Efficient, generic scan image

edit

then , just do it:

Mat img = ...
img = img * alpha + beta;

in other words, opencv already has overloaded operators for this. if you catch yourself writing your own your own per-pixel-loops, you're doing it the wrong way.

Thank you for answer. Sorry I provided probably but example. I know that we have also converTo image.convertTo(new_image, -1, alpha, beta);

But What about below code. Join two images base on alpha channel of image2.

    void join(cv::Mat& image, const cv::Mat& image2)
    {
        assert(image.rows == image2.rows);
        assert(image.cols == image2.cols);

        auto it = image.begin<cv::Vec4b>();
        auto it2 = image2.begin<cv::Vec4b>();
        const auto itEnd = image.end<cv::Vec4b>();
        for (; it != itEnd; ++it, ++it2)
        {
            double c1 = (255 - (*it2)[3]) / 255.0;
            double c2 = (*it2)[3] / 255.0;
            (*it)[0] = (*it)[0] * c1 + (*it2)[0] * c2;
            (*it)[1] = (*it)[1] * c1 + (*it2)[1] * c2;
            (*it)[2] = (*it)
(*it)[3] = 255;
    }
    }

As @berak said, just use overloaded operators, in this case applied to the corresponding channel.

I do not know how to use operators instead of below three lines. Could you write it for me?

double c1 = (255 - (*it2)[3]) / 255.0;
double c2 = (*it2)[3] / 255.0;
(*it)[0] = (*it)[0] * c1 + (*it2)[0] * c2;

Thank you in advance.