Merging JPGs with GDI in C#

Question

My scenario:

• I have one color background image JPG.
• I have one black text on white background JPG.
• Both images are the same size (height and width)

I want to overlay the image with black text and white background over the color background image, i.e. the white background becomes transparent to see the color background beneath it.

How can I do this with GDI in C#?

Thanks!

Answer 1

If the images are the same size, iterate over them and "AND" the colors for each pixel. For the white pixels, you should get the color of the other image, and for the black ones you should get black.

If they're not the same size, scale first.

I'm making this up off the top of my head, but something like:

Color destColor = Color.FromArgb(pixel1.ToArgb() & pixel2.ToArgb());

Answer 2

check out this article. it give you code for making a specified color as the transparent color in your image http://www.codedblog.com/2007/08/28/generating-a-transparent-gif-image-using-c/

Answer 3

Thanks to GalacticCowboy I was able to come up with this solution:

using (Bitmap background = (Bitmap)Bitmap.FromFile(backgroundPath))
{
     using (Bitmap foreground = (Bitmap)Bitmap.FromFile(foregroundPath))
     {
          // check if heights and widths are the same
          if (background.Height == foreground.Height & background.Width == foreground.Width)
          {
               using (Bitmap mergedImage = new Bitmap(background.Width, background.Height))
               {
                    for (int x = 0; x < mergedImage.Width; x++)
                    {
                         for (int y = 0; y < mergedImage.Height; y++)
                         {
                              Color backgroundPixel = background.GetPixel(x, y);
                              Color foregroundPixel = foreground.GetPixel(x, y);
                              Color mergedPixel = Color.FromArgb(backgroundPixel.ToArgb() & foregroundPixel.ToArgb());
                              mergedImage.SetPixel(x, y, mergedPixel);
                          }
                    }
                    mergedImage.Save("filepath");
               }

          }
     }
}

Works like a charm. Thanks!

Answer 4

There exist easier and faster way. You should use ImageAttributes when you draw image that must be partially visible.

Image BackImage = Image.FromFile(backgroundPath);
using (Graphics g = Graphics.FromImage(BackImage))
{
    using (ForeImage = Image.FromFile(foregroundPath))
    {   
        ImageAttributes imageAttr = new ImageAttributes();
        imageAttr.SetColorKey(Color.FromArgb(245, 245, 245), Color.FromArgb(255, 255, 255),
            ColorAdjustType.Default);
        g.DrawImage(ForeImage, new Rectangle(0, 0, BackImage.Width, BackImage.Height),
            0, 0, BackImage.Width, BackImage.Height, GraphicsUnit.Pixel, imageAttr);
    }
}

SetColorKey method will make color from specified range transparent, so you can make your white bitmap pixels transparent, including all pixels that are affected to jpeg compression artefacts.