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

imagePickerController.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");

                     }

                 });

             }];