([rwVideoSerializationQueueDescription UTF8String], NULL);
队列mainSerializationQueue 用于asset reader 和 writer 的启动,停止和取消. 其他两个队列用于output/input的读取和写入.
接着, 加载asset中的track, 并开始重编码.
self.asset = <#AVAsset that you want to reencode#>;
self.cancelled = NO;
self.outputURL = <#NSURL representing desired output URL for file generated by asset writer#>;
// Asynchronously load the tracks of the asset you want to read.
[self.asset loadValuesAsynchronouslyForKeys:@[@"tracks"] completionHandler:^{
// Once the tracks have finished loading, dispatch the work to the main serialization queue.
dispatch_async(self.mainSerializationQueue, ^{
// Due to asynchronous nature, check to see if user has already cancelled.
if (self.cancelled)
return;
BOOL success = YES;
NSError *localError = nil;
// Check for success of loading the assets tracks.
success = ([self.asset statusOfValueForKey:@"tracks" error:&localError] == AVKeyValueStatusLoaded);
if (success)
{
// If the tracks loaded successfully, make sure that no file exists at the output path for the asset writer.
NSFileManager *fm = [NSFileManager defaultManager];
NSString *localOutputPath = [self.outputURL path];
if ([fm fileExistsAtPath:localOutputPath])
success = [fm removeItemAtPath:localOutputPath error:&localError];
}
if (success)
success = [self setupAssetReaderAndAssetWriter:&localError];
if (success)
success = [self startAssetReaderAndWriter:&localError];
if (!success)
[self readingAndWritingDidFinishSuccessfully:success withError:localError];
});
}];
剩下的工作就是实现取消的处理, 并实现三个自定义方法.
初始化Asset Reader 和 Writer
自定义方法setupAssetReaderAndAssetWriter实现了asset Reader 和 writer的初始化和配置. 在这个示例中, audio先被asset reader解压为 Linear PCM, 然后被asset write压缩为128 kbps AAC. video被asset reader 解压为YUV, 然后被asset writer 压缩为H.264:
- (BOOL)setupAssetReaderAndAssetWriter:(NSError **)outError
{
// Create and initialize the asset reader.
self.assetReader = [[AVAssetReader alloc] initWithAsset:self.asset error:outError];
BOOL success = (self.assetReader != nil);
if (success)
{
// If the asset reader was successfully initialized, do the same for the asset writer.
self.assetWriter = [[AVAssetWriter alloc] initWithURL:self.outputURL fileType:AVFileTypeQuickTimeMovie error:outError];
success = (self.assetWriter != nil);
}
if (success)
{
// If the reader and writer were successfully initialized, grab the audio and video asset tracks that will be used.
AVAssetTrack *assetAudioTrack = nil, *assetVideoTrack = nil;
NSArray *audioTracks = [self.asset tracksWithMediaType:AVMediaTypeAudio];
if ([audioTracks count] > 0)
assetAudioTrack = [audioTracks objectAtIndex:0];
NSArray *videoTracks = [self.asset tracksWithMediaType:AVMediaTypeVideo];
if ([videoTracks count] > 0)
assetVideoTrack = [videoTracks objectAtIndex:0];
if (assetAudioTrack)
{
// If there is an audio track to read, set the decompression settings to Linear PCM and create the asset reader output.
NSDictionary *decompressionAudioSettings = @{ AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatLinearPCM] };
self.assetReaderAudioOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetAudioTrack outputSettings:decompressionAudioSettings];
[self.assetReader addOutput:self.assetReaderAudioOutput];
// Then, set the compression settings to 128kbps AAC and create the asset writer input.
AudioChannelLayout stereoChannelLayout = {
.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo,
.mChannelBitmap = 0,
.mNumberChannelDescriptions = 0
};
NSData *channelLayoutAsData = [NSData dataWithBytes:&stereoChannelLayout length:offsetof(AudioChannelLayout, mChannelDescriptions)];
NSDictionary *compressionAudioSettings = @{
AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatMPEG4AAC],
AVEncoderBitRateKey : [NSNumber numberWithInteger:128000],
AVSampleRateKey : [NSNumber numberWithInteger:44100],
AVChannelLayoutKey : channelLayoutAsData,
AVNumberOfChannelsKey : [NSNumber numberWithUnsignedInteger:2]
};
self.assetWriterAudioInput = [AVAssetWriterInput assetWriterInputWithMediaType:[assetAudioTrack mediaType] outputSettings:compressionAudioSettings];
[self.assetWriter addInput:self.assetWriterAudioInput];
}
if (assetVideoTrack)
{
// If there is a video track to read, set the decompression settings for YUV and create the asset reader output.
