Ios DrawView保存组合图像（倍增）

我有两个UIImageView，一个在底部，在第二个UIImageView上显示默认图像（如照片），您可以在其中绘制

我想从两个图像创建一个UIImage，并将其另存为新图像

我该怎么做？我曾尝试过：

 func saveImage() {

    print("save combined image")

    let topImage = self.canvasView.image
    let bottomImage = self.backgroundImageView.image

    let size = CGSizeMake(topImage!.size.width, topImage!.size.height)
    UIGraphicsBeginImageContextWithOptions(size, false, 0.0)

    [topImage!.drawInRect(CGRectMake(0,0,size.width, topImage!.size.height))];
    [bottomImage!.drawInRect(CGRectMake(0,0,size.width, bottomImage!.size.height))];

    let newImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    UIImageWriteToSavedPhotosAlbum(newImage, self, #selector(CanvasViewController.image(_:didFinishSavingWithError:contextInfo:)), nil)

}

但结果不正确（拉伸且无覆盖）

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有什么想法吗？

好的，我会找到一个解决方案，只需要添加“乘法”作为混合模式

    let topImage = self.canvasView.image
    let bottomImage = self.backgroundImageView.image

    let size = CGSizeMake(bottomImage!.size.width, bottomImage!.size.height)
    UIGraphicsBeginImageContextWithOptions(size, false, 0.0)


    [bottomImage!.drawInRect(CGRectMake(0,0,size.width, bottomImage!.size.height))];

    [topImage!.drawInRect(CGRectMake(0,0,size.width, bottomImage!.size.height), blendMode: CGBlendMode.Multiply , alpha: 1.0)];

    let newImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()