C# Bitmap SetPixel

Question

I am trying to do some assignment for my course and i am having issues with the SetPixel method on the Bitmap object.

            // Retrieve the image.
            image = new Bitmap(FileLocation);
            SobelImage = image;

            int x, y;

            // Loop through the images pixels to reset color.
            for (x = 0; x < image.Width; x++)
            {
                for (y = 0; y < image.Height; y++)
                {

                    Console.WriteLine(String.Format("At {0} : {1}, Brightness : {2}", x, y, image.GetPixel(x, y).GetBrightness()));


                    Pixel pixel = new Pixel(image.GetPixel(x, y).GetBrightness());

                    SobelImage.SetPixel(x, y, Color.FromArgb(image.GetPixel(x, y).R, 0, 0)); 
                 }
             }

Any help please, this is the exception : An unhandled exception of type 'System.InvalidOperationException' occurred in System.Drawing.dll

Additional information: SetPixel is not supported for images with indexed pixel formats.

As a side-note: If you want decent performance, don't use `GetPixel` and `SetPixel`. They're *very* slow. See [SetPixel is too slow. Is there a faster way to draw to bitmap?](http://stackoverflow.com/questions/7768711/setpixel-is-too-slow-is-there-a-faster-way-to-draw-to-bitmap) — CodesInChaos, Oct 14 '15 at 17:00
Note that most lockbits implementation will not use indexed formats. For these you may need to analyze and change the palette in addition to the pixel indices.. — TaW, Oct 14 '15 at 17:27

score 1 · Answer 1 · answered Oct 14 '15 at 16:53

As the error message says the SetPixel method is not supported for that kind of image.

You have an image format with indexed colors, for example a GIF or an 8-bit PNG, where the color of each pixel is an index into a color palette. You can't change the color of a pixel, because the color is not actually stored in the pixel.

To use the SetPixel method you can create a new empty bitmap, which will be in 32-bit argb format, and draw the image on it:

Bitmap newImage = new Bitmap(image.Width, image.Height);
using (Graphics graphics = Graphics.FromImage(newImage)) {
  graphics.DrawImage(image, 0, 0);
}

The new image will support SetPixel. Note that the image format will be different when you save it.

Ah Thanks, got it working. Where then is the colour stored? As you can ask for the R, B and G value with no issues? — RobouteGuiliman, Oct 14 '15 at 17:04
The colors are in a color palette, and each pixel contains a color index in the palette. When the color is read the index in the pixel is used to get the color from the palette. — Guffa, Oct 14 '15 at 17:16

score 0 · Accepted Answer · answered Oct 14 '15 at 16:59

You can not set individual pixels in an indexed bitmap because the of the way data stored. For an indexed bitmap the RGB values are coming from a table by index, but it is a one way operation - you can't just get an index from RGB value because the table could not have a suitable value by the time of a PutPixel operation. A new table could be created from all the RGB values in an RGB bitmap, while processing the entire picture and this is quite expensive operation.

You need to create a new Bitmap in an RGB format, and put pixels there.

Bitmap newImage = new Bitmap(sourceImage.Width, sourceImagr.Height);

As a side note, you should use Bitmap objects inside using construct.

C# Bitmap SetPixel

2 Answers2