OpenCV (Emgu.CV) — compositing images with alpha

Question

I\'m using Emgu.CV to perform some basic image manipulation and composition. My images are loaded as Image.

Question #1:

Answer 1

Using Osiris's suggestion as a starting point, and having checked out alpha compositing on Wikipedia, i ended up with the following which worked really nicely for my purposes.

This was used this with Emgucv. I was hoping that the opencv gpu::AlphaComposite methods were available in Emgucv which I believe would have done the following for me, but alas the version I am using didn't appear to have them implemented.

static public Image Overlay( Image image1, Image image2 )
        {

            Image result = image1.Copy();
            Image src = image2;
            Image dst = image1;

            int rows = result.Rows;
            int cols = result.Cols;
            for (int y = 0; y < rows; ++y)
            {
                for (int x = 0; x < cols; ++x)
                {
                    // http://en.wikipedia.org/wiki/Alpha_compositing
                    double  srcA = 1.0/255 * src.Data[y, x, 3];
                    double dstA = 1.0/255 * dst.Data[y, x, 3];
                    double outA = (srcA + (dstA - dstA * srcA));
                    result.Data[y, x, 0] = (Byte)(((src.Data[y, x, 0] * srcA) + (dst.Data[y, x, 0] * (1 - srcA))) / outA);  // Blue
                    result.Data[y, x, 1] = (Byte)(((src.Data[y, x, 1] * srcA) + (dst.Data[y, x, 1] * (1 - srcA))) / outA);  // Green
                    result.Data[y, x, 2] = (Byte)(((src.Data[y, x, 2] * srcA) + (dst.Data[y, x, 2] * (1 - srcA))) / outA); // Red
                    result.Data[y, x, 3] = (Byte)(outA*255);
                }
            }
            return result;
        }

A newer version, using emgucv methods. rather than a loop. Not sure it improves on performance. double unit = 1.0 / 255.0; Image[] dstS = dst.Split(); Image[] srcS = src.Split(); Image[] rs = result.Split();

        Image srcA = srcS[3] * unit;
        Image dstA = dstS[3] * unit;
        Image outA = srcA.Add(dstA.Sub(dstA.Mul(srcA)));// (srcA + (dstA - dstA * srcA));

        // Red.
        rs[0] = srcS[0].Mul(srcA).Add(dstS[0].Mul(1 - srcA)).Mul(outA.Pow(-1.0)); // Mul.Pow is divide.
        rs[1] = srcS[1].Mul(srcA).Add(dstS[1].Mul(1 - srcA)).Mul(outA.Pow(-1.0));
        rs[2] = srcS[2].Mul(srcA).Add(dstS[2].Mul(1 - srcA)).Mul(outA.Pow(-1.0));
        rs[3] = outA.Mul(255);

        // Merge image back together.
        CvInvoke.cvMerge(rs[0], rs[1], rs[2], rs[3], result);
        return result.Convert();