// Drawing against a transparent black background blends
// the 'alpha' pixels against black, leaving a black outline.
// Interpolate between black and white
// based on it's intensity to covert this 'outline' to
// it's grayscale equivalent.
public static void ConvertToGrey(Bitmap bitmap)
{
var transBlack = Color.Transparent;
using (var bitmapData = new PixelAccessor(bitmap, ImageLockMode.ReadWrite))
{
for (int y = 0; y < bitmap.Height; y++)
{
for (int x = 0; x < bitmap.Width; x++)
{
System.Drawing.Color color = bitmapData[x, y];
if (color.ColorsEqual(transBlack))
{
continue;
}
var alpha = color.A / (255.0f);
var col = Color.Lerp(Color.Transparent, Color.White, alpha);
color = System.Drawing.Color.FromArgb(color.A, col.R, col.G, col.B);
bitmapData [x, y] = color;
}
}
}
}
