the pixel values changed while using imwrite to jpg files in c++

Question

I'm writing a code like this in c++: I wish to have a 100% same copy image of test1.jpg. Unfortunately, I find lots of pixel values change after cv::imwrite.

int main()
{
    cv::Mat img1 = cv::imread("./test1.jpg");
    cv::imwrite("test2.jpg", img1);
    cv::Mat img2 = cv::imread("./test2.jpg");
    int count = 0;
    for (int i = 0; i < 250; i++) {
        for (int j = 0; j < 250; j++) {
            
                if (img1.at<uchar>(i, j) != img2.at<uchar>(i, j)) {
                    count++;
                
            }
        }
    }
    std::cout << count << std::endl;
    return 0;
}

I use count in this program to see how many differences are there between these two images, although both images (test1.jpg and test2.jpg) have the same size at 46kb, the count's value is as high as 16768!

Is there any method to avoid the change of pixel? I'm only going to use jpg files in the program. Thanks a lot!

so can png files or other types of images avoid this problem? — Happy_Dunckey, Jan 21 '22 at 14:42
@Happy_Dunckey yeah a .png will work. i don't get why you're being downvoted this is an easy mistake to make when you're starting out — Hersh Joshi, Jan 21 '22 at 14:48

score 0 · Accepted Answer · answered Jan 21 '22 at 14:47

0

If you want non-lossy compression, you can't use jpg's and have to use a .png (there's .bmp as well but its uncompressed)

jpg = cv.imread("../resources/fisheye/1_1.jpg")
cv.imwrite("1_1.png", jpg)
png = cv.imread("1_1.png")
np.sum(np.where(jpg != png, 1, 0)) # number of differing pixels between images

Output: 0

answered Jan 21 '22 at 14:47

Hersh Joshi

419
3
13

Thanks a lot. I'll try it now! – Happy_Dunckey Jan 21 '22 at 14:52

the pixel values changed while using imwrite to jpg files in c++

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