从AVAssetExportSession

Question

对于ios，我使用AVAssetExportSession将AVAsset压缩为mp4文件。 （6秒视频）。

如果我保留原始屏幕尺寸（1280 * 720），则视频大小为645362字节 当我设置videoComposition并将视频大小裁剪为（720 * 405）时，输出视频大小为631862字节。

为什么视频尺寸超过一半，视频大小几乎相同？ 代码如下：

NSURL * saveURL = [NSURL fileURLWithPath:savePath];

//create an avassetrack with our asset
AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

CGSize videoSz = clipVideoTrack.naturalSize;
CGRect cropRect = CGRectMake(0, 0, videoSz.width, 320.0/568.0*videoSz.width);

//create a video composition and preset some settings
AVMutableVideoComposition* videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.frameDuration = CMTimeMake(1, 30);

CGFloat cropOffX = cropRect.origin.x;
CGFloat cropOffY = cropRect.origin.y;
CGFloat cropWidth = cropRect.size.width;
CGFloat cropHeight = cropRect.size.height;

videoComposition.renderSize = CGSizeMake(cropWidth, cropHeight);

//create a video instruction
AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, asset.duration);

AVMutableVideoCompositionLayerInstruction* transformer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:clipVideoTrack];

UIImageOrientation videoOrientation = [self getVideoOrientationFromAsset:asset];

CGAffineTransform t1 = CGAffineTransformIdentity;
CGAffineTransform t2 = CGAffineTransformIdentity;

switch (videoOrientation) {
    case UIImageOrientationUp:
        t1 = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.height - cropOffX, 0 - cropOffY );
        t2 = CGAffineTransformRotate(t1, M_PI_2 );
        break;
    case UIImageOrientationDown:
        t1 = CGAffineTransformMakeTranslation(0 - cropOffX, clipVideoTrack.naturalSize.width - cropOffY ); // not fixed width is the real height in upside down
        t2 = CGAffineTransformRotate(t1, - M_PI_2 );
        break;
    case UIImageOrientationRight:
        t1 = CGAffineTransformMakeTranslation(0 - cropOffX, 0 - cropOffY );
        t2 = CGAffineTransformRotate(t1, 0 );
        break;
    case UIImageOrientationLeft:
        t1 = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.width - cropOffX, clipVideoTrack.naturalSize.height - cropOffY );
        t2 = CGAffineTransformRotate(t1, M_PI  );
        break;
    default:
        NSLog(@"no supported orientation has been found in this video");
        break;
}

CGAffineTransform finalTransform = t2;
[transformer setTransform:finalTransform atTime:kCMTimeZero];

//add the transformer layer instructions, then add to video composition
instruction.layerInstructions = [NSArray arrayWithObject:transformer];
videoComposition.instructions = [NSArray arrayWithObject: instruction];

AVAssetExportSession *avAssetExportSession = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetMediumQuality];
if (!keepOrigin) {
    [avAssetExportSession setVideoComposition:videoComposition];
}
[avAssetExportSession setOutputFileType:AVFileTypeMPEG4];
[avAssetExportSession setShouldOptimizeForNetworkUse:YES];
[avAssetExportSession exportAsynchronouslyWithCompletionHandler:^(void){

    NSData * data = [NSData dataWithContentsOfURL:[NSURL fileURLWithPath:savePath]];
    NSUInteger size = data.length;
    NSLog(@"print video length = %lu", (unsigned long)size);

}];