How can merge two images to be one image with the two images combined one transparent over the second one?

Question

Bitmap bmp1 = new Bitmap(@"c:\coneimages\Cone_Images1.gif");
Bitmap bmp2 = new Bitmap(@"c:\coneimages\PictureBox_Images1.gif");
Bitmap bmp3 = new Bitmap(MergeTwoImages(bmp1,bmp2));
bmp3.Save(@"c:\coneimages\merged.bmp");

And

public static Bitmap MergeTwoImages(Image firstImage, Image secondImage)
        {
            if (firstImage == null)
            {
                throw new ArgumentNullException("firstImage");
            }

            if (secondImage == null)
            {
                throw new ArgumentNullException("secondImage");
            }

            int outputImageWidth = firstImage.Width > secondImage.Width ? firstImage.Width : secondImage.Width;

            int outputImageHeight = firstImage.Height + secondImage.Height + 1;

            Bitmap outputImage = new Bitmap(outputImageWidth, outputImageHeight, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

            using (Graphics graphics = Graphics.FromImage(outputImage))
            {
                graphics.DrawImage(firstImage, new Rectangle(new Point(), firstImage.Size),
                    new Rectangle(new Point(), firstImage.Size), GraphicsUnit.Pixel);
                graphics.DrawImage(secondImage, new Rectangle(new Point(0, firstImage.Height + 1), secondImage.Size),
                    new Rectangle(new Point(), secondImage.Size), GraphicsUnit.Pixel);
            }

            return outputImage;
        }

But that's not what i wanted i'm not sure how and what to look for in google. What i want is that the bmp1 will over bmp2 like layer. The bmp1 is transparent and i want it be like layer over bmp2.

So in bmp3 i will see the whole regular bmp2 with bmp1 on it.

Answer 1

Assuming bmp1 has alpha in it, draw bmp2 first , THEN with compositing mode set to SourceOver (the default) draw bmp1 . This should accomplish the correct alpha blend order.

In other words...

Bitmap bmp3 = new Bitmap(MergeTwoImages(bmp2,bmp1)); // Swapped arguments.

If bmp1 does not contain alpha, you will need to use a color matrix to change the transparency.

Answer 2

Bitmap m_back = new Bitmap(bmp2.Width, bmp2.Height);
for (int y = 0; y < bmp2.Height; y++)
{
     for (int x = 0; x < bmp2.Width; x++)
     {
          Color temp = Color.FromArgb(80, bmp2.GetPixel(x, y));
          m_back.SetPixel(x, y, temp);
     }
}

Bitmap bmp3 = new Bitmap(MergeTwoImages(bmp1, m_back));