AVCaptureVideoPreviewLayer错误的帧捕获

Question

我有代码：

    previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];
[previewLayer setBackgroundColor:[[UIColor blackColor] CGColor]];
previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
CALayer *rootLayer = [self.previewView layer];
[rootLayer setMasksToBounds:NO];
[previewLayer setFrame:CGRectMake([rootLayer bounds].origin.x, [rootLayer bounds].origin.y-20, [rootLayer bounds].size.width, [rootLayer bounds].size.height+40)];
[rootLayer addSublayer:previewLayer];

[previewLayer setBounds:previewLayer.frame];
[session startRunning];

当我开始捕捉图像时，飞溅白色视图在20px（状态栏高度）上移至顶部。并且snaped图片有错误的框架（-20到顶部，20到向下或在顶部是正确的，-20px向下）。实际的previewImage框架是

CGRectMake(15, 59, 290, 200).

现在它在帧CGRect中捕获图像（白色飞溅视图）=（15,59,290,170）。但是，snaped图像具有类似CGRect =（15,59,290,230）的帧。我做错了什么？请帮忙..