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QImage and OpenCV Mat problem



  • Hi all.

    I'm using Windows 10 and Linux 18.04 with Qt 5.12 and OpenCV 4.0.1.
    I have a problem when loading a QImage and then converting it to an OpenCV Mat.
    It seems that QImage has more data in its array than the Mat.

    Properties:
    Qimage: width=1001px, height=1001px, type=QImage::Format_Grayscale, bytesPerLine=1004
    Mat: width=1001px, height=1001px, type=CV_8UC1, bytesPerLine=1001

    This problem does only occur when the image width is odd.

    Does anyone know why and how to solve it?

    The result images are confusing. The pixels are shifted.

    711e428e-4798-47e1-bd81-7c5969e274bb-grafik.png

    fe6c9da0-67c7-4e2e-ba19-d0e352090a63-grafik.png

    Regards,
    Oliver


  • Lifetime Qt Champion

    Hi,

    IIRC, it will depend on the image format and size. There can be padding involved.

    In any case, it's usually done the other way around: load image with OpenCV, apply processing and move then to QImage.

    What are you currently doing ?



  • Hi,

    currently I am loading a QImage from a file.
    Then I encode it to Base64-Format and insert the Base64-String into an xml-structure.

    This xml is sended through tcp/ip to a server where the server is decoding the Base64-String back to an image. And then I am processing the image with OpenCV.

    My current workaround is to save the QImage to hard disk and load again it with OpenCV. That works.

    Regards,
    Oliver


  • Lifetime Qt Champion

    That might be a silly question but why not directly encode the file content ?



  • @stvokr
    QImage uses a 32-bit alignment, I also had some issues with the bytes: https://forum.qt.io/topic/105321/different-bytes-count-of-qimage



  • @SGaist
    I will try that, but I need that QImage object directly. It should be possible to display the base64 string directly in a web browser as HTML content.

    @beecksche
    Thanks for that.


  • Lifetime Qt Champion

    There's no problem with that but you didn't gave much information about your application so it's not easy to provide recommendations.



  • @SGaist
    I figured out that the main problem is not transferring the image but loading the image with QImage. Even when I just load the image from harddisk the bytesPerLine is 1004 for an 8 bit pixel image with 1001px width. So as already mentioned from @beecksche the 32-bit alignment will prevent the correct direct conversion from QImage to cv::Mat.


  • Qt Champions 2019

    Afaics OpenCV::Mat also supports padding. Images are always padded so it would be better to teach OpenCV::Mat about it.


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