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

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.

Answer 2

You dont need a color matrix to achive this, just simply change encoding to CCITT! That only Black & White. Result remains correct and result file size is very small. Also much more efficient and faster than System.DrawImage.

This is the perfect solution:

public void toCCITT(string tifURL)
{
    byte[] imgBits = File.ReadAllBytes(tifURL);

    using (MemoryStream ms = new MemoryStream(imgBits))
    {
        using (Image i = Image.FromStream(ms))
        {
            EncoderParameters parms = new EncoderParameters(1);
            ImageCodecInfo codec = ImageCodecInfo.GetImageDecoders()
                                                 .FirstOrDefault(decoder => decoder.FormatID == ImageFormat.Tiff.Guid);

            parms.Param[0] = new EncoderParameter(Encoder.Compression, (long)EncoderValue.CompressionCCITT4);

            i.Save(@"c:\test\result.tif", codec, parms);
        }
    }
}

Answer 3

VB.NET version:

Using gr As Graphics = Graphics.FromImage(SourceImage) 'SourceImage is a Bitmap object'
  Dim gray_matrix As Single()() = {
    New Single() {0.299F, 0.299F, 0.299F, 0, 0},
    New Single() {0.587F, 0.587F, 0.587F, 0, 0},
    New Single() {0.114F, 0.114F, 0.114F, 0, 0},
    New Single() {0, 0, 0, 1, 0},
    New Single() {0, 0, 0, 0, 1}
  }
  Dim ia As New System.Drawing.Imaging.ImageAttributes
  ia.SetColorMatrix(New System.Drawing.Imaging.ColorMatrix(gray_matrix))
  ia.SetThreshold(0.8)
  Dim rc As New Rectangle(0, 0, SourceImage.Width, SourceImage.Height)
  gr.DrawImage(SourceImage, rc, 0, 0, SourceImage.Width, SourceImage.Height, GraphicsUnit.Pixel, ia)
End Using

Answer 4

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 5

If unsafe is acceptable and ColorMatrix not required, you could use the following.

It's approx. 3 times faster than DrawImage and ColorMatrix.

    // make in place Bitmap 24bpp B&W from a Bitmap 24bpp color
    public static unsafe void Bmp24bppColorToBmp24bppBW(ref Bitmap bmp) {
        // sanity check
        if ( bmp.PixelFormat != PixelFormat.Format24bppRgb ) {
            return;
        }
        // lock bmp
        BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height),
                                          ImageLockMode.ReadWrite,
                                          PixelFormat.Format24bppRgb);
        byte* scan0 = (byte*)bmpData.Scan0.ToPointer();
        // loop bmp length
        int bmpLen = bmpData.Stride * bmp.Height;
        for ( int i = 0; i < bmpLen; i += 3 ) {
            // current pixel value
            byte pixVal = (byte)((scan0[i] + scan0[i + 1] + scan0[i + 2]) / 3.0f + 0.5f);
            // make B & W --> adjust threshold, here 200
            pixVal = (byte)(pixVal > 200 ? 255 : 0);
            // write to bmp
            scan0[i + 0] = pixVal;
            scan0[i + 1] = pixVal;
            scan0[i + 2] = pixVal;
        }
        // unlock bmp
        bmp.UnlockBits(bmpData);
    }