Detect bounding box in OCR? - c++

Question

I have an image and I obtained a binary image of it. I would expect a rectangular bounding box, but i didn't get it. This is my code:
vector<vector<Point>> contours;
Vec4i hierarchy;
findContours(binary, contours, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
/*Mat drawing = Mat::zeros(binary.size(), CV_8UC3);
for (int i = 0; i < contours.size(); i++)
{
Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
drawContours(drawing, contours, i, color, 1, 8, hierarchy, 0, Point());
}
imshow("contours", drawing);*/
vector<Point> approx, approxRectangle;
Rect bounding_rect(0, 0, 0, 0);
double max_area = 0;
for (int i = 0; i < contours.size(); i++)// xet tung contour
{
approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);
if (approx.size() == 4 && isContourConvex(Mat(approx)))
{
Rect box = boundingRect(contours[i]);
if (bounding_rect.area() == 0){
bounding_rect = box;
approxRectangle = approx;
}
else{
if (bounding_rect.area() < box.area()){
bounding_rect = box;
approxRectangle = approx;
}
}
}
}`
This is my image:

Answer 1

You don't get the desired result, because you're looking for almost rectangular contours, but this won't work since the contours you're interested in is not rectanglar. You can see (in blue) the approximation of that contour (obtained on my binarized image):
This shows you that this is not a reliable constraint.
You can easily solve this, in this case, computing the bounding box for each contour, and keep the largest (in green):
Code:
#include <opencv2/opencv.hpp>
#include <iostream>
#include <algorithm>
using namespace std;
using namespace cv;
int main()
{
// Load image
Mat3b img = imread("path_to_image");
// Convert to grayscale
Mat1b binary;
cvtColor(img, binary, COLOR_BGR2GRAY);
// Binarize (remove anti-aliasing artifacts)
binary = binary > 200;
// Find contours
vector<vector<Point>> contours;
findContours(binary.clone(), contours, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE, Point(0, 0));
// Compute the bounding boxes
vector<Rect> boxes;
for (int i = 0; i < contours.size(); ++i)
{
boxes.push_back(boundingRect(contours[i]));
}
// Find index of largest contours
int idx_largest_box = distance(boxes.begin(), max_element(boxes.begin(), boxes.end(), [](const Rect& lhs, const Rect& rhs) {
return lhs.area() < rhs.area();
}));
// Draw largest box
rectangle(img, boxes[idx_largest_box], Scalar(0,255,0));
imshow("Result", img);
waitKey();
return 0;
}