watermeter_deskewing.txt

#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;

void hough_transform(Mat& im, Mat& orig, double* skew)
{
	double max_r = sqrt(pow(.5*im.cols, 2) + pow(.5*im.rows, 2));
	int angleBins = 180;
	Mat acc = Mat::zeros(Size(2 * max_r, angleBins), CV_32SC1);
	int cenx = im.cols / 2;
	int ceny = im.rows / 2;
	for (int x = 1; x<im.cols - 1; x++)
	{
		for (int y = 1; y<im.rows - 1; y++)
		{
			if (im.at<uchar>(y, x) == 255)
			{
				for (int t = 0; t<angleBins; t++)
				{
					double r = (x - cenx)*cos((double)t / angleBins*CV_PI) + (y - ceny)*sin((double)t / angleBins*CV_PI);
					r += max_r;
					acc.at<int>(t, int(r))++;
				}
			}
		}
	}
	Mat thresh;
	normalize(acc, acc, 255, 0, NORM_MINMAX);
	convertScaleAbs(acc, acc);
	/*debug
	Mat cmap;
	applyColorMap(acc,cmap,COLORMAP_JET);
	imshow("cmap",cmap);
	imshow("acc",acc);*/

	Point maxLoc;
	minMaxLoc(acc, 0, 0, 0, &maxLoc);
	double theta = (double)maxLoc.y / angleBins*CV_PI;
	double rho = maxLoc.x - max_r;
	if (abs(sin(theta))<0.000001)//check vertical
	{
		//when vertical, line equation becomes
		//x = rho
		double m = -cos(theta) / sin(theta);
		Point2d p1 = Point2d(rho + im.cols / 2, 0);
		Point2d p2 = Point2d(rho + im.cols / 2, im.rows);
		line(orig, p1, p2, Scalar(0, 0, 255), 1);
		*skew = 90;
		cout << "skew angle " << " 90" << endl;
	}
	else
	{
		//convert normal form back to slope intercept form
		//y = mx + b
		double m = -cos(theta) / sin(theta);
		double b = rho / sin(theta) + im.rows / 2. - m*im.cols / 2.;
		Point2d p1 = Point2d(0, b);
		Point2d p2 = Point2d(im.cols, im.cols*m + b);
		line(orig, p1, p2, Scalar(0, 0, 255), 1);
		double skewangle;
		skewangle = p1.x - p2.x>0 ? (atan2(p1.y - p2.y, p1.x - p2.x)*180. / CV_PI) : (atan2(p2.y - p1.y, p2.x - p1.x)*180. / CV_PI);
		*skew = skewangle;
		cout << "skew angle " << skewangle << endl;
	}
	imshow("orig", orig);
}

Mat preprocess1(Mat& im)
{
	Mat ret = Mat::zeros(im.size(), CV_32SC1);

	for (int x = 1; x<im.cols - 1; x++)
	{
		for (int y = 1; y<im.rows - 1; y++)
		{

			int gy = (im.at<uchar>(y - 1, x + 1) - im.at<uchar>(y - 1, x - 1))
				+ 2 * (im.at<uchar>(y, x + 1) - im.at<uchar>(y, x - 1))
				+ (im.at<uchar>(y + 1, x + 1) - im.at<uchar>(y + 1, x - 1));
			int gx = (im.at<uchar>(y + 1, x - 1) - im.at<uchar>(y - 1, x - 1))
				+ 2 * (im.at<uchar>(y + 1, x) - im.at<uchar>(y - 1, x))
				+ (im.at<uchar>(y + 1, x + 1) - im.at<uchar>(y - 1, x + 1));
			int g2 = (gy*gy + gx*gx);
			ret.at<int>(y, x) = g2;
		}
	}
	normalize(ret, ret, 255, 0, NORM_MINMAX);
	ret.convertTo(ret, CV_8UC1);
	threshold(ret, ret, 50, 255, THRESH_BINARY);
	return ret;
}

Mat preprocess2(Mat& im)
{
	// 1) assume white on black and does local thresholding
	// 2) only allow voting top is white and buttom is black(buttom text line)
	Mat thresh;
	//thresh=255-im;
	thresh = im.clone();
	adaptiveThreshold(thresh, thresh, 255, CV_ADAPTIVE_THRESH_GAUSSIAN_C, THRESH_BINARY, 15, -2);
	Mat ret = Mat::zeros(im.size(), CV_8UC1);
	for (int x = 1; x<thresh.cols - 1; x++)
	{
		for (int y = 1; y<thresh.rows - 1; y++)
		{
			bool toprowblack = thresh.at<uchar>(y - 1, x) == 0 || thresh.at<uchar>(y - 1, x - 1) == 0 || thresh.at<uchar>(y - 1, x + 1) == 0;
			bool belowrowblack = thresh.at<uchar>(y + 1, x) == 0 || thresh.at<uchar>(y + 1, x - 1) == 0 || thresh.at<uchar>(y + 1, x + 1) == 0;

			uchar pix = thresh.at<uchar>(y, x);
			if ((!toprowblack && pix == 255 && belowrowblack))
			{
				ret.at<uchar>(y, x) = 255;
			}
		}
	}
	return ret;
}
Mat rot(Mat& im, double thetaRad)
{
	cv::Mat rotated;
	double rskew = thetaRad* CV_PI / 180;
	double nw = abs(sin(thetaRad))*im.rows + abs(cos(thetaRad))*im.cols;
	double nh = abs(cos(thetaRad))*im.rows + abs(sin(thetaRad))*im.cols;
	cv::Mat rot_mat = cv::getRotationMatrix2D(Point2d(nw*.5, nh*.5), thetaRad * 180 / CV_PI, 1);
	Mat pos = Mat::zeros(Size(1, 3), CV_64FC1);
	pos.at<double>(0) = (nw - im.cols)*.5;
	pos.at<double>(1) = (nh - im.rows)*.5;
	Mat res = rot_mat*pos;
	rot_mat.at<double>(0, 2) += res.at<double>(0);
	rot_mat.at<double>(1, 2) += res.at<double>(1);
	cv::warpAffine(im, rotated, rot_mat, Size(nw, nh), cv::INTER_LANCZOS4);
	return rotated;
}

int main(int argc, char** argv)
{
	string src = "C:/Users/praveen_gadi/Pictures/ocrknn/watermeter.jpg";
	Mat im = imread(src);
	Mat gray;
	cvtColor(im, gray, CV_BGR2GRAY);

	Mat preprocessed = preprocess2(gray);
	imshow("preprocessed2", preprocessed);
	double skew;
	hough_transform(preprocessed, im, &skew);
	Mat rotated = rot(im, skew* CV_PI / 180);
	imwrite("C:/Users/praveen_gadi/Pictures/ocrknn/watermeter2.jpg",rotated );
	imshow("corrected", rotated);

	waitKey(0);
	return 0;
}