OpenCV canny; output image is pure gray - c

Question

I am learning opencv and reading a book and following examples. The book introduced the canny filter. However there is some problem with my output. As an input image I have given a 512x512 gray scale image but the filter output is pure gray image. Here is the image:
This is the input image.
And this is the output image.
And here is the snippets:
#include <opencv\cv.h>
#include <opencv2\highgui\highgui.hpp>
#include "Resources.h"
IplImage* doCanny(
IplImage* in,
double lowThresh,
double highThresh,
double aperture
) {
if (in->nChannels != 1)
{
return 0; // Canny only handle gray scale images.
}
IplImage* out = cvCreateImage(
CvSize(cvGetSize(in)),
IPL_DEPTH_8U,
1
);
cvCanny(in, out, lowThresh, highThresh, aperture);
return out;
}
int main(int argc, char** argv)
{
IplImage* image = cvLoadImage(IMAGE_FRUIT);
IplImage* output = doCanny(image, 200, 201, 1);
cvNamedWindow("Canny", CV_WINDOW_AUTOSIZE);
cvShowImage("Canny", output);
cvWaitKey(0);
cvReleaseImage(&output);
cvDestroyWindow("Canny");
return 0;
}
Visual Studio 2015, OpenCV version 2.4.13

Answer 1

I think if you step through your code, you will realize the cvCanny function never gets triggered, the returned output from doCanny is a null pointer.
OpenCV's Canny edge detection algorithm only accepts gray scale image, which is why the original code has the "if (in->nChannels != 1)" check, so you need to convert your input image into a grayscale image first.
// Convert to grayscale first
IplImage* gray_image = cvCreateImage(cvGetSize(image), IPL_DEPTH_8U, 1);
cvCvtColor(image, gray_image, CV_BGR2GRAY);
// Perform Canny
IplImage* output = doCanny(gray_image, 200, 201, 3);
Additional, I think your "aperture" parameter for cvCanny is also invalid, try to use the default value 3 (or 5, 7), and you should be able to see the result.
I would also recommend using the C++ interface instead of the deprecated C interface.