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Merge source channel to final image including alpha.

  //Laod source and create alpha   
  Mat src=imread("red.png",1);   
  Mat alpha;   
  inRange(src, Scalar(0,0,250),  Scalar(0,0,255),alpha);    
  bitwise_not(alpha,alpha);

  //split source  
  Mat bgr[3];   
  split(src,bgr);   

  //Merge to final image including alpha   
  Mat tmp[4] = { bgr[0],bgr[1],bgr[2],alpha};   
  Mat dst;   
  merge(tmp,4,dst);   
  imwrite("dst.png",dst);