OpenCV Alpha Blending: Blend two cv::Mat with alpha mask. Poorly optimised.

gistfile1.cpp

void blendWithMask(cv::Mat &base, cv::Mat &src, cv::Mat &mask, cv::Mat &out){
    char ch = base.channels();
    double alpha = 0;
    for( int y = 0; y < base.rows; y++ ){
        uchar* pBS = base.ptr<uchar>(y);
        uchar* pSR = src.ptr<uchar>(y);
        uchar* pMK = mask.ptr<uchar>(y);
        uchar* pOU = out.ptr<uchar>(y);
        for( int x = 0; x < base.cols*ch; x++ ){
            int ix = x / ch;
            if(pMK[ix] == 255){
                pOU[x] = pBS[x];
            } else if(pMK[ix] == 0){
                pOU[x] = pSR[x];
            } else {
                alpha = pMK[ix] / 255.0;
                pOU[x] = pSR[x] * (1-alpha) + pBS[x] * alpha;
            }
        }
    }
}