Question

I need to do some thresholding for my image. The threshold filter function just accepts 8-16bpp grayscale. My bitmap picture has the 32bppRGB pixelformat. Please suggest some code for the same. (I also want to know if it is possible without pixel by pixel operations)

p.s. I am using the Aforge.NET for thresholding.

Thanks

-Sagar

Answer 1

Easiest way:

public static Bitmap MakeGrayscale(Bitmap original)
 {
    //make an empty bitmap the same size as original
    Bitmap newBitmap = new Bitmap(original.Width, original.Height);

   for (int i = 0; i < original.Width; i++)
    {
       for (int j = 0; j < original.Height; j++)
       {
          //get the pixel from the original image
          Color originalColor = original.GetPixel(i, j);

         //create the grayscale version of the pixel
          int grayScale = (int)((originalColor.R * .3) + (originalColor.G * .59)
              + (originalColor.B * .11));

         //create the color object
          Color newColor =  Color.FromArgb(grayScale, grayScale, grayScale);

          //set the new image's pixel to the grayscale version
          newBitmap.SetPixel(i, j, newColor);
         }
     }

    return newBitmap;
 }

Faster way:

public static Bitmap MakeGrayscale2(Bitmap original)
 {
    unsafe
    {
       //create an empty bitmap the same size as original
       Bitmap newBitmap = new Bitmap(original.Width, original.Height);

      //lock the original bitmap in memory
       BitmapData originalData = original.LockBits(
          new Rectangle(0, 0, original.Width, original.Height),
          ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb);

      //lock the new bitmap in memory
       BitmapData newData = newBitmap.LockBits(
          new Rectangle(0, 0, original.Width, original.Height), 
         ImageLockMode.WriteOnly, PixelFormat.Format24bppRgb);

       //set the number of bytes per pixel
       int pixelSize = 3;

      for (int y = 0; y < original.Height; y++)
       {
          //get the data from the original image
          byte* oRow = (byte*)originalData.Scan0 + (y * originalData.Stride);

         //get the data from the new image
          byte* nRow = (byte*)newData.Scan0 + (y * newData.Stride);

         for (int x = 0; x < original.Width; x++)
          {
             //create the grayscale version
             byte grayScale = 
               (byte)((oRow[x * pixelSize] * .11) + //B
                (oRow[x * pixelSize + 1] * .59) +  //G
                (oRow[x * pixelSize + 2] * .3)); //R

            //set the new image's pixel to the grayscale version
             nRow[x * pixelSize] = grayScale; //B
             nRow[x * pixelSize + 1] = grayScale; //G
             nRow[x * pixelSize + 2] = grayScale; //R
          }
       }

      //unlock the bitmaps
       newBitmap.UnlockBits(newData);
       original.UnlockBits(originalData);

      return newBitmap;
    }
 }

Fastest way:

public static Bitmap MakeGrayscale3(Bitmap original)
 {
    //create a blank bitmap the same size as original
    Bitmap newBitmap = new Bitmap(original.Width, original.Height);

    //get a graphics object from the new image
    Graphics g = Graphics.FromImage(newBitmap);

   //create the grayscale ColorMatrix
    ColorMatrix colorMatrix = new ColorMatrix(
       new float[][] 
      {
          new float[] {.3f, .3f, .3f, 0, 0},
          new float[] {.59f, .59f, .59f, 0, 0},
          new float[] {.11f, .11f, .11f, 0, 0},
          new float[] {0, 0, 0, 1, 0},
          new float[] {0, 0, 0, 0, 1}
       });

   //create some image attributes
    ImageAttributes attributes = new ImageAttributes();

   //set the color matrix attribute
    attributes.SetColorMatrix(colorMatrix);

   //draw the original image on the new image
    //using the grayscale color matrix
    g.DrawImage(original, new Rectangle(0, 0, original.Width, original.Height),
       0, 0, original.Width, original.Height, GraphicsUnit.Pixel, attributes);

   //dispose the Graphics object
    g.Dispose();
    return newBitmap;
 }