What would be a good TRUE black and white colormatrix?

Question

I want to convert an image from color to B/W (i.e. no grayscale, just black and white). Does anyone have a good colormatrix to achieve this?

Answer 1

If you want it to look halfway decent, you'll probably want to apply some form of dithering.

Here's a full discussion, if a bit dated:

http://www.efg2.com/Lab/Library/ImageProcessing/DHALF.TXT

Answer 2

I've finally found a solution to my problem:

1. Transform the image to grayscale, using well a known colormatrix.
2. Use SetThreshold method of the ImageAttributes class to set the threshold that separates black from white.

Here is the C# code:

using (Graphics gr = Graphics.FromImage(SourceImage)) // SourceImage is a Bitmap object
        {                
            var gray_matrix = new float[][] { 
                new float[] { 0.299f, 0.299f, 0.299f, 0, 0 }, 
                new float[] { 0.587f, 0.587f, 0.587f, 0, 0 }, 
                new float[] { 0.114f, 0.114f, 0.114f, 0, 0 }, 
                new float[] { 0,      0,      0,      1, 0 }, 
                new float[] { 0,      0,      0,      0, 1 } 
            };

            var ia = new System.Drawing.Imaging.ImageAttributes();
            ia.SetColorMatrix(new System.Drawing.Imaging.ColorMatrix(gray_matrix));
            ia.SetThreshold(0.8); // Change this threshold as needed
            var rc = new Rectangle(0, 0, SourceImage.Width, SourceImage.Height);
            gr.DrawImage(SourceImage, rc, 0, 0, SourceImage.Width, SourceImage.Height, GraphicsUnit.Pixel, ia);                
        }

I've benchmarked this code and it is approximately 40 times faster than pixel by pixel manipulation.