混合后甚至想要透明图像

Question

我正在尝试混合两个图像here。

这是我的全部代码

#include <cv.h>
#include <highgui.h>
#include <iostream>

using namespace cv;

int main( int argc, char** argv )
{
 double beta; double input;

 Mat src1, src2, dst;

 /// Ask the user enter alpha
 std::cout<<" Simple Linear Blender "<<std::endl;
 std::cout<<"-----------------------"<<std::endl;

 src1 = imread("face.jpg");
 src2 = imread("necklace1.png");

 if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src2.data ) { printf("Error loading src2 \n"); return -1; }

 double alpha = 0.1; // something
 int min_x = ( (src1.cols - src2.cols)/2 );
 int min_y = ( (src1.rows - src2.rows)/2 );
 int width, height;

 // min_x, min_y should be valid in src1 and [width height] = size(src2)
 cv::Rect roi = cv::Rect(min_x, min_y, src2.cols, src2.rows);  

 // "out_image" is the output ; i.e. src1 with a part of it blended with  src2
 cv::Mat out_image = src1.clone();

 // Set the ROIs for the selected sections of src1 and out_image (the same at the moment)
 cv::Mat src1_roi= src1(roi);
 cv::Mat out_image_roi = out_image(roi);

 // Blend the ROI of src1 with src2 into the ROI of out_image
 cv::addWeighted(src1_roi,alpha,src2,1-alpha,0.0,out_image_roi);

 imshow( "Linear Blend", out_image );

 waitKey(0);
 return 0;
}

我的输入图片是

但输出结果是

我希望这条项链像以前一样透明。怎么做？

我在cvtColor( out_image_roi,out_image_roi, CV_BGR2BGRA );

之前尝试了cv::addWeighted(src1_roi,alpha,src2,1-alpha,0.0,out_image_roi);

但是输出只是面部图像。