Implementation Question: How to create bounding boxes around answers on worksheets

edit

For the fun of it I did a quick and dirty approach, considering that you will have this "result" separation lines. Have a look below:

#include <iostream>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main()
{
    Mat src = imread("worksheet.jpg");

    if(!src.data)
    {
        cerr << "Problem loading image!!!" << endl;
        return EXIT_FAILURE;
    }

//        imshow("src", src);

    // resizing for practical reasons
    Mat rsz;
    Size size(800, 1132);
    resize(src, rsz, size);

//        imshow("rsz", rsz);

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

    // Apply adaptiveThreshold at the bitwise_not of gray, notice the ~ symbol
    Mat bw;
    adaptiveThreshold(~gray, bw, 255, CV_ADAPTIVE_THRESH_MEAN_C, THRESH_BINARY, 15, -2);

    // Dilate a bit in order to correct possible gaps
    Mat kernel = Mat::ones(2, 2, CV_8UC1);
    dilate(bw, bw, kernel);

    // Show binary image
    imshow("bin", bw);

    // Create the images that will use to extract the horizontal lines
    Mat horizontal = bw.clone();

    // Specify size on horizontal axis
    int horizontalsize = horizontal.cols / 30;

    // Create structure element for extracting horizontal lines through morphology operations
    Mat horizontalStructure = getStructuringElement(MORPH_RECT, Size(horizontalsize,1));

    // Apply morphology operations
    erode(horizontal, horizontal, horizontalStructure, Point(-1, -1));
    dilate(horizontal, horizontal, horizontalStructure, Point(-1, -1));

    // Show extracted horizontal lines
    imshow("horizontal", horizontal);

    // Find external contour
    vector<Vec4i> hierarchy;
    std::vector<std::vector<cv::Point> > contours;
    cv::findContours(horizontal, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

    vector<vector<Point> > contours_poly( contours.size() );
    vector<Rect> boundRect( contours.size() );
    for( size_t i = 0; i < contours.size(); i++ )
    {
        approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
        boundRect[i] = boundingRect( Mat(contours_poly[i]) );
    }

    // Draw the contour as a solid blob filling also any convexity defect with the extracted hulls
    for (size_t i = 0; i < contours.size(); i++)
    {
//        cout << boundRect[i].tl() << endl;
//        cout << boundRect[i].br() << endl << endl;

//        cout << arcLength(cv::Mat(contours[i]), true) << endl;
        double length = arcLength(cv::Mat(contours[i]), true);
        // skip any noise lines
        if(length < 75)
            continue;

        if(length > 200) // filter long with short lines
        {
            boundRect[i] += Size(0, -40); // expanding rectangle by a certain amount
            boundRect[i] -= Point(0, 3); // shifting rectangle by a certain offset
        }else{
            boundRect[i] += Size(0, 40);
            boundRect[i] -= Point(0, -4);
        }

        drawContours( rsz, contours, i, Scalar(0, 0, 255), 1, 8, vector<Vec4i>(), 0, Point() );
        rectangle( rsz, boundRect[i].tl(), boundRect[i].br(), Scalar(0, 255, 0), 1, 8, 0 );
    }

    imshow("src", rsz);

    /* Now you can do whatever post processing you want
     * with the data within the rectangles. */

    waitKey(0);
    return 0;
}

enjoy. But be aware that might some optimization is needed to fit all your use cases.