ios - 自定义 AVVideoComposition 的 AVAsset 首选转换

Question

我创建了一个自定义 AVVideoComposition 类并像这样使用它：

AVAsset *asset = ...
AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoCompositionWithPropertiesOfAsset:asset];
videoComposition.customVideoCompositorClass = [MyCustomCompositor class];
MyCustomInstruction *instruction = // custom instruction holding CIFilter that is applied to every video frame
videoComposition.instructions = @[instruction];

导出会话后使用如下：

AVAssetExportSession *session = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetHighestQuality];
session.outputURL = ...
session.outputFileType = AVFileTypeQuickTimeMovie;
session.videoComposition = videoComposition;

[session exportAsynchronouslyWithCompletionHandler:^{
    ...
}];

根据文档，如果我正在使用AVVideoComposition，轨道preferredTransform将不起作用。同样使用自定义AVVideoCompostion指令，我无法AVMutableVideoCompositionLayerInstruction使用setTransform:atTime:进行设置。

如何获得正确方向的视频？