NSDictionary *decompressionVideoSettings = @{
(id)kCVPixelBufferPixelFormatTypeKey : [NSNumber numberWithUnsignedInt:kCVPixelFormatType_422YpCbCr8],
(id)kCVPixelBufferIOSurfacePropertiesKey : [NSDictionary dictionary]
};
self.assetReaderVideoOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetVideoTrack outputSettings:decompressionVideoSettings];
[self.assetReader addOutput:self.assetReaderVideoOutput];
CMFormatDescriptionRef formatDescription = NULL;
// Grab the video format descriptions from the video track and grab the first one if it exists.
NSArray *videoFormatDescriptions = [assetVideoTrack formatDescriptions];
if ([videoFormatDescriptions count] > 0)
formatDescription = (__bridge CMFormatDescriptionRef)[formatDescriptions objectAtIndex:0];
CGSize trackDimensions = {
.width = 0.0,
.height = 0.0,
};
// If the video track had a format description, grab the track dimensions from there. Otherwise, grab them direcly from the track itself.
if (formatDescription)
trackDimensions = CMVideoFormatDescriptionGetPresentationDimensions(formatDescription, false, false);
else
trackDimensions = [assetVideoTrack naturalSize];
NSDictionary *compressionSettings = nil;
// If the video track had a format description, attempt to grab the clean aperture settings and pixel aspect ratio used by the video.
if (formatDescription)
{
NSDictionary *cleanAperture = nil;
NSDictionary *pixelAspectRatio = nil;
CFDictionaryRef cleanApertureFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_CleanAperture);
if (cleanApertureFromCMFormatDescription)
{
cleanAperture = @{
AVVideoCleanApertureWidthKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureWidth),
AVVideoCleanApertureHeightKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHeight),
AVVideoCleanApertureHorizontalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHorizontalOffset),
AVVideoCleanApertureVerticalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureVerticalOffset)
};
}
CFDictionaryRef pixelAspectRatioFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_PixelAspectRatio);
if (pixelAspectRatioFromCMFormatDescription)
{
pixelAspectRatio = @{
AVVideoPixelAspectRatioHorizontalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioHorizontalSpacing),
AVVideoPixelAspectRatioVerticalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioVerticalSpacing)
};
}
// Add whichever settings we could grab from the format description to the compression settings dictionary.
if (cleanAperture || pixelAspectRatio)
{
NSMutableDictionary *mutableCompressionSettings = [NSMutableDictionary dictionary];
if (cleanAperture)
[mutableCompressionSettings setObject:cleanAperture forKey:AVVideoCleanApertureKey];
if (pixelAspectRatio)
[mutableCompressionSettings setObject:pixelAspectRatio forKey:AVVideoPixelAspectRatioKey];
[mutableCompressionSettings setObject:[NSNumber numberWithFloat:bps] forKey:AVVideoAverageBitRateKey];//比特率
[mutableCompressionSettings setObject:@(24) forKey:AVVideoExpectedSourceFrameRateKey];//帧率
[mutableCompressionSettings setObject:@(1) forKey:AVVideoMaxKeyFrameIntervalKey];//帧间隔
[mutableCompressionSettings setObject:AVVideoProfileLevelH264Main31 forKey:AVVideoProfileLevelKey];
compressionSettings = mutableCompressionSettings;
}
}
// Create the video settings dictionary for H.264.
NSMutableDictionary *videoSettings = (NSMutableDictionary *) @{
AVVideoCodecKey : AVVideoCodecH264,
AVVideoWidthKey : [NSNumber numberWithDouble:trackDimensions.width],
AVVideoHeightKey : [NSNumber numberWithDouble:trackDimensions.height]
};//设置视频尺寸，及h.264的编码格式
// Put the compression settings into the video settings dictionary if we were able to grab them.
if (compressionSettings)
[videoSettings setObject:compressionSettings forKey:AVVideoCompressionPropertiesKey];
// Create the asset writer input and add it to the asset writer.
self.assetWriterVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:[videoTrack mediaType] outputSettings:videoSettings];
[self.assetWriter addInput:self.assetWriterVideoInput];
}
}
return success;
}
重编码Asset
方法startAssetReaderAndWriter负责读取和写入asset:
- (BOOL)startAssetReaderAndWriter:(NSError **)outError
{
BOOL success = YES;
// Attempt to start the asset reader.
success = [self.assetReader startReading];
if (!success)
*outError = [self.assetReader error];
if (success)
{
// If the reader started successfully, attempt to start the asset writer.
success = [self.assetWriter startWriting];
if (!success)
*outError = [self.assetWriter error];
}
if (success)
{
// If the asset reader and writer both started successfully, create the dispatch group where the reencoding will take place and start a sample-writing session.
self.dispatchGroup = dispatch_group_create();
[self.assetWriter startSessionAtSourceTime:kCMTimeZero];
self.audioFinished = NO;
self.videoFinished = NO;
if (self.assetWriterAudioInput)
{
// If there is audio to reencode, enter the dispatch group before beginning the work.
dispatch_group_enter(self.dispatchGroup);
// Specify the block to execute when the asset writer is ready for audio media data, and specify the queue to call it on.
