One image (kinect depth map) as opacityMask for other image

Hi, I have two image controls.

I want to use Img1 as a OpacityMask of Img2.

Img1 - image to apply mask to

Img2 - image to apply mask from

The source of Img1 is being read from kinect depth image: (I'm using OpenCV)

var depthBmp = depthFrame.SliceDepthImage((int)sliderMin.Value, (int)sliderMax.Value);
Image openCVImg = new Image(depthBmp.ToBitmap()); outImg.Source = ImageHelpers.ToBitmapSource(openCVImg);

Img1.Source = ImageHelpers.ToBitmapSource(openCVImg);

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When I create ImageBrush, set its Imagesource to Img1.Source and apply OpacityMask from Brush, nothing happens, Img2 is shown without mask:

brush = new ImageBrush();

brush.ImageSource = Img1.Source;
Img2.OpacityMask = brush;

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For example, the mask is applied when I use BitmapImage as ImageSource of ImageBrush:

BitmapImage Img3 = new BitmapImage(new Uri(@"c:\temp\turtle.png", UriKind.Absolute));

brush = new ImageBrush();

brush.ImageSource = Img3.Source;
Img2.OpacityMask = brush;

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Tnx.

Zvonimir