How to create a semi transparent shape?

Question

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/)

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?

Answer 1

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);
}