Search small images on one images.

edit

This should extracts and straighten the small images

1. Convert to gray scale
2. Threshold
3. Find Contours
4. Get Bounding Rect and Rotated Rect
5. Extracts the rect with mask for a default background
6. straighten the small image

and so on

If the contour isn't a rectangle however its the Bounding Rect will be extracted like this

EDIT: Ok now you can play with trackbar to understand parameters

namespace ExtractStamps {
#define USE_COPY_MASK 0
int useEqualize, useNormalize, threshold, blurSize;
int extract;
Mat src, src_gray;
const std::string winName = "Extract Stamp";
void FindStamps(Mat &bw, Mat &dst)
{
    // find external contours ignores holes in the fish
    vector<vector<cv::Point> > contours;
    vector<cv::Vec4i> hierarchy;
    cv::findContours(bw, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
    //draw all contours
    vector<Rect>  stampsRect;
    vector<RotatedRect>  stampsRectRotated;
#if USE_COPY_MASK==1
    Mat copyMask(bw.rows, bw.cols, bw.type(), Scalar(0));
#endif
    int minContourSize = cvRound(0.05*src.cols);
    for (int i = 0; i< contours.size(); i++) {
        Rect rect = boundingRect(contours[i]);
        if (rect.width < minContourSize) {
            if (rect.width > minContourSize / 2.0)
                // draw not so small contours in RED 
                drawContours(dst, contours, i, cv::Scalar(0, 0, 255), 1);
            continue;
        }
        // selected contours in GREEN
        drawContours(dst, contours, i, cv::Scalar(0, 255, 0), 1);
        //store contour boundingRect
        cv::rectangle(dst, rect, Scalar(255, 0, 0), 4); //draw it in BLUE
        stampsRect.push_back(rect);
        stampsRectRotated.push_back(minAreaRect(contours[i]));
#if USE_COPY_MASK==1
        // prepare the copy mask 
        //drawContours(copyMask, contours, i, 255, FILLED, 8, hierarchy);
#endif
    }
    if (extract) {
        Mat rot_mat(2, 3, CV_32FC1);
        RotatedRect rr;
        double angle;
        for (int i = 0; i< stampsRect.size(); i++) {
            RotatedRect  rr = stampsRectRotated[i];
            Rect rect = stampsRect[i];
            Point center(rect.width / 2, rect.height / 2);
            // create an empty image with a default background
            Scalar bkg(0, 255, 0);
            Mat stamp(rect.height, rect.width, src.type(), bkg);
            // copy stamp from source 
#if USE_COPY_MASK==1
            // using a contour mask
            src(rect).copyTo(stamp, copyMask(rect));
#else
            src(rect).copyTo(stamp);
#endif
            //straighten
            if (rr.angle != 0) {
                if (rr.angle < 0)
                    angle = rr.angle;
                if (rr.angle < -45)
                    angle = 90 + rr.angle;
                if (abs(angle) > 0.5) {
                    rot_mat = getRotationMatrix2D(center, angle, 1);
                    warpAffine(stamp, stamp, rot_mat, stamp.size(), 1, 0, bkg);
                }
            }
            //imwrite("../img/Stamp" + to_string(idx) + ".jpg", stamp);
            imshow("Stamp" + to_string(i), stamp);
        }
    }
}
void UseThreshold(const Mat &gray,Mat &bw)
{
    std::string win;

    // threshold to select bright
    cv::threshold(gray, bw, threshold, 255, cv::THRESH_BINARY);
    win = "threshold"; namedWindow(win, WINDOW_NORMAL);
    imshow(win, bw);
}
void Preprocessing(const Mat &imgIn, Mat &imgOut)
{
    if (blurSize >= 3) {
        blurSize += (1 - blurSize % 2);
        GaussianBlur(imgIn, imgOut, cv::Size(blurSize, blurSize), 0);
    }
    else
        imgIn.copyTo(imgOut);

    if (useEqualize)
        equalizeHist(imgOut, imgOut);
    if (useNormalize)
        normalize(imgOut, imgOut,0, 255, NORM_MINMAX);
    //BrightnessAndContrastAuto(img, img, 1);
}

void onTrackbar(int, void*)
{
    Mat tmp,bw, dst;

    Preprocessing(src_gray,tmp);
    UseThreshold(tmp,bw);

    src.copyTo(dst); // keep a copy to show the result
    FindStamps(bw, dst);
    imshow(winName, dst);
}

void Main()
{
    Mat img;
    //img = imread("../img/stamp-big.png");
    img = imread("../img/stamp-rot.png");
    // remove some unwanted cols and rows
    int skipRowTop = 0.01*img ...