Convert an image to grayscale

Question

Is there a way to convert an image to grayscale 16 bits per pixel format, rather than setting each of the r,g and b components to luminance. I currently have a bmp from file.

Bitmap c = new Bitmap("filename");

I want a Bitmap d, that is grayscale version of c. I do see a constructor that includes System.Drawing.Imaging.PixelFormat, but I don't understand how to use that. I'm new to Image Processing and the relevant C# libraries, but have a moderate experience with C# itself.

Any help, reference to an online source, hint or suggestion will be appreciated.

EDIT: d is the grayscale version of c.

Answer 1

"I want a Bitmap d, that is grayscale. I do see a consructor that includes System.Drawing.Imaging.PixelFormat, but I don't understand how to use that."

Here is how to do this

Bitmap grayScaleBP = new 
         System.Drawing.Bitmap(2, 2, System.Drawing.Imaging.PixelFormat.Format16bppGrayScale);

EDIT: To convert to grayscale

             Bitmap c = new Bitmap("fromFile");
             Bitmap d;
             int x, y;

             // Loop through the images pixels to reset color.
             for (x = 0; x < c.Width; x++)
             {
                 for (y = 0; y < c.Height; y++)
                 {
                     Color pixelColor = c.GetPixel(x, y);
                     Color newColor = Color.FromArgb(pixelColor.R, 0, 0);
                     c.SetPixel(x, y, newColor); // Now greyscale
                 }
             }
            d = c;   // d is grayscale version of c  

Faster Version from switchonthecode follow link for full analysis:

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
   using(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
       using(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);
       }
   }
   return newBitmap;
}

Answer 2

Bitmap d = new Bitmap(c.Width, c.Height);

for (int i = 0; i < c.Width; i++)
{
    for (int x = 0; x < c.Height; x++)
    {
        Color oc = c.GetPixel(i, x);
        int grayScale = (int)((oc.R * 0.3) + (oc.G * 0.59) + (oc.B * 0.11));
        Color nc = Color.FromArgb(oc.A, grayScale, grayScale, grayScale);
        d.SetPixel(i, x, nc);
    }
}

This way it also keeps the alpha channel.
Enjoy.

Answer 3

There's a static method in ToolStripRenderer class, named CreateDisabledImage. Its usage is as simple as:

Bitmap c = new Bitmap("filename");
Image d = ToolStripRenderer.CreateDisabledImage(c);

It uses a little bit different matrix than the one in the accepted answer and additionally multiplies it by a transparency of value 0.7, so the effect is slightly different than just grayscale, but if you want to just get your image grayed, it's the simplest and best solution.

Answer 4

The code below is the simplest solution:

Bitmap bt = new Bitmap("imageFilePath");

for (int y = 0; y < bt.Height; y++)
{
    for (int x = 0; x < bt.Width; x++)
    {
        Color c = bt.GetPixel(x, y);

        int r = c.R;
        int g = c.G;
        int b = c.B;
        int avg = (r + g + b) / 3;
        bt.SetPixel(x, y, Color.FromArgb(avg,avg,avg));
    }   
}

bt.Save("d:\\out.bmp");

Answer 5

None of the examples above create 8-bit (8bpp) bitmap images. Some software, such as image processing, only supports 8bpp. Unfortunately the MS .NET libraries do not have a solution. The PixelFormat.Format8bppIndexed format looks promising but after a lot of attempts I couldn't get it working.

To create a true 8-bit bitmap file you need to create the proper headers. Ultimately I found the Grayscale library solution for creating 8-bit bitmap (BMP) files. The code is very simple:

Image image = Image.FromFile("c:/path/to/image.jpg");
GrayBMP_File.CreateGrayBitmapFile(image, "c:/path/to/8bpp/image.bmp");

The code for this project is far from pretty but it works, with one little simple-to-fix problem. The author hard-coded the image resolution to 10x10. Image processing programs do not like this. The fix is open GrayBMP_File.cs (yeah, funky file naming, I know) and replace lines 50 and 51 with the code below. The example sets the resolution to 200x200 but you should change it to the proper number.

int resX = 200;
int resY = 200;
// horizontal resolution
Copy_to_Index(DIB_header, BitConverter.GetBytes(resX * 100), 24);
// vertical resolution 
Copy_to_Index(DIB_header, BitConverter.GetBytes(resY * 100), 28);

Answer 6

To summarize a few items here: There are some pixel-by-pixel options that, while being simple just aren't fast.

@Luis' comment linking to: (archived) https://web.archive.org/web/20110827032809/http://www.switchonthecode.com/tutorials/csharp-tutorial-convert-a-color-image-to-grayscale is superb.

He runs through three different options and includes timings for each.