cropImage.cpp

#include <QCoreApplication>
#include <opencv.hpp>
#include <QDebug>

double compute_skew(QString filename);
void deskew(QString filename, double angle);

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    QString filename = "m8.jpg";
    deskew(filename,compute_skew(filename));

    return a.exec();
}

double compute_skew(QString filename){
    cv::Mat src = cv::imread(filename.toStdString(),0);
    cv::Size size = src.size();

    // inverting black & white
    cv::bitwise_not(src,src);

    std::vector<cv::Vec4i> lines;
    cv::HoughLinesP(src,lines,1,CV_PI/180,100,size.width/2.f,20);

    cv::Mat disp_lines(size, CV_8UC1, cv::Scalar(0,0,0));
    double angle = 0;
    unsigned nb_lines = lines.size();
    for(unsigned i = 0; i< nb_lines; i++){
        cv::line(disp_lines,cv::Point(lines[i][0],lines[i][1]),cv::Point(lines[i][2],lines[i][3]),cv::Scalar(255,0,0));
        angle += atan2((double)lines[i][3] - lines[i][1],(double)lines[i][2] - lines[i][0]);
    }
    angle /= nb_lines; // mean angle, in radians.


    angle = ( angle * 180 )/ CV_PI;
    qDebug() << "File " << filename << ": " << angle;

    return angle;
    // can be used for debug purpose
//    cv::imshow(filename.toStdString(), disp_lines);
//    cv::waitKey(0);
//    cv::destroyWindow(filename.toStdString());
}

void deskew(QString filename, double angle){
    cv::Mat img = cv::imread(filename.toStdString(),0);
    cv::bitwise_not(img,img);

    std::vector<cv::Point> points;
    cv::Mat_<uchar>::iterator it = img.begin<uchar>();
    cv::Mat_<uchar>::iterator end = img.end<uchar>();
    for(;it != end;it++){
        if(*it){
            points.push_back(it.pos());
        }
    }

    // rotated box
    cv::RotatedRect box = cv::minAreaRect(cv::Mat(points));

    cv::Mat rot_mat = cv::getRotationMatrix2D(box.center, angle, 1);
    cv::Mat rotated;
    cv::warpAffine(img, rotated, rot_mat, img.size(), cv::INTER_CUBIC);

    cv::Size box_size = box.size;
    if (box.angle < -45.){
        std::swap(box_size.width, box_size.height);
    }
    cv::Mat cropped;
    cv::getRectSubPix(rotated, box_size, box.center, cropped);
    // cropped mat will contain the text with minimal margins

    cv::imwrite("ms_cropped.jpg",cropped);
    qDebug() << "image written";
}

you can convert to c++ if u don't hv Qt installed..
i) just change all the instances of QString with std::string
ii) remove all calls to ".toStdString()".
iii) replace qDebug() with std::cout
iv) finally remove QCoreApplication line..

The above code gives you a rotated image with all the margins cut down to minimum.