[self.assetWriterAudioInput requestMediaDataWhenReadyOnQueue:self.rwAudioSerializationQueue usingBlock:^{
// Because the block is called asynchronously, check to see whether its task is complete.
if (self.audioFinished)
return;
BOOL completedOrFailed = NO;
// If the task isn't complete yet, make sure that the input is actually ready for more media data.
while ([self.assetWriterAudioInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next audio sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [self.assetReaderAudioOutput copyNextSampleBuffer];
if (sampleBuffer != NULL)
{
BOOL success = [self.assetWriterAudioInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
if (completedOrFailed)
{
// Mark the input as finished, but only if we haven't already done so, and then leave the dispatch group (since the audio work has finished).
BOOL oldFinished = self.audioFinished;
self.audioFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterAudioInput markAsFinished];
}
dispatch_group_leave(self.dispatchGroup);
}
}];
}
if (self.assetWriterVideoInput)
{
// If we had video to reencode, enter the dispatch group before beginning the work.
dispatch_group_enter(self.dispatchGroup);
// Specify the block to execute when the asset writer is ready for video media data, and specify the queue to call it on.
[self.assetWriterVideoInput requestMediaDataWhenReadyOnQueue:self.rwVideoSerializationQueue usingBlock:^{
// Because the block is called asynchronously, check to see whether its task is complete.
if (self.videoFinished)
return;
BOOL completedOrFailed = NO;
// If the task isn't complete yet, make sure that the input is actually ready for more media data.
while ([self.assetWriterVideoInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next video sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [self.assetReaderVideoOutput copyNextSampleBuffer];
if (sampleBuffer != NULL)
{
BOOL success = [self.assetWriterVideoInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
if (completedOrFailed)
{
// Mark the input as finished, but only if we haven't already done so, and then leave the dispatch group (since the video work has finished).
BOOL oldFinished = self.videoFinished;
self.videoFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterVideoInput markAsFinished];
}
dispatch_group_leave(self.dispatchGroup);
}
}];
}
// Set up the notification that the dispatch group will send when the audio and video work have both finished.
dispatch_group_notify(self.dispatchGroup, self.mainSerializationQueue, ^{
BOOL finalSuccess = YES;
NSError *finalError = nil;
// Check to see if the work has finished due to cancellation.
if (self.cancelled)
{
// If so, cancel the reader and writer.
[self.assetReader cancelReading];
[self.assetWriter cancelWriting];
}
else
{
// If cancellation didn't occur, first make sure that the asset reader didn't fail.
if ([self.assetReader status] == AVAssetReaderStatusFailed)
{
finalSuccess = NO;
finalError = [self.assetReader error];
}
// If the asset reader didn't fail, attempt to stop the asset writer and check for any errors.
if (finalSuccess)
{
finalSuccess = [self.assetWriter finishWriting];
if (!finalSuccess)
finalError = [self.assetWriter error];
}
}
// Call the method to handle completion, and pass in the appropriate parameters to indicate whether reencoding was successful.
[self readingAndWritingDidFinishSuccessfully:finalSuccess withError:finalError];
});
}
// Return success here to indicate whether the asset reader and writer were started successfully.
return success;
}
在重编码过程中, 为了提升性能, 音频处理和视频处理在两个不同队列中进行. 但这两个队列在一个dispatchGroup中, 当每个队列的任务都完成后, 会调用readingAndWritingDidFinishSuccessfully。
处理编码结果
对重编码的结果进行处理并同步到UI:
- (void)readingAndWritingDidFinishSuccessfully:(BOOL)success withError:(NSError *)error
{
if (!success)
{
// If the reencoding process failed, we need to cancel the asset reader and writer.
[self.assetReader cancelReading];
[self.assetWriter cancelWriting];
dispatch_async(dispatch_get_main_queue(), ^{
// Handle any UI tasks here related to failure.
