QRCodeDetector on android does not decode

[EddyTheCo]

System Information

OpenCV version: 4.8.0
Android 11
Android NDK

Detailed description

I am using precompiled libraries of OpenCV taken from these releases.

The precompiled libraries are used by my library using this CMakeLists.txt

This library works well in wasm, and desktop(only tested in linux). But when using Android, I can check that the QRCodeDetector detects the QRCODE but never decodes it.
Also when using auto str=detectAndDecodeCurved(greyImg,corners); corners becomes a 1x4 matrix, what are those numbers?
It is not clear to me what the documentation means with 'vertices of the found QR code quadrangle'. In what unit, what is the reference point? How can you express four vertices in a 2D plane with 4 numbers?

I see that when pointing the camera to a QRCODE corners becomes a 1x4 matrix, so it is detecting something.

Steps to reproduce

std::string MyQRCodeDetector::decode_grey(unsigned char* img,int rows ,int cols)
{
    cv::Mat greyImg = cv::Mat(rows,cols, CV_8UC1, img);

    cv::Mat corners;
    auto str=detectAndDecodeCurved(greyImg,corners);

    qDebug()<<"cols:"<<corners.cols;
    qDebug()<<"rows:"<<corners.rows;
    qDebug()<<"depth:"<<corners.depth();

    for(int i = 0 ; i < corners.rows ; i++)
    {
        for(int j = 0 ; j < corners.cols ; j++)
        {
            qDebug()<<corners.at<float>(i,j);

        }
    }
    if(corners.rows&&corners.cols)
    {
        cv::line(greyImg, cv::Point2i(corners.at<float>(0,0),corners.at<float>(0,1)), cv::Point2i(corners.at<float>(0,2),corners.at<float>(0,1)), cv::Scalar(100,200,155), 11);
        cv::line(greyImg, cv::Point2i(corners.at<float>(0,2),corners.at<float>(0,1)),cv::Point2i(corners.at<float>(0,2),corners.at<float>(0,3)), cv::Scalar(30,100,0), 8);
        cv::line(greyImg, cv::Point2i(corners.at<float>(0,0),corners.at<float>(0,1)), cv::Point2i(corners.at<float>(0,0),corners.at<float>(0,3)), cv::Scalar(150,4,255), 5);
        cv::line(greyImg, cv::Point2i(corners.at<float>(0,0),corners.at<float>(0,3)),cv::Point2i(corners.at<float>(0,2),corners.at<float>(0,3)), cv::Scalar(75,75,75), 2);
    }
    qDebug()<<"STTTTRRR:"<<str;
    return str;
}

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)

[asmorkalov]

please attach test image.

[EddyTheCo]

I am using the cellphone camera I have tried with many qr codes(for example the first one from Wikipedia).
There is no easy way to print the greyImg Mat on the Android debugger. And I think imshow(),imwrite() will not work on Android so easy.

I can see the resulting image after doing cv::line(greyImg,...
It is a clear image containing a QRCODE with segments(created with cv::line ), one on top of each other from the top-left to the top-right corner of the QRCODE. If there is no QRCODE on the picture no segment is shown.
The segments on top of each other are shown also in Linux desktop with a webcam, but there the QRCODE is decoded.

[EddyTheCo]

I was using OpenCV 4.5.5 on the desktop and 4.8.0 on Android. When using OpenCV 4.8.0 on the Desktop the app prints Library QUIRC is not linked. No decoding is performed. Take it to the OpenCV repository. This is not printed on the Android debugger(as expected from the debugger).
So I am changing -DWITH_QUIRC=OFF to -DWITH_QUIRC=ON when building OpenCV. Let's see if that solves the issue.

[AleksandrPanov]

"Also when using auto str=detectAndDecodeCurved(greyImg,corners); corners becomes a 1x4 matrix, what are those numbers?
It is not clear to me what the documentation means with 'vertices of the found QR code quadrangle'. In what unit, what is the reference point? How can you express four vertices in a 2D plane with 4 numbers?
I see that when pointing the camera to a QRCODE corners becomes a 1x4 matrix, so it is detecting something."

@EddyTheCo, cv::Mat uses 1x4 matrix with two channels in this case. You can just cout<<corners to check this.
You can use this code to get value of i corner from 1x4 Mat with 2 channels: Point2f corner_i = corners.at<Point2f>(0,i);
Also you can just use std::vector<Point> corners, detectAndDecodeCurved return std::vector in this case.

Uses channels() method to get num channels.

How is the image matrix stored in memory?

[AleksandrPanov]

@EddyTheCo, thank you, I think that we need to update docs for detectAndDecode()