Unable to write Grayscale image with imwrite in OpenCV

Question

I tried to convert RGB image into grayscale, conversion is successfull and i can display it with imshow. Yet, i cannot write it with imwrite, imwrite function returns NULL. My original image is 1920*1080 JPEG file with UINT8. Here is the code

#include <opencv\cv.h>
#include <opencv\highgui.h>

using namespace cv;

int main(int argc, char** argv)
{
char* imageName = argv[1];

Mat image;
image = imread(imageName, 1);

if (argc != 2 || !image.data)
{
    printf(" No image data \n ");
    return -1;
}

Mat gray_image;
cvtColor(image, gray_image, CV_BGR2GRAY);

if (imwrite("../../images/Gray_Image.jpg", gray_image) ==NULL) {
    printf( "Writing image is not successfull!");       
}

namedWindow(imageName, CV_WINDOW_AUTOSIZE);
namedWindow("Gray image", CV_WINDOW_AUTOSIZE);

imshow(imageName, image);
imshow("Gray image", gray_image);

waitKey(0);
return 0;
}

Why imwrite returns NULL?

Are you on windows? I noticed you used backslashes for the #include paths, but forward slashes for the image path you are writing. — Vaughn Cato
– Vaughn Cato, Commented Mar 9, 2014 at 20:22

Cheers and hth. - Alf · Accepted Answer · 2014-03-09 20:30:30Z

2

The #include directive supports forward slashes. Use forward slashes there, for portability. And for sanity.

File handling functions generally don't support forward slashes in Windows. Use backward slashes there. Or, better, use some path handling class (such classes often support forward slashes, since they need to be portable, and automatically convert to backward slash in Windows).

In short,

/ → \ to fix the immediate problem.

That said, the OpenCV imwrite probably does not support automatic folder creation, so you'd better make sure that …

the specified folder exists, and
for a relative path (as you have), that the program execution's current folder is where you imagined the relative path starting.

answered Mar 9, 2014 at 20:30

Cheers and hth. - Alf

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1 Comment

Gorkem Polat Over a year ago

Thank you very much, as you said it is related with backslash and automatic folder creation

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