private async Task <CompositionSurfaceBrush> CreateOpacityMaskBrush(Uri opacityMaskImageUri)
{
// load in our opacity mask image.
// this is created in a graphic tool such as paint.net
var opacityMaskSurface = await _surfaceFactory.CreateSurfaceFromUriAsync(opacityMaskImageUri);
// create surfacebrush with ICompositionSurface that contains the gradient opacity mask asset
var opacityBrush = _compositor.CreateSurfaceBrush(opacityMaskSurface);
opacityBrush.Stretch = CompositionStretch.UniformToFill;
return(opacityBrush);
}