iOS：AVFoundation视频拼接-自适应旋转角度

网上有很多视频合成/拼接的代码，大部分并没有适应视频自己的旋转角度，
所以如果有竖屏资源，合成出的视频会被旋转90度。

花了点时间，做出了一个自适应旋转角度的视频合成demo

最终的做出的效果图：

主要使用下面的类：
AVMutableVideoCompositionLayerInstruction：对各个视频资源做处理：旋转，透明，裁剪。

核心代码：

- (AVMutableVideoCompositionInstruction*)getCompositionInstructions:(NSArray<AVMutableCompositionTrack*>*)compositionTracks tracks:(NSArray<AVAssetTrack*>*)tracks assets:(NSArray<AVAsset*>*)assets maxDuration:(CMTime)maxDuration naturalSize:(CGSize)naturalSize{
    
    //资源动画范围
    __block CMTime atTime = kCMTimeZero;

    NSMutableArray* layerInstructions = [NSMutableArray array];

    //视频轨道中的一个视频，可以缩放、旋转等
    AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, maxDuration);
    
    [compositionTracks enumerateObjectsUsingBlock:^(AVMutableCompositionTrack * _Nonnull compositionTrack, NSUInteger idx, BOOL * _Nonnull stop) {
        AVAssetTrack *assetTrack = tracks[idx];
        AVAsset *asset = assets[idx];

        AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionTrack];
        
        //设置旋转角度
        CGAffineTransform transfrom = [self getTransfromFromTrack:assetTrack naturalSize:naturalSize];
        [layerInstruction setTransform:transfrom atTime:kCMTimeZero];
        
        //设置透明
        atTime = CMTimeAdd(atTime, asset.duration);
        [layerInstruction setOpacity:0.0 atTime:atTime];
        
        [layerInstructions addObject:layerInstruction];
    }];
    
    mainInstruction.layerInstructions = layerInstructions;
    
    return mainInstruction;
}

设置视频的旋转

- (CGAffineTransform)getTransfromFromTrack:(AVAssetTrack*)track naturalSize:(CGSize)naturalSize{
    
    UIImageOrientation assetOrientation = UIImageOrientationUp;
    BOOL isPortrait = NO;
    CGAffineTransform transfrom = track.preferredTransform;
    
    if(transfrom.a == 0 && transfrom.b == 1.0 && transfrom.c == -1.0 && transfrom.d == 0) {
        assetOrientation = UIImageOrientationRight;
        isPortrait = YES;
    }
    if(transfrom.a == 0 && transfrom.b == -1.0 && transfrom.c == 1.0 && transfrom.d == 0) {
        assetOrientation = UIImageOrientationLeft;
        isPortrait = YES;
    }
    if(transfrom.a == 1.0 && transfrom.b == 0 && transfrom.c == 0 && transfrom.d == 1.0) {
        assetOrientation = UIImageOrientationUp;
    }
    if(transfrom.a == -1.0 && transfrom.b == 0 && transfrom.c == 0 && transfrom.d == -1.0) {
        assetOrientation = UIImageOrientationDown;
    }
        
    CGFloat assetScaleToFitRatio = naturalSize.width / track.naturalSize.width;
    
    if(isPortrait) {
        assetScaleToFitRatio = naturalSize.width / track.naturalSize.height;
        CGAffineTransform assetScaleFactor = CGAffineTransformMakeScale(assetScaleToFitRatio,assetScaleToFitRatio);
        transfrom = CGAffineTransformConcat(track.preferredTransform, assetScaleFactor);
    }else{
        CGAffineTransform assetScaleFactor = CGAffineTransformMakeScale(assetScaleToFitRatio,assetScaleToFitRatio);
        transfrom = CGAffineTransformConcat(CGAffineTransformConcat(track.preferredTransform, assetScaleFactor),CGAffineTransformMakeTranslation(0, naturalSize.width/2));
    }
    return transfrom;
}

GitHub：
https://github.com/qw9685/composeVideos