Can you change one colour to another in an Bitmap image?

Question

For Bitmap, there is a MakeTransparent method, is there one similar for changing one color to another?

// This sets Color.White to          


        

Answer 1

Lifting the code from this answer:

public static class BitmapExtensions
{
    public static Bitmap ChangeColor(this Bitmap image, Color fromColor, Color toColor)
    {
        ImageAttributes attributes = new ImageAttributes();
        attributes.SetRemapTable(new ColorMap[]
        {
            new ColorMap()
            {
                OldColor = fromColor,
                NewColor = toColor,
            }
        }, ColorAdjustType.Bitmap);

        using (Graphics g = Graphics.FromImage(image))
        {
            g.DrawImage(
                image,
                new Rectangle(Point.Empty, image.Size),
                0, 0, image.Width, image.Height,
                GraphicsUnit.Pixel,
                attributes);
        }

        return image;
    }
}

While I haven't benchmarked it, this should be faster than any solution that's doing GetPixel/SetPixel in a loop. It's also a bit more straightforward.

Answer 2

Through curiosity to Yorye Nathan's comment, This is an extension that I created by modifying http://msdn.microsoft.com/en-GB/library/ms229672(v=vs.90).aspx.

It can turn all pixels in a bitmap from one colour to another.

public static class BitmapExt
{
    public static void ChangeColour(this Bitmap bmp, byte inColourR, byte inColourG, byte inColourB, byte outColourR, byte outColourG, byte outColourB)
    {
        // Specify a pixel format.
        PixelFormat pxf = PixelFormat.Format24bppRgb;

        // Lock the bitmap's bits.
        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        BitmapData bmpData =
        bmp.LockBits(rect, ImageLockMode.ReadWrite,
                     pxf);

        // Get the address of the first line.
        IntPtr ptr = bmpData.Scan0;

        // Declare an array to hold the bytes of the bitmap. 
        // int numBytes = bmp.Width * bmp.Height * 3; 
        int numBytes = bmpData.Stride * bmp.Height;
        byte[] rgbValues = new byte[numBytes];

        // Copy the RGB values into the array.
        Marshal.Copy(ptr, rgbValues, 0, numBytes);

        // Manipulate the bitmap
        for (int counter = 0; counter < rgbValues.Length; counter += 3)
        {
            if (rgbValues[counter] == inColourR &&
                rgbValues[counter + 1] == inColourG &&
                rgbValues[counter + 2] == inColourB)

             {
                rgbValues[counter] = outColourR;
                rgbValues[counter + 1] = outColourG;
                rgbValues[counter + 2] = outColourB;
             }
        }

        // Copy the RGB values back to the bitmap
        Marshal.Copy(rgbValues, 0, ptr, numBytes);

        // Unlock the bits.
        bmp.UnlockBits(bmpData);
    }
}

called by bmp.ChangeColour(0,128,0,0,0,0);

Answer 3

You can use SetPixel for that:

private void ChangeColor(Bitmap s, System.Drawing.Color source, System.Drawing.Color target)
{
    for (int x = 0; x < s.Width; x++)
    {
        for (int y = 0; y < s.Height; y++)
        {
            if (s.GetPixel(x, y) == source)
                s.SetPixel(x, y, target);
        }                
    }
}

GetPixel and SetPixel are wrappers around gdiplus.dll functions GdipBitmapGetPixel and GdipBitmapSetPixel accordingly

Remarks:

Depending on the format of the bitmap, GdipBitmapGetPixel might not return the same value as was set by GdipBitmapSetPixel. For example, if you call GdipBitmapSetPixel on a Bitmap object whose pixel format is 32bppPARGB, the pixel's RGB components are premultiplied. A subsequent call to GdipBitmapGetPixel might return a different value because of rounding. Also, if you call GdipBitmapSetPixel on a Bitmap object whose color depth is 16 bits per pixel, information could be lost during the conversion from 32 to 16 bits, and a subsequent call to GdipBitmapGetPixel might return a different value.

Answer 4

You need a library that provides a way to modify the color space of an image without having to work with pixels. LeadTools has a pretty extensive image library that you can use that supports color space modifications, including swapping colors.