OpenCV outputing odd pixel values

Question

I want to do processing on images with C++ and have been using OpenCV. I've gotten a bit stuck though when it comes to trying to access each pixel individually. I can output the whole gray scaled image just fine using:

cout << image;

and get the expected output of values like so:

[143, 147, 164, 177, 177, 185, 196, 195, 185, 186, 178, 190, 178, 163, 183...

But when I try to output one pixel at a time using:

for (int y = 0; y < image.rows; ++y) {
    for (int x = 0;x < image.cols; ++x) {
        std::cout<<image.at<double>(y, x)<<" ";
    }
            cout << endl;}

my output is a bunch of large numbers like these:

-2.98684e+18 -1.21685e-83 -1.91543e-113 -1.8525e-59 -2.73052e-127 -2.08731e-35 -3.72066e-103 ...

Any thoughts as to what I am missing or doing wrong?

where does `image` come from? what datatype is it? most probably `` is just the wrong datatype to interpret the values. Try `image.at(y,x)` or if it is not grayscale but BGR try `image.at(y,x)`. — Micka, Mar 21 '14 at 21:50
did you check your image.type() that should be CV_8U=0 or CV_8UC3=16? Reading video sometimes produces mistakes in terms of number of channels. — Vlad, Mar 22 '14 at 00:40
@Micka I'm getting my image from a video frame and then converting it to grayscale like this: `Mat frame;Mat image;` `VideoCapture vid(".../test_file.mp4");` `vid.read(frame);` `cvtColor( frame, gray_image, CV_BGR2GRAY );` — user3448181, Mar 22 '14 at 00:41
@Vlad I ran `cout << image.type();' and it outputs `0`. Does this mean it is CV_8U? — user3448181, Mar 22 '14 at 01:33

score 1 · Answer 1 · answered Mar 21 '14 at 23:08

since your values [143, 147, 164, 177...] look like uchar type your Mat type should be either CV_8U=0 or CV_8UC3=16, which you can check with image.type(). So your output should be (as @Micka noted)

std::cout<<image.at<uchar>(y, x)<<" "; // gray
or
std::cout<<image.at<Vec3b>(y, x)<<" "; // color

In the future just use this in order to stop worrying about the type:

Rect rect(0, 0, 10, 10); // print 10x10 block
cout<<image(rect)<<endl;

Not to say that knowing type isn't important though.

score 0 · Accepted Answer · edited May 23 '17 at 10:29

0

If you want to print all pixels of the image, you can simply use:

cout << image << endl;

If you want to print a pixel, use:

cout << image.at<Vec3b>(y, x) << endl; // for color image

or

cout << (int) image.at<uchar>(y, x) << endl; // for gray-scale image

Note that, in the last line, casting it to int is needed. For details, check out Why "cout" works weird for "unsigned char"?.

edited May 23 '17 at 10:29

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answered Mar 22 '14 at 02:11

herohuyongtao

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OpenCV outputing odd pixel values

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