OpenCV: Unable to run Canny edge detection in C++ - c++

Question

I have written a simple code to perform canny edge detection on a live stream. The code is as shown below,
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <iostream>
using namespace cv;
using namespace std;
int lowThreshold=0;
int const max_lowThreshold = 100;
int kernel_size = 3;
int ratio = 3;
Mat img;
Mat display;
void CannyThreshold()
{
cvtColor(img, display, COLOR_RGB2GRAY);
// GaussianBlur(display,display,Size(7,7),3,3);
GaussianBlur(display, display, Size(1, 1), 1,1);
printf("%d\n",lowThreshold);
Canny(display,display,lowThreshold,3);
imshow("Canny",display);
}
int main()
{
VideoCapture cap(0);
namedWindow("Canny");
createTrackbar("Min Threshold: ","Canny",&lowThreshold,max_lowThreshold);
while(1)
{
cap.read(img);
int ret = waitKey(1);
CannyThreshold();
if(ret == 'q')
break;
}
cap.release();
return 0;
}
I get the following run-time error when I run the code. (I'm using OpenCV 4)
error: (-215:Assertion failed) ksize.width > 0 && ksize.width % 2 == 1 && ksize.height > 0 && ksize.height % 2 == 1 in function 'createGaussianKernels'
Any suggestions on how I can solve this error?

Answer 1

The issue is GaussianBlur cant accept kernel size of 1. Correct it to 3x3 or 5x5 in your code as follows
#include <opencv2/core/utility.hpp>
#include "opencv2/video/tracking.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/videoio.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>
#include <ctype.h>
using namespace cv;
using namespace std;
int main(int argc, const char** argv)
{
VideoCapture cap;
Mat frame;
Mat image; // from cap to image
Mat src_gray;
Mat dst;
Mat detected_edges;
const String window_name = "Canny Edge Detector - VideoCapture";
int lowThreshold = 10;
const int max_lowThreshold = 100;
const int ratio = 3;
const int kernel_size = 3;
int FPS;
int frame_width, frame_height;
int camNum = 0;
cap.open(camNum);
if (!cap.isOpened())
{
cout << "***Could not initialize capturing...***\n";
cout << "Current parameter's value: \n";
return -1;
}
FPS = cap.get(CAP_PROP_FPS);
frame_width = cap.get(CAP_PROP_FRAME_WIDTH);
frame_height = cap.get(CAP_PROP_FRAME_HEIGHT);
dst.create(frame_width, frame_height, CV_8UC3);
//cout << CV_8UC3;
while (true)
{
cap >> frame;
if (frame.empty())
break;
frame.copyTo(image);
// Convert the image to grayscale
cvtColor(image, src_gray, COLOR_BGR2GRAY);
//![reduce_noise]
/// Reduce noise with a kernel 3x3
blur(src_gray, detected_edges, Size(3, 3));
//![reduce_noise]
//![canny]
/// Canny detector
Canny(detected_edges, detected_edges, lowThreshold, lowThreshold*ratio, kernel_size);
//![canny]
/// Using Canny's output as a mask, we display our result
//![fill]
dst = Scalar::all(0);
//![fill]
//![copyto]
image.copyTo(dst, detected_edges);
//![copyto]
//![display]
imshow(window_name, dst);
if (waitKey(1000 / FPS) >= 0)
break;
}
return 0;
}