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Please find the code below

Mat image;
// read the input image
image = imread("D:\\omr.jpg");

Mat gray;
cvtColor(image, gray, CV_BGR2GRAY);

GaussianBlur(gray, gray, Size(5,5) , 0, 0);

Mat thres;
threshold(gray, thres, 130, 255, CV_THRESH_BINARY_INV);
imshow("thres", thres);
waitKey();


//  apply a connected component analysis with statistics
//  (from opencv 3.0 onwards)
// cca
Mat labels, stats, centroids;
int numObjects = connectedComponentsWithStats(thres, labels, stats, centroids);

// Draw bounding boxes for objects in the original image
// exclude background = 0
int i;
for(i=1; i<numObjects; i++)
{
    int left = stats.at<int>(i, CC_STAT_LEFT);
    int top = stats.at<int>(i, CC_STAT_TOP);
    double width = stats.at<int>(i, CC_STAT_WIDTH);
    double height = stats.at<int>(i, CC_STAT_HEIGHT);
    int area = stats.at<int>(i, CC_STAT_AREA);

    double k = width/height;

    if(area > 500 && area < 2000 && k > 0.9 && k < 1.1)
    {
        // draw rectangle
        rectangle(image, Rect(left, top, width, height), Scalar(0,255,0), 2);
    }
}

imshow("BoundingBoxes", image);

The output image

Regards

Amal

Please find the code below

Mat image;
// read the input image
image = imread("D:\\omr.jpg");

Mat gray;
cvtColor(image, gray, CV_BGR2GRAY);

GaussianBlur(gray, gray, Size(5,5) , 0, 0);

Mat thres;
threshold(gray, thres, 130, 255, CV_THRESH_BINARY_INV);
imshow("thres", thres);
waitKey();


//  apply a connected component analysis with statistics
//  (from opencv 3.0 onwards)
// cca
Mat labels, stats, centroids;
int numObjects = connectedComponentsWithStats(thres, labels, stats, centroids);

// Draw bounding boxes for objects in the original image
// exclude background = 0
int i;
for(i=1; i<numObjects; i++)
{
    int left = stats.at<int>(i, CC_STAT_LEFT);
    int top = stats.at<int>(i, CC_STAT_TOP);
    double width = stats.at<int>(i, CC_STAT_WIDTH);
    double height = stats.at<int>(i, CC_STAT_HEIGHT);
    int area = stats.at<int>(i, CC_STAT_AREA);

    double k = width/height;

    if(area > 500 && area < 2000 && k > 0.9 && k < 1.1)
    {
        // draw rectangle
        rectangle(image, Rect(left, top, width, height), Scalar(0,255,0), 2);
    }
}

imshow("BoundingBoxes", image);

The output image

Here is the code using contours. I hope you are comfortable using it in Java.

Mat image;
// read the input image
image = imread("D:\\omr.jpg");

Mat gray;
cvtColor(image, gray, CV_BGR2GRAY);

GaussianBlur(gray, gray, Size(5,5) , 0, 0);

Mat thres;
threshold(gray, thres, 130, 255, CV_THRESH_BINARY_INV);
imshow("thres", thres);
waitKey();


vector<vector<Point> > contours;
// find contours
findContours(thres, contours, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
for( int i = 0; i < contours.size(); i++ )
{
    Rect brect = boundingRect(Mat(contours[i]));

    double k = (brect.width+0.0)/brect.height;

    if(brect.area() > 500 && brect.area() < 2000 && k > 0.9 && k < 1.1)
    {

        RotatedRect minRect = minAreaRect( Mat(contours[i]) );

        // rotated rectangle
        Point2f rect_points[4];
        minRect.points( rect_points );
        for( int j = 0; j < 4; j++ )
            line( image, rect_points[j], rect_points[(j+1)%4], Scalar(0,255,0), 2, 8 );
    }
}


imshow("BoundingBoxes", image);

Regards

Amal