How to create a semi transparent shape?

In OpenCV 3 this code worked for me:

cv::Mat source = cv::imread("IMG_2083s.png");
cv::Mat overlay;
double alpha = 0.3;

// copy the source image to an overlay
source.copyTo(overlay);

// draw a filled, yellow rectangle on the overlay copy
cv::rectangle(overlay, cv::Rect(100, 100, 300, 300), cv::Scalar(0, 125, 125), -1);

// blend the overlay with the source image
cv::addWeighted(overlay, alpha, source, 1 - alpha, 0, source);

Source/Inspired by: http://bistr-o-mathik.org/2012/06/13/simple-transparency-in-opencv/

The image below illustrates transparency using OpenCV. You need to do an alpha blend between the image and the rectangle. Below is the code for one way to do this.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>

int main( int argc, char** argv )
{
    cv::Mat image = cv::imread("IMG_2083s.png"); 
    cv::Mat roi = image(cv::Rect(100, 100, 300, 300));
    cv::Mat color(roi.size(), CV_8UC3, cv::Scalar(0, 125, 125)); 
    double alpha = 0.3;
    cv::addWeighted(color, alpha, roi, 1.0 - alpha , 0.0, roi); 

    cv::imshow("image",image);
    cv::waitKey(0);
}