How to create a semi transparent shape?

I would like to know how to draw semi-transparent shapes in OpenCV, similar to those in the image below (from http://tellthattomycamera.wordpress.com/)

enter image description here

I don't need those fancy circles, but I would like to be able to draw a rectangle, e.g, on a 3 channel color image and specify the transparency of the rectangle, something like

rectangle (img, Point (100,100), Point (300,300), Scalar (0,125,125,0.4), CV_FILLED);

where 0,125,125 is the color of the rectangle and 0.4 specifies the transparency. However OpenCV doesn't have this functionality built into its drawing functions. How can I draw shapes in OpenCV so that the original image being drawn on is partially visible through the shape?

Answers:

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.

enter image description here

#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);
}

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