Ios 如何设置人像视频的背景图像？

我一直在尝试为人像视频设置背景图像/CALayer，这样我就可以避免黑条。 我可以设置图像，但它显示黑色背景，而不是图像

CALayer *videoLayer = [CALayer layer]
CGRect frame = CGRectMake(0, 0, size.with, size.height);
videoLayer.frame = frame;
videoLayer.backgroundColor = [UIColor clearColor].CGColor;
CALayer *backgroundImageLalyer = [CALayer layer]
CALayer *outputLayer = [CALayer layer]
[outputLayer addSublayer:backgroundImageLayer]
[outputLayer addSublayer:videoLayer]

[backgroundLayer setContents:(id)[backgroundImage CGImage]];
backgroundLayer.frame = frame;
[backgroundLayer setMasksToBounds:YES];

composition.animationTool = [AVVideoCompositionCoreAnimationTool
                               videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:outputLayer]

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有人能建议我为人像视频设置背景图像吗？

你用什么来显示视频？我已经创建了AVMutableComposition，其中包含视频曲目。