Removing lines from image - c++

Question

I am a beginner in OpenCV, I need to remove the horizontal and vertical lines in the image so that only the text remains ( The lines were causing trouble when extracting text in ocr ). I am trying to extract text from the Nutrient Fact Table. Can anyone help me?

Answer 1

This was an interesting question, so I gave it a shot. Below I will show you how to extract and remove horizontal and vertical lines. You could extrapolate from it. Also, for sake of saving time, I did not preprocess your image to crop out the background as one should, which is an avenue for improvement.
The result:
The code (edit: added vertical lines):
#include <iostream>
#include <opencv2/opencv.hpp>
using namespace std;
using namespace cv;
int main(int, char** argv)
{
// Load the image
Mat src = imread(argv[1]);
// Check if image is loaded fine
if(!src.data)
cerr << "Problem loading image!!!" << endl;
Mat gray;
if (src.channels() == 3)
{
cvtColor(src, gray, CV_BGR2GRAY);
}
else
{
gray = src;
}
//inverse binary img
Mat bw;
//this will hold the result, image to be passed to OCR
Mat fin;
//I find OTSU binarization best for text.
//Would perform better if background had been cropped out
threshold(gray, bw, 0, 255, THRESH_BINARY_INV | THRESH_OTSU);
threshold(gray, fin, 0, 255, THRESH_BINARY | THRESH_OTSU);
imshow("binary", bw);
Mat dst;
Canny( fin, dst, 50, 200, 3 );
Mat str = getStructuringElement(MORPH_RECT, Size(3,3));
dilate(dst, dst, str, Point(-1, -1), 3);
imshow("dilated_canny", dst);
//bitwise_and w/ canny image helps w/ background noise
bitwise_and(bw, dst, dst);
imshow("and", dst);
Mat horizontal = dst.clone();
Mat vertical = dst.clone();
fin = ~dst;
//Image that will be horizontal lines
Mat horizontal = bw.clone();
//Selected this value arbitrarily
int horizontalsize = horizontal.cols / 30;
Mat horizontalStructure = getStructuringElement(MORPH_RECT, Size(horizontalsize,1));
erode(horizontal, horizontal, horizontalStructure, Point(-1, -1));
dilate(horizontal, horizontal, horizontalStructure, Point(-1, -1), 1);
imshow("horizontal_lines", horizontal);
//Need to find horizontal contours, so as to not damage letters
vector<Vec4i> hierarchy;
vector<vector<Point> >contours;
findContours(horizontal, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_NONE);
for (const auto& c : contours)
{
Rect r = boundingRect(c);
float percentage_height = (float)r.height / (float)src.rows;
float percentage_width = (float)r.width / (float)src.cols;
//These exclude contours that probably are not dividing lines
if (percentage_height > 0.05)
continue;
if (percentage_width < 0.50)
continue;
//fills in line with white rectange
rectangle(fin, r, Scalar(255,255,255), CV_FILLED);
}
int verticalsize = vertical.rows / 30;
Mat verticalStructure = getStructuringElement(MORPH_RECT, Size(1,verticalsize));
erode(vertical, vertical, verticalStructure, Point(-1, -1));
dilate(vertical, vertical, verticalStructure, Point(-1, -1), 1);
imshow("verticalal", vertical);
findContours(vertical, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_NONE);
for (const auto& c : contours)
{
Rect r = boundingRect(c);
float percentage_height = (float)r.height / (float)src.rows;
float percentage_width = (float)r.width / (float)src.cols;
//These exclude contours that probably are not dividing lines
if (percentage_width > 0.05)
continue;
if (percentage_height < 0.50)
continue;
//fills in line with white rectange
rectangle(fin, r, Scalar(255,255,255), CV_FILLED);
}
imshow("Result", fin);
waitKey(0);
return 0;
}
The limitations of this approach are that the lines need to be straight. Due to the curve in the bottom line, it cuts slightly into "E" in "Energy". Perhaps with a hough line detection like suggested (I've never used it), a similar but more robust approach could be devised. Also, filling in the lines with rectangles probably is not the best approach.