具有透明视频层的AVVideoCompositionCoreAnimationTool

Question

我在视频图层的顶部和后面制作带有图形的视频合成。问题是我无法将视频图层背景颜色设置为清晰（透明），它始终显示黑色背景颜色。我想在它背后展示一些图形。这是代码。我将视频层移动了-200px只是为了确保显示它背后的图形。如何设置视频图层以显示透明背景？

var backLayer = new CALayer
{
    Contents = s.CGImage,
    BackgroundColor = UIColor.White.CGColor,
    Bounds = new CGRect(0, 0, size.Width, size.Height),
    Frame = new CGRect(0, 0, size.Width, size.Height),
    Hidden = false
};

var animationLayer = new CALayer { Contents = s.CGImage };
animationLayer.BackgroundColor = UIColor.Clear.CGColor;
animationLayer.Bounds = new CGRect(size.Width - 100, 50, 50, 50);
animationLayer.Frame = new CGRect(size.Width - 100, 50, 50, 50);
animationLayer.Hidden = false;

var videoLayer = new CALayer();
videoLayer.BackgroundColor = UIColor.Clear.CGColor;
videoLayer.ShadowColor = UIColor.Clear.CGColor;
videoLayer.BorderColor = UIColor.Clear.CGColor;
videoLayer.Frame = new CGRect(0, -200, size.Width, size.Height);
videoLayer.Bounds = new CGRect(0, -200, size.Width, size.Height);

parentLayer.AddSublayer(backLayer);
parentLayer.AddSublayer(videoLayer);
parentLayer.AddSublayer(animationLayer);
parentLayer.BackgroundColor = UIColor.Clear.CGColor;

videoComposition.AnimationTool = AVVideoCompositionCoreAnimationTool.FromLayer(videoLayer, parentLayer);