近期项目由于要使用拍摄视频，用的是UIImagePickerViewController，要求是分辨率640x480，但是试了各种

p_w_picpathPickerController.videoQuality

拍出来的视频导出来依然达不到640x480.

并且旋转设备拍摄后拍摄出来的视频导出来也是反的。

最后查阅了各种网站找了各种资料之后，看到国外大牛在视频拍摄完成之后使用AVFoundation做的压缩和视频调整。整理了一下代码做了部分调整，感觉到AVfoundation的强大之处，哈哈

这些代码用在拍摄之后的回掉中，

AVAsset *firstAsset = [AVAsset assetWithURL:[info objectForKey:UIImagePickerControllerMediaURL]];

if(firstAsset !=nil && [[firstAsset tracksWithMediaType:AVMediaTypeVideo] count]>0){

//Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.

AVMutableComposition* mixComposition = [[AVMutableComposition alloc] init];

//VIDEO TRACK

AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

[firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

AVMutableVideoCompositionInstruction * MainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];

MainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, firstAsset.duration);

if ([[firstAsset tracksWithMediaType:AVMediaTypeAudio] count]>0) {

//AUDIO TRACK

AVMutableCompositionTrack *firstAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

[firstAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];

}else{

NSLog(@"warning: video has no audio");

}

//FIXING ORIENTATION//

AVMutableVideoCompositionLayerInstruction *FirstlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:firstTrack];

AVAssetTrack *FirstAssetTrack = [[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

UIImageOrientation FirstAssetOrientation_ = UIImageOrientationUp;

BOOL isFirstAssetPortrait_ = NO;

CGAffineTransform firstTransform = FirstAssetTrack.preferredTransform;

if(firstTransform.a == 0 && firstTransform.b == 1.0 && firstTransform.c == -1.0 && firstTransform.d == 0)

{

FirstAssetOrientation_= UIImageOrientationRight; isFirstAssetPortrait_ = YES;

}

if(firstTransform.a == 0 && firstTransform.b == -1.0 && firstTransform.c == 1.0 && firstTransform.d == 0)

{

FirstAssetOrientation_ = UIImageOrientationLeft; isFirstAssetPortrait_ = YES;

}

if(firstTransform.a == 1.0 && firstTransform.b == 0 && firstTransform.c == 0 && firstTransform.d == 1.0)

{

FirstAssetOrientation_ = UIImageOrientationUp;

}

if(firstTransform.a == -1.0 && firstTransform.b == 0 && firstTransform.c == 0 && firstTransform.d == -1.0)

{

FirstAssetOrientation_ = UIImageOrientationDown;

}

CGFloat FirstAssetScaleToFitRatio = 768.0/FirstAssetTrack.naturalSize.width;

if(isFirstAssetPortrait_)

{

FirstAssetScaleToFitRatio = 768.0/FirstAssetTrack.naturalSize.height;

CGAffineTransform FirstAssetScaleFactor = CGAffineTransformMakeScale(FirstAssetScaleToFitRatio,FirstAssetScaleToFitRatio);

[FirstlayerInstruction setTransform:CGAffineTransformConcat(FirstAssetTrack.preferredTransform, FirstAssetScaleFactor) atTime:kCMTimeZero];

}

else

{

CGAffineTransform FirstAssetScaleFactor = CGAffineTransformMakeScale(FirstAssetScaleToFitRatio,FirstAssetScaleToFitRatio);

[FirstlayerInstruction setTransform:CGAffineTransformConcat(CGAffineTransformConcat(FirstAssetTrack.preferredTransform, FirstAssetScaleFactor),CGAffineTransformMakeTranslation(0, 0)) atTime:kCMTimeZero];

}

[FirstlayerInstruction setOpacity:0.0 atTime:firstAsset.duration];

MainInstruction.layerInstructions = [NSArray arrayWithObjects:FirstlayerInstruction,nil];;

AVMutableVideoComposition *MainCompositionInst = [AVMutableVideoComposition videoComposition];

MainCompositionInst.instructions = [NSArray arrayWithObject:MainInstruction];

MainCompositionInst.frameDuration = CMTimeMake(1, 30);

if(isFirstAssetPortrait_)

{

MainCompositionInst.renderSize = CGSizeMake(480.0, 640.0);

}else{

MainCompositionInst.renderSize = CGSizeMake(640.0, 480.0);

}

// MainCompositionInst.renderSize = CGSizeMake(640.0, 480.0);

NSURL *url = [NSURL fileURLWithPath:videoPath];

exporter = nil;

exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPreset640x480];

exporter.outputURL=url;

exporter.outputFileType = AVFileTypeMPEG4;

exporter.videoComposition = MainCompositionInst;

exporter.shouldOptimizeForNetworkUse = YES;

[exporter exportAsynchronouslyWithCompletionHandler:^

{

dispatch_async(dispatch_get_main_queue(), ^{

// [self exportDidFinish:exporter];

if(exporter.status == AVAssetExportSessionStatusCompleted){

//视频保存成功之后的处理

}else{

NSLog(@"error fixing orientation");

}

});

}];