});
}
else
{
// Reencoding was successful, reset booleans.
self.cancelled = NO;
self.videoFinished = NO;
self.audioFinished = NO;
dispatch_async(dispatch_get_main_queue(), ^{
// Handle any UI tasks here related to success.
});
}
}
当然，还可以取消重编码
使用多个串行队列, 可以很轻松的取消对asset的重编码. 可以将下面的代码与UI上的”取消”按钮关联起来:
- (void)cancel
{
// Handle cancellation asynchronously, but serialize it with the main queue.
dispatch_async(self.mainSerializationQueue, ^{
// If we had audio data to reencode, we need to cancel the audio work.
if (self.assetWriterAudioInput)
{
// Handle cancellation asynchronously again, but this time serialize it with the audio queue.
dispatch_async(self.rwAudioSerializationQueue, ^{
// Update the Boolean property indicating the task is complete and mark the input as finished if it hasn't already been marked as such.
BOOL oldFinished = self.audioFinished;
self.audioFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterAudioInput markAsFinished];
}
// Leave the dispatch group since the audio work is finished now.
dispatch_group_leave(self.dispatchGroup);
});
}
if (self.assetWriterVideoInput)
{
// Handle cancellation asynchronously again, but this time serialize it with the video queue.
dispatch_async(self.rwVideoSerializationQueue, ^{
// Update the Boolean property indicating the task is complete and mark the input as finished if it hasn't already been marked as such.
BOOL oldFinished = self.videoFinished;
self.videoFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterVideoInput markAsFinished];
}
// Leave the dispatch group, since the video work is finished now.
dispatch_group_leave(self.dispatchGroup);
});
}
// Set the cancelled Boolean property to YES to cancel any work on the main queue as well.
self.cancelled = YES;
});
}
如果需要重编码相册中的视频的话，还需要一下步骤：
-
导出视频资源
PHVideoRequestOptions* options = [[PHVideoRequestOptions alloc] init]; options.version = PHVideoRequestOptionsVersionOriginal; options.deliveryMode = PHVideoRequestOptionsDeliveryModeAutomatic; options.networkAccessAllowed = YES; [[PHImageManager defaultManager] requestAVAssetForVideo:asset options:options resultHandler:^(AVAsset* avasset, AVAudioMix* audioMix, NSDictionary* info){ // NSLog(@"Info:\n%@",info); AVURLAsset *videoAsset = (AVURLAsset*)avasset; NSLog(@"AVAsset URL: %@",videoAsset.URL); NSString *videoPath = videoAsset.URL.path; NSLog(@"原视频大小:%llu", [[NSFileManager defaultManager] attributesOfItemAtPath:videoPath error:nil].fileSize); [self startExportVideoWithVideoAsset:videoAsset completion:completion]; -
设置导出视频是压缩系数等
NSArray *presets = [AVAssetExportSession exportPresetsCompatibleWithAsset:videoAsset]; if ([presets containsObject:AVAssetExportPreset640x480]) { AVAssetExportSession *session = [[AVAssetExportSession alloc]initWithAsset:videoAsset presetName:AVAssetExportPreset640x480]; NSDateFormatter *formater = [[NSDateFormatter alloc] init]; [formater setDateFormat:@"yyyy-MM-dd-HH:mm:ss"]; NSString *outputPath = [[CACHES_FOLDER stringByAppendingPathComponent:@"video"] stringByAppendingFormat:@"/%@.mp4",[formater stringFromDate:[NSDate date]]]; NSLog(@"video outputPath = %@",outputPath); session.outputURL = [NSURL fileURLWithPath:outputPath]; // Optimize for network use. session.shouldOptimizeForNetworkUse = true; NSArray *supportedTypeArray = session.supportedFileTypes; if ([supportedTypeArray containsObject:AVFileTypeMPEG4]) { session.outputFileType = AVFileTypeMPEG4; } else if (supportedTypeArray.count == 0) { NSLog(@"No supported file types 视频类型暂不支持导出"); return; } else { session.outputFileType = [supportedTypeArray objectAtIndex:0]; } if (![[NSFileManager defaultManager] fileExistsAtPath:[CACHES_FOLDER stringByAppendingPathComponent:@"video"]]) { [[NSFileManager defaultManager] createDirectoryAtPath:[CACHES_FOLDER stringByAppendingPathComponent:@"video"] withIntermediateDirectories:YES attributes:nil error:nil]; } AVMutableVideoComposition *videoComposition = [self fixedCompositionWithAsset:videoAsset]; if (videoComposition.renderSize.width) { // 修正视频转向 session.videoComposition = videoComposition; } // Begin to export video to the output path asynchronously. [session exportAsynchronouslyWithCompletionHandler:^(void) { switch (session.status) { case AVAssetExportSessionStatusUnknown: NSLog(@"AVAssetExportSessionStatusUnknown"); break; case AVAssetExportSessionStatusWaiting: NSLog(@"AVAssetExportSessionStatusWaiting"); break; case AVAssetExportSessionStatusExporting: NSLog(@"AVAssetExportSessionStatusExporting"); break; case AVAssetExportSessionStatusCompleted: { NSLog(@"AVAssetExportSessionStatusCompleted"); dispatch_async(dispatch_get_main_queue(), ^{ if (completion) { completion(outputPath); } NSLog(@"导出的视频大小:%llu", [[NSFileManager defaultManager] attributesOfItemAtPath:outputPath error:nil].fileSize); }); } break; case AVAssetExportSessionStatusFailed: NSLog(@"AVAssetExportSessionStatusFailed"); break; default: break; } }]; } -
获取优化后的视频转向信息
-(AVMutableVideoComposition *)fixedCompositionWithAsset:(AVAsset *)videoAsset { AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition]; // 视频转向 int degrees = [self degressFromVideoFileWithAsset:videoAsset]; if (degrees != 0) { CGAffineTransform translateToCenter; CGAffineTransform mixedTransform; videoComposition.frameDuration = CMTimeMake(1, 30); NSArray *tracks = [videoAsset tracksWithMediaType:AVMediaTypeVideo]; AVAssetTrack *videoTrack = [tracks objectAtIndex:0]; if (degrees == 90) { // 顺时针旋转90° translateToCenter = CGAffineTransformMakeTranslation(videoTrack.naturalSize.height, 0.0); mixedTransform = CGAffineTransformRotate(translateToCenter,M_PI_2); videoComposition.renderSize = CGSizeMake(videoTrack.naturalSize.height,videoTrack.naturalSize.width); } else if(degrees == 180){ // 顺时针旋转180° translateToCenter = CGAffineTransformMakeTranslation(videoTrack.naturalSize.width, videoTrack.naturalSize.height); mixedTransform = CGAffineTransformRotate(translateToCenter,M_PI); videoComposition.renderSize = CGSizeMake(videoTrack.naturalSize.width,videoTrack.naturalSize.height); } else if(degrees == 270){ // 顺时针旋转270° translateToCenter = CGAffineTransformMakeTranslation(0.0, videoTrack.naturalSize.width); mixedTransform = CGAffineTransformRotate(translateToCenter,M_PI_2*3.0); videoComposition.renderSize = CGSizeMake(videoTrack.naturalSize.height,videoTrack.naturalSize.width); } AVMutableVideoCompositionInstruction *roateInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction]; roateInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, [videoAsset duration]); AVMutableVideoCompositionLayerInstruction *roateLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack]; [roateLayerInstruction setTransform:mixedTransform atTime:kCMTimeZero]; roateInstruction.layerInstructions = @[roateLayerInstruction]; // 加入视频方向信息 videoComposition.instructions = @[roateInstruction]; } return videoComposition; -
获取视频角度
-(int)degressFromVideoFileWithAsset:(AVAsset *)asset { int degress = 0; NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeVideo]; if([tracks count] > 0) { AVAssetTrack *videoTrack = [tracks objectAtIndex:0]; CGAffineTransform t = videoTrack.preferredTransform; if(t.a == 0 && t.b == 1.0 && t.c == -1.0 && t.d == 0){ // Portrait degress = 90; } else if(t.a == 0 && t.b == -1.0 && t.c == 1.0 && t.d == 0){ // PortraitUpsideDown degress = 270; } else if(t.a == 1.0 && t.b == 0 && t.c == 0 && t.d == 1.0){ // LandscapeRight degress = 0; } else if(t.a == -1.0 && t.b == 0 && t.c == 0 && t.d == -1.0){ // LandscapeLeft degress = 180; } } return degress; }
在实现中发现，对于导出的压缩过后的视频，如果小于一定程度，再进行编解码的话，不但体积没有变小，反而增大了，无论改什么参数都不会变小体积，这个需要进一步研究。另外，这个是对于压缩过后的视频进行重编码的，还可以交换下步骤，先获取视频资源AVAsset，在将AVAsset重编码，然后进行压缩。经过试验，俩者得出的结果是一样的，在一定程度都能压缩，到达一定程度了，无论改什么参数都压缩不了大小了
参考文献：http://www.devzhang.cn/2016/09/20/Asset%E7%9A%84%E9%87%8D%E7%BC%96%E7%A0%81%E5%8F%8A%E5%AF%BC%E5%87%BA/