CODEWITHSUNDEEP
c++opencvopencv3.0
user9870
user9870

Reputation: 45

Why is the input image findContours() in openCV has to be converted to grayscale?

I am following the tutorial on the following site:

http://harismoonamkunnu.blogspot.co.uk/2013/06/opencv-find-biggest-contour-using-c.html

If I follow the code, it works perfectly. However, what I did is I input a greyscaled image (greyscaled by photoshop and saved as jpg) and I try to skip the step:

cvtColor(src,thr,CV_BGR2GRAY); //Convert to gray

when I run the same code in Visual Studio again, I received a runtime error showing:

Unhandled exception at 0x00007FFE56A79E08 in image_capture.exe: Microsoft C++ exception: cv::Exception at memory location 0x000000C1D58FE1F0. occurred

at countours.cpp file :

scanner = cvStartFindContours( img, storage, cntHeaderSize, mode, method, offset );

I want to try to avoid using the cvtColor because in my application, my input image is a grayscaled picture. My code is as following:

#include "stdafx.h"
#include <iostream>
#include "opencv2\highgui\highgui.hpp"
#include "opencv\cv.h"
#include "opencv2\imgproc\imgproc.hpp"

using namespace cv;
using namespace std;
int main()
{
 int largest_area=0;
 int largest_contour_index=0;
 Rect bounding_rect;

 Mat src = imread("src.jpg"); //Load source image
 Mat thr(src.rows,src.cols,CV_8UC1); 
 Mat dst(src.rows,src.cols,CV_8UC1,Scalar::all(0));
 //cvtColor(src,thr,CV_BGR2GRAY); //Convert to gray
 threshold(src, src,25, 255,THRESH_BINARY); //Threshold the gray

    vector<vector<Point>> contours; // Vector for storing contour
    vector<Vec4i> hierarchy;

    findContours( src, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE ); // Find the contours in the image

     for( int i = 0; i< contours.size(); i++ ) // iterate through each contour. 
      {
       double a=contourArea( contours[i],false);  //  Find the area of contour
       if(a>largest_area){
       largest_area=a;
       largest_contour_index=i;                //Store the index of largest contour
       bounding_rect=boundingRect(contours[i]); // Find the bounding rectangle for biggest contour
       }

      }

 Scalar color( 255,255,255);
 drawContours( dst, contours,largest_contour_index, color, CV_FILLED, 8, hierarchy ); // Draw the largest contour using previously stored index.
 rectangle(src, bounding_rect,  Scalar(0,255,0),1, 8,0);  
 imshow( "src", src );
 imshow( "largest Contour", dst );
 waitKey(0);
}

Any idea how to avoid cvtColor()?

Thank you

Upvotes: 3

Views: 5046

Answers (1)

tuan.tran
tuan.tran

Reputation: 1881

According to opencv documentation, input image to findContour function must be an 8-bit single channel so you must change your input image to that type by threshold, cvtColor, etc. And if your image is an gray image, simply load it in CV_LOAD_IMAGE_GRAYSCALE mode.

cv::Mat rawImg = imread(imgName, CV_LOAD_IMAGE_GRAYSCALE);

Upvotes: 1

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