Creating a drop shadow

Requirement: Take a PNG image that has an alpha mask and from it generate a new bitmap which has the same alpha mask but every pixel is black.

Solution:

private Bitmap GenerateDropShadowImage(Image image)
{
  Bitmap bitmap = new Bitmap(image);
  BitmapData bits = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), System.Drawing.Imaging.ImageLockMode.ReadWrite, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

  IntPtr bottomScanLine = bits.Scan0;
  int bytesPerRow = bits.Width * 4;

  unsafe
  {
    byte* pixelValue = (byte*)bottomScanLine.ToPointer();
    for (int count = 0; count < bits.Width * bits.Height; count++)
    {
      pixelValue[0] = 0;
      pixelValue[1] = 0;
      pixelValue[2] = 0;
      pixelValue = pixelValue + 4;
    }
  }
  bitmap.UnlockBits(bits);
  return bitmap;
}