Billboard transparency

Hi,

I create a new Billboard and set an image on it using a DifuseMaterial:

var material = new DiffuseMaterial(new ImageBrush(new BitmapImage(new Uri($"Images/target.png", UriKind.Relative))));
_billboard1.Material = material;

The image is a PNG file with transparency. But the transparent pixels are showing as white on the HelixViewport3D:

Are there any secrets when working with transparency or is it not supported?

Thanks,

Rodrigo