C# – Convert 32-bit Bitmap to 8-bit (both color and grayscale)

cnetsystem.drawing

I have a System.Drawing.Bitmap with PixelFormat of Format32bppRgb.
I want this image to be converted to a bitmap of 8bit.

The following is the code to convert a 32-bit image to an 8-bit grayscale image:

        public static Bitmap ToGrayscale(Bitmap bmp)
        {
            int rgb;
            System.Drawing.Color c;

            for (int y = 0; y < bmp.Height; y++)
                for (int x = 0; x < bmp.Width; x++)
                {
                    c = bmp.GetPixel(x, y);
                    rgb = (int)((c.R + c.G + c.B) / 3);
                    bmp.SetPixel(x, y, System.Drawing.Color.FromArgb(rgb, rgb, rgb));
                }
            return bmp;
        }

However, the Bitmap I end up with still has the PixelFormat Property of Format32bppRgb.

Also,

  • How can I convert a 32-bit color image into an 8-bit color image?

Thanks for any input!

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Best Answer

You must create (and return) new instance of Bitmap.

PixelFormat is specified in constructor of Bitmap and can not be changed.

EDIT: Sample code based on this answer on MSDN:

    public static Bitmap ToGrayscale(Bitmap bmp) {
        var result = new Bitmap(bmp.Width, bmp.Height, PixelFormat.Format8bppIndexed);

        BitmapData data = result.LockBits(new Rectangle(0, 0, result.Width, result.Height), ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

        // Copy the bytes from the image into a byte array
        byte[] bytes = new byte[data.Height * data.Stride];
        Marshal.Copy(data.Scan0, bytes, 0, bytes.Length);

        for (int y = 0; y < bmp.Height; y++) {
            for (int x = 0; x < bmp.Width; x++) {
                var c = bmp.GetPixel(x, y);
                var rgb = (byte)((c.R + c.G + c.B) / 3);

                bytes[x * data.Stride + y] = rgb;
            }
        }

        // Copy the bytes from the byte array into the image
        Marshal.Copy(bytes, 0, data.Scan0, bytes.Length);

        result.UnlockBits(data);

        return result;
    }
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