FormatConvertedBitmap poor Black and White results - wpf

Question

This seems to be the standard method to convert the pixel format of a bitmap but it's giving me a very poor result when converting to black and white format:
// convert to black & white bitmap
FormatConvertedBitmap monoBitmap = new FormatConvertedBitmap(bitmap, PixelFormats.BlackWhite, null, 0);
// save black & white bitmap to file
fileName = string.Format("{0}.bmp", data.SelectedProduct.Code);
bitmapEncoder = new BmpBitmapEncoder();
bitmapEncoder.Frames.Add(BitmapFrame.Create(monoBitmap));
using (Stream stream = File.Create(fileName))
{
bitmapEncoder.Save(stream);
}
The resulting image file is very grainy and pixelated. I need to send it to a line printer so I need sharp edges.
The original before conversion looks fine (its 32BPP color) and if I use something like IrfanView to manually convert the original to black and white it also comes out much better than whatever .NET is doing.
Is there another alternative for doing this in .NET instead of FormatConvertedBitmap?

Answer 1

It are using PixelFormats.BlackWhite, which uses only black and white colors in the resulting image, which probably can make it look grainy if there are light pixels surrounded by darker ones. I suppose the effect can be better, if you convert the image to use the shades of gray. In order to do this, you this line of code instead of the one in the code you've posted:
FormatConvertedBitmap monoBitmap = new FormatConvertedBitmap(bitmap, PixelFormats.Gray32Float, null, 0);
This should convert the image to use only only shades of gray, so no color will be needed to print it.
Let me know if it's the effect you want to achieve.
You can find some more information about PixelFormats class here: http://msdn.microsoft.com/en-us/library/system.windows.media.pixelformats.aspx.