如何使用CGAffineTransform使旋转的视频居中

Question

我正在使用AVMutableComposition合并多个视频。我希望主要渲染比例为16：9。现在，我正在使用以下代码转换以人像拍摄的视频，以使其适合此宽高比。问题是最终结果使它们旋转但位于视频的左侧。如何将它们放置在中央？

BOOL isVideoAssetPortrait_  = NO;


CGAffineTransform videoTransform = videoAssetTrack.preferredTransform;
if (videoTransform.a == 0 && videoTransform.b == 1.0 && videoTransform.c == -1.0 && videoTransform.d == 0) {
    videoAssetOrientation_ = UIImageOrientationRight;
    isVideoAssetPortrait_ = YES;
}
if (videoTransform.a == 0 && videoTransform.b == -1.0 && videoTransform.c == 1.0 && videoTransform.d == 0) {
    videoAssetOrientation_ =  UIImageOrientationLeft;
    isVideoAssetPortrait_ = YES;
}
if (videoTransform.a == 1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == 1.0) {
    videoAssetOrientation_ =  UIImageOrientationUp;
}
if (videoTransform.a == -1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == -1.0) {
    videoAssetOrientation_ = UIImageOrientationDown;
}

 CGFloat FirstAssetScaleToFitRatio = 720 / videoAssetTrack.naturalSize.width;

    if(isVideoAssetPortrait_) {
        FirstAssetScaleToFitRatio = 720/videoAssetTrack.naturalSize.width;
        CGAffineTransform FirstAssetScaleFactor = CGAffineTransformMakeScale(FirstAssetScaleToFitRatio,FirstAssetScaleToFitRatio);
        [videolayerInstruction setTransform:CGAffineTransformConcat(videoAssetTrack.preferredTransform, FirstAssetScaleFactor) atTime:kCMTimeZero];
    }else{
        CGAffineTransform FirstAssetScaleFactor = CGAffineTransformMakeScale(FirstAssetScaleToFitRatio,FirstAssetScaleToFitRatio);
        [videolayerInstruction setTransform:CGAffineTransformConcat(CGAffineTransformConcat(videoAssetTrack.preferredTransform, FirstAssetScaleFactor),CGAffineTransformMakeTranslation(0, 160)) atTime:kCMTimeZero];
    }

谢谢