CAPTCHA formation in OpenCV

A CAPTCHA (an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart") is a type of challenge-response test used in computing to determine whether or not the user is human. It protects your website from spam by means of math logic, easily understood by human beings. You will not have to spend your precious time on annoying attempts to understand hard-to-read words, combinations of letters or pictures that make your eyes pop up.

I have borrowed this source code from here.

Example:

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#include <stdio.h>
#include <stdlib.h>
#include <fstream>
#include <opencv2/opencv.hpp>
#include <cstdlib>
#include <ctime>

using namespace cv;
using namespace std;

const Scalar WHITE = CV_RGB(255,255,255);
const int       CAPTCHA_LENGTH = 8;
const int         CHAR_HEIGHT = 100;
const int         CHAR_WIDTH  = 80;
const string     CHARACTERS[] = {"a", "A", "b", "B", "c", "C", "D", "d", "e", "E", "f", "F", "g", "G",
    "h", "H", "j", "J", "k", "K", "m", "M", "n", "N", "q", "Q", "R", "t", "T", "w", "W", "x", "X", "y", "Y",
    "1", "2", "3", "4", "5", "6", "7", "8", "9"};

void rotate(Mat &input, Mat &output) {

    int sign = rand() % 2;
    if ( sign == 0 ) {
        sign = -1;
    }

    int angle = ( rand() % 30 ) * sign; // between -30 and 30

    Point2f center(input.cols/2.0F, input.rows/2.0F);
    Mat rotationMatrix = getRotationMatrix2D(center, angle, 1.0);

    warpAffine(input, output, rotationMatrix, input.size(),
        INTER_LINEAR, BORDER_CONSTANT, WHITE);

}

void scale(Mat &input, Mat &output, float height, float width) {

    height = ( ( rand() % 20 ) * -1 ) + height;
    width  = ( ( rand() % 20 ) * -1 ) + width;

    Size s(width, height);
    cv::resize(input, output, s);
}

void addLines(Mat &image) {
    rand();
    RNG rng(rand());
    int numLines = CAPTCHA_LENGTH;
    for ( int i = 0; i < numLines; ++i) {
        int startX = rand() % image.cols;
        int endX = rand()   % image.cols;
        int startY = rand() % image.rows;
        int endY = rand()   % image.rows;

        cv::line(image,
            cv::Point(startX, startY),
            cv::Point(endX, endY),
            Scalar(rng.uniform(0, 256),rng.uniform(0, 256),rng.uniform(0, 256)),
            rng.uniform(0, 3), CV_AA); // anti-alias
    }
}

void transform(Mat &charImage) {

    cv::Point2f src[4];
    src[0] = cv::Point2f(0, 0);
    src[1] = cv::Point2f(0, CHAR_HEIGHT);
    src[2] = cv::Point2f(CHAR_WIDTH, 0);
    src[3] = cv::Point2f(CHAR_WIDTH, CHAR_HEIGHT);

    cv::Point2f dst[4];
    dst[0] = cv::Point2f(0, 0);
    dst[1] = cv::Point2f(0, CHAR_HEIGHT);
    dst[2] = cv::Point2f(CHAR_WIDTH, 0);

    int varWidth  = CHAR_WIDTH / 2;
    int varHeight = CHAR_HEIGHT / 2;
    int widthWarp  = CHAR_WIDTH - varWidth + (rand() % varWidth);
    int heightWarp = CHAR_HEIGHT - varHeight + (rand() % varHeight);
    dst[3] = cv::Point2f(widthWarp, heightWarp);

    Mat perspectiveTranx = cv::getPerspectiveTransform(src, dst);
    cv::warpPerspective(charImage, charImage, perspectiveTranx,
        cv::Size(charImage.cols, charImage.rows),
        INTER_CUBIC,
        BORDER_CONSTANT,
        WHITE);
}

void addNoise(Mat &image) {
    rand();
    RNG rng(rand());
    int i,j;
    for (int n = 0; n < 100; n++) {

        i= rand() % image.cols;
        j= rand() % image.rows;

        Point center(i, j);
        circle(image, center, rng.uniform(1, 3),     // radius,
            Scalar(rng.uniform(0, 256),rng.uniform(0, 256),rng.uniform(0, 256)),  // color of noise points
            -2,                  // thickness, negative for filled
            CV_AA);                
    }
}

int main(int argc, char *argv[])
{
    int charactersSize = sizeof(CHARACTERS) / sizeof(CHARACTERS[0]);

    Mat outImage(CHAR_HEIGHT, CHAR_WIDTH * CAPTCHA_LENGTH, CV_8UC3, WHITE);

    srand((unsigned)time(0));
    rand();
    RNG rng(rand());

    
    Scalar color = CV_RGB(0, 0, 0); //255, 127, 80);

    for ( int i = 0; i < CAPTCHA_LENGTH; ++i ) {
        cv::Mat charImage(CHAR_HEIGHT, CHAR_WIDTH, CV_8UC3, WHITE);

        string c = CHARACTERS[rand() % charactersSize];

        putText(charImage, c, Point(10, CHAR_HEIGHT - 10), rng.uniform(1, 6), rng.uniform(3.0, 4.0), color, rng.uniform(1, 5), CV_AA);
        imshow("1st look",charImage);
        transform(charImage);
        imshow("transform",charImage);
        rotate(charImage, charImage);
        imshow("rotate",charImage);
        scale(charImage, charImage, CHAR_HEIGHT, CHAR_WIDTH);
        imshow("scale",charImage);

        charImage.copyTo(outImage(cv::Rect(CHAR_WIDTH * i, 0, charImage.cols, charImage.rows)));
        //waitKey(0);
    }

    addLines(outImage);
    addNoise(outImage);
    double a = rng.uniform(0, 2);
    cout<<"   "<<a<<endl;
    imshow("Captcha11", outImage);
    waitKey(0);

    return EXIT_SUCCESS;
}

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Result:

Sources:
http://en.wikipedia.org/wiki/CAPTCHA