# Detecting ID Card

Hi,

I'm trying to detect the following card

And warp it, rotate it, to be centered on the screen.

I tried the following approach but it doesn't work at all.

#include "opencv2/imgproc.hpp"
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;
using namespace std;

#define EPSILON 1E-5

//! Return the maximum of the provided numbers
static double maximum(double number1, double number2, double number3) {
return std::max(std::max(number1, number2), number3);
}

//! Check if the two numbers are equal (almost)
/*!
* The expression for determining if two real numbers are equal is:
* if (Abs(x - y) <= EPSILON * Max(1.0f, Abs(x), Abs(y))).
*
* @Param number1 First number
* @Param number2 Second number
*/
static bool almostEqual(double number1, double number2) {
return (std::abs(number1 - number2) <= (EPSILON * maximum(1.0, std::abs(number1), std::abs(number2))));
}

//! Determine the intersection point of two lines, if this point exists
/*! Two lines intersect if they are not parallel (Parallel lines intersect at
* +/- infinity, but we do not consider this case here).
*
* The lines are specified by a pair of points each. If they intersect, then
* the function returns true, else it returns false.
*
* Lines can be specified in the following form:
*      A1x + B1x = C1
*      A2x + B2x = C2
*
* If det (= A1*B2 - A2*B1) == 0, then lines are parallel
*                                else they intersect
*
* If they intersect, then let us denote the intersection point with P(x, y) where:
*      x = (C1*B2 - C2*B1) / (det)
*      y = (C2*A1 - C1*A2) / (det)
*
* @Param a1 First point for determining the first line
* @Param b1 Second point for determining the first line
* @Param a2 First point for determining the second line
* @Param b2 Second point for determining the second line
* @Param intersection The intersection point, if this point exists
*/
static bool lineIntersection(const cv::Point2f &a1, const cv::Point2f &b1, const cv::Point2f &a2,
const cv::Point2f &b2, cv::Point2f &intersection) {
double A1 = b1.y - a1.y;
double B1 = a1.x - b1.x;
double C1 = (a1.x * A1) + (a1.y * B1);

double A2 = b2.y - a2.y;
double B2 = a2.x - b2.x;
double C2 = (a2.x * A2) + (a2.y * B2);

double det = (A1 * B2) - (A2 * B1);

if (!almostEqual(det, 0)) {
intersection.x = static_cast<float>(((C1 * B2) - (C2 * B1)) / (det));
intersection.y = static_cast<float>(((C2 * A1) - (C1 * A2)) / (det));

return true;
}

return false;
}

struct vector_sorter
{
bool operator ()(const std::vector<cv::Point>& a, const std::vector<cv::Point> & b)
{
double dist_a = norm(a[0] - a[1]);
double dist_b = norm(b[0] - b[1]);
return dist_a > dist_b;
}
};

void sortCorners(std::vector<cv::Point2f>& corners)
{
std::vector<cv::Point2f> top, bot;
cv::Point2f center;
// Get mass center
for (int i = 0; i < corners.size(); i++)
center += corners[i];
center *= (1. / corners.size());

for (int i = 0; i < corners.size(); i++)
{
if (corners[i].y < center.y)
top.push_back(corners[i]);
else
bot.push_back(corners[i]);
}
corners.clear();

if (top.size() == 2 && bot.size() == 2) {
cv::Point2f tl = top[0].x > top[1].x ? top ...
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