//in safari, when canvas is captured, that has transparency, it fills transparent areas with 255 not black
//we force transparent areas to be black instead
static void PreProcessBitmap(RasterImage derivedImage)
{
using (RasterImage alpha = derivedImage.CreateAlphaImage())
{
alpha.AddColorRgbRangeToRegion(new RasterColor(0, 0, 0), new RasterColor(3, 3, 3), RasterRegionCombineMode.Set);
derivedImage.SetRegion(null, alpha.GetRegion(null), RasterRegionCombineMode.Set);
FillCommand fill = new FillCommand(new RasterColor(0, 0, 0));
fill.Run(derivedImage);
}
}