先获得原视频的方向,url为视频路径

-(NSUInteger)degressFromVideoFileWithURL:(NSURL *)url
{
    NSUInteger degress = 0;
    
    AVAsset *asset = [AVAsset assetWithURL:url];
    NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeVideo];
    if([tracks count] > 0) {
        AVAssetTrack *videoTrack = [tracks objectAtIndex:0];
        CGAffineTransform t = videoTrack.preferredTransform;
        
        if(t.a == 0 && t.b == 1.0 && t.c == -1.0 && t.d == 0){
            // Portrait
            degress = 90;
        }else if(t.a == 0 && t.b == -1.0 && t.c == 1.0 && t.d == 0){
            // PortraitUpsideDown
            degress = 270;
        }else if(t.a == 1.0 && t.b == 0 && t.c == 0 && t.d == 1.0){
            // LandscapeRight
            degress = 0;
        }else if(t.a == -1.0 && t.b == 0 && t.c == 0 && t.d == -1.0){
            // LandscapeLeft
            degress = 180;
        }
    }
    
    return degress;
}

然后修改movieWriter的transform属性

        NSInteger a= [self degressFromVideoFileWithURL:self.pathURL];
        CGAffineTransform rotate = CGAffineTransformMakeRotation(a / 180.0 * M_PI );
        movieWriter.transform=rotate;

这样就可以保证美颜之后的视频方向与原视频方向相同。