How to convert QImage to opencv Mat c++

[Malikov]

I have searched on the internet but I cannot find a method in C++ of converting a QImage ( QImage(2048, 1148, QImage::Format_Grayscale8) ) to a OpenCV Mat for write frames to cv::VideoWriter("output.avi",cv::VideoWriter::fourcc('D','I','V','X'),10, size,0);

This is the method I'm using but it throws exception when write to cv::VideoWriter :

cv::Mat QImageToCvMat(QImage &inImage)
{
cv::Mat mat(inImage.height(), inImage.width(),
CV_16UC1, inImage.bits(), inImage.bytesPerLine());
return mat;
}

The exception:

OpenCV Error: Image step is wrong () in cvSetData, file /build/opencv-XDqSFW/opencv-3.2.0+dfsg/modules/core/src/array.cpp, line 939
terminate called after throwing an instance of 'cv::Exception'
what(): /build/opencv-XDqSFW/opencv-3.2.0+dfsg/modules/core/src/array.cpp:939: error: (-13) in function cvSetData

How would I do this the right way ?.

Any help is appreciated.

[J.Hilk]

@Malikov
https://github.com/dbzhang800/QtOpenCV

[Malikov]

Is their simpler way? In one method?

[Pl45m4]

@Malikov said in How to convert QImage to opencv Mat c++:

Image step is wrong () in cvSetData

Set correct imageStep and it should work.

See here:
https://stackoverflow.com/questions/17127762/cvmat-to-qimage-and-back/17137998

or here:
https://stackoverflow.com/questions/5026965/how-to-convert-an-opencv-cvmat-to-qimage

[JKSH]

@Malikov said in How to convert QImage to opencv Mat c++:

cv::Mat mat(inImage.height(), inImage.width(),
CV_16UC1, inImage.bits(), inImage.bytesPerLine());

CV_16UC1 is for 16-bit data. QImage::Format_Grayscale8 is for 8-bit data. If I'm not mistaken, these are incompatible. (Try converting your QImage to a 16-bit format first)

Also, see this answer: https://stackoverflow.com/a/17137998/1144539

[Pl45m4]

@JKSH said in How to convert QImage to opencv Mat c++:

Try converting your QImage to a 16-bit format first

Just using CV_8UC1 type Mat would be the easier way, since @Malikov wants to get a cv::Mat from existing QImage :)

[Malikov]

@Pl45m4

Hi

I'm not sure what format type I need to set in the Mat contractor.
Each frame has 1 channels and 2 elem bytes. I initialize QImage variable this way:

        img = QImage(width, height, QImage::Format_Grayscale8);
        uchar *pSrc = (uchar *)m_frame.pBuffer + m_frame.usHeader;
        uchar *pDst = (uchar *)img.bits();

       int pixels = frame.width * frame.height * frame.channels;
       for (int i = 0; i < pixels; ++i)
       {
                    *pDst++ = *pSrc;
                    pSrc += elementBytes;
       } 

My issue is to convert this QImage img to Mat variable in order to write frames to cv::VideoWriter.

[eyllanesc]

@Malikov See this answer: https://stackoverflow.com/a/50848100/6622587

static cv::Mat QImageToMat(const QImage & image){
    cv::Mat out;
    switch(image.format()) {
    case QImage::Format_Invalid:
    {
        cv::Mat empty;
        empty.copyTo(out);
        break;
    }
    case QImage::Format_RGB32:
    {
        cv::Mat view(image.height(),image.width(),CV_8UC4,(void *)image.constBits(),image.bytesPerLine());
        view.copyTo(out);
        break;
    }
    case QImage::Format_RGB888:
    {
        cv::Mat view(image.height(),image.width(),CV_8UC3,(void *)image.constBits(),image.bytesPerLine());
        cv::cvtColor(view, out, cv::COLOR_RGB2BGR);
        break;
    }
    default:
    {
        QImage conv = image.convertToFormat(QImage::Format_ARGB32);
        cv::Mat view(conv.height(),conv.width(),CV_8UC4,(void *)conv.constBits(),conv.bytesPerLine());
        view.copyTo(out);
        break;
    }
    }
    return out;
}

[Malikov]

Sorry its not helping me

The format is QImage::Format_Grayscale8 and Im not sire what format I need for Mat.

[Bonnie]

@Malikov
As @JKSH and @Pl45m4 already said, just replace CV_16UC1 with CV_8UC1 like

cv::Mat mat(image.height(), image.width(), CV_8UC1, image.bits(), image.bytesPerLine());

Have you tried that?
You need to keep image valid and unchanged during the use of mat because they are sharing internal data.

[Malikov]

tnx.
It worked