Creating separate image using a Contour from an image?

Question

I want to create a separate image using Contours from an image.

I have already seen the answers here and here, but by using them the background becomes black.

But i want to have a transparent background because i have to process those images further where black will create problem.

Question: How can i get a transparent background for the extracted image.

Currently i am using the following code though which i am able to create a separate imge but with black background:

Mat findRect::extractImage( int min_x, int min_y , int rows, int cols , Mat frame, vector<Point> ROI_Poly)
{
    Mat mask = Mat::zeros(frame.rows, frame.cols, CV_8UC1);

    // Fill polygon white
    fillConvexPoly(mask, &ROI_Poly[0], ROI_Poly.size(), 255, 8, 0);                 

    // Create new image for result storage
    Mat imageDest = cvCreateMat(frame.rows, frame.cols, CV_8UC3);

    // Cut out ROI and store it in imageDest
    frame.copyTo(imageDest, mask);  

    // Extracting the ROI only 
    roi = Rect (min_x, min_y, cols, rows);
    Mat detectedSquare;


    if( 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= frame.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= frame.rows )
        detectedSquare= imageDest(roi);

    //imshow("extracted image" , detectedSquare);
    return detectedSquare;
}

Answer 1

So you have already the contour and 3 channel source image.

1. Now just create single channel image by drawing contour and inverting it, this will reprent your alpha channel.

2. Suppose you already copied source image to new Mat with black background, just split this image to a Mat array of size 3.

3. Now you got three primary channel R, G, B, and Alpha.

4. Now just merge all these to a new Mat.

Done! .

See below code, here I created alpha using thresholding, instead you have to use contour drawing.

Mat src=imread("src.png",1);
Mat dst;
Mat tmp,alpha;

cvtColor(src,tmp,CV_BGR2GRAY);
threshold(tmp,alpha,10,255,THRESH_BINARY);

Mat rgb[3];
split(src,rgb);

Mat rgba[4]={rgb[0],rgb[1],rgb[2],alpha};
merge(rgba,4,dst);

BGR source

Alpha for background(0 = full transparency, and 255 = no transparency )

Result with background alpha