public static async Task <CompositionBrush> GetAlphaMaskAsync(this UIElement element)
{
CompositionBrush mask = null;
try
{
//For some reason, using TextBlock and getting the AlphaMask
//generates a shadow with a size more smaller than the control size.
if (element is TextBlock textElement)
{
mask = textElement.GetAlphaMask();
}
// We also use this option with images and shapes, even though have the option to
// get the AlphaMask directly (in case it is clipped).
else if (element is FrameworkElement frameworkElement)
{
var height = (int)frameworkElement.ActualHeight;
var width = (int)frameworkElement.ActualWidth;
if (height > 0 && width > 0)
{
var visual = ElementCompositionPreview.GetElementVisual(element);
var elementVisual = visual.Compositor.CreateSpriteVisual();
elementVisual.Size = element.RenderSize.ToVector2();
var bitmap = new RenderTargetBitmap();
await bitmap.RenderAsync(
element,
width,
height);
var pixels = await bitmap.GetPixelsAsync();
using (var softwareBitmap = SoftwareBitmap.CreateCopyFromBuffer(
pixels,
BitmapPixelFormat.Bgra8,
bitmap.PixelWidth,
bitmap.PixelHeight,
BitmapAlphaMode.Premultiplied))
{
var brush = CompositionImageBrush.FromBGRASoftwareBitmap(
visual.Compositor,
softwareBitmap,
new Size(bitmap.PixelWidth, bitmap.PixelHeight));
mask = brush.Brush;
}
}
}
}
catch (Exception exc)
{
Debug.WriteLine($"Failed to get AlphaMask {exc}");
mask = null;
}
return(mask);
}
public static CompositionImageBrush FromBGRASoftwareBitmap(
Compositor compositor,
SoftwareBitmap bitmap,
Size outputSize)
{
CompositionImageBrush brush = new CompositionImageBrush();
brush.CreateDevice(compositor);
brush.CreateDrawingSurface(outputSize);
brush.DrawSoftwareBitmap(bitmap, outputSize);
brush.CreateSurfaceBrush(compositor);
return(brush);
}
