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Resource path for non-qt functions



  • I've encountered a problem, that i can't use my relative file pathes to anything but qt functions.

    For example, such files path(which got from resources), working fine when i use it for my QGraphicsPixmapItem:

    ":/myFiles/files/lego.jpg"
    

    And it is useless for other functions, which demand path as std::string, and not related to qt. I using .toStdString() ,and so far can not understand how should i write relative path, so i only use absolute path for now:

    "D:/stud/Qt/testApp02/files/lego.jpg"
    

    How should i specify correct relative filepath for qt app?



  • Problem "resolved". From this topic i understand that i can't acces .res files of my qt app, it's okay. But then i found that i'm not able to simply use relative path's in my app for some strange qt reason.

    So i found such wonderful static function - QString QCoreApplication::applicationDirPath(); which returns directory of running app, then simply add desired "relative" path QString and here we go!



  • @Engelard
    You misunderstand.

    ":/..." paths are Qt resources. They are available to Qt functionality. If you want to do anything "external" on them, it is your job to extract them to a physical file (use QFile) and pass that to the external thing to access.



  • @JonB why it is sound so complex, for such a simple thing as specifying file path...



  • @Engelard
    :) It's not that. There isn't any physical file for that ":/myFiles/files/lego.jpg", is there? It's buried away in Qt resources, which only Qt can access, and it uses ":/..." to signify that.

    So you have to get it out to hand it to other programs (which you don't usually do). If you don't want to have to do the extraction, don't compile it in as a resource, supply it as an external file instead.


  • Lifetime Qt Champion

    Hi,

    @Engelard said in Resource path for non-qt functions:

    @JonB why it is sound so complex, for such a simple thing as specifying file path...

    Qt resources are embedded in your executable/library. From the looks of it, you are trying to pass these files to some external library. That library has absolutely no idea on how to access the resources. Hence you will have to copy them somewhere on your hard drive to process it further.



  • @Engelard said in Resource path for non-qt functions:

    why it is sound so complex, for such a simple thing as specifying file path...

    You may want to read about the Qt Resource System which "is a platform-independent mechanism for storing binary files in the application's executable"

    You'll see then that it's just more than specifying a path...



  • @JonB said in Resource path for non-qt functions:

    If you don't want to have to do the extraction, don't compile it in as a resource, supply it as an external file instead.

    Thats the first thought that i had, and i understand that i can't simply use resource path. But i also don't know where should i put my "files" folder with all my stuff for correct deployment, so my program will use it "externally", should i place it in project folder with my .pro .cpp .h files? Or should i put it build folder directly? And after that, how should i specify that relative path properly.


  • Lifetime Qt Champion

    Where are these files supposed to live ?
    What are they used for ?



  • @SGaist said in Resource path for non-qt functions:

    Where are these files supposed to live ?

    In program folder i suppose when it deployed and ready to use.

    @SGaist said in Resource path for non-qt functions:

    What are they used for ?

    For in-program calculations, image files mostly.



  • If i want to use in my program relative path like:

    QString tempPath = "files/lego.jpg";
    

    where then i should place my physical folder "files" then?



  • @Engelard said in Resource path for non-qt functions:

    where then i should place my physical folder "files" then?

    as a subfolder under the root (or installation) folder of your Qt app, i.e. relative to your Qt app executable

    /path/to/your/fantastic/app/myAppExecutable
    /path/to/your/fantastic/app/files
    /path/to/your/fantastic/app/files/myicon.jpg
    


  • @Engelard
    You can get into trouble using relative paths. If the current directory of your program is not what you think it is or changes, it may be wrong for you. When you open files at runtime you would be best making a full path to them. If you plan to pass this path to an external program, you may be advised to pass the full path. There are Qt methods to provide strings for the path, e.g. https://doc.qt.io/qt-5/qcoreapplication.html#applicationDirPath.



  • @Pablo-J-Rogina said in Resource path for non-qt functions:

    as a subfolder under the root (or installation) folder of your Qt app, i.e. relative to your Qt app executable

    So for the time i'm developing app i can place it to the build folder, right?
    :D\Qt\build-testApp02-MSVC_for_x32-Debug\debug\files
    But it is still a problem with specifying such relative path with string, i probably still writing it incorrectly:

    QString tempPath = "/files/lego.jpg";
    


  • @JonB I'm trying do as you firstly suggested with QFile, but i don't get what should i do. Get filepath with QFileInfo? but it is senseless, it simply give me back my path to /resources.



  • @Engelard
    Just use QFile to open the ":/..." file for read, since it is QFile which understands this path syntax to mean read from resources. Write that to a physical, external file (also probably via a QFile) to export to file for outside world.

    But a while ago I thought you were saying you were giving up on resource files and switching over to physical files anyway?



  • @JonB said in Resource path for non-qt functions:

    Just use QFile to open the ":/..." file for read

    I do not need read that, i need only it's path in string type so i could pass that as the parameter for OpenCV functions :) Which will read/write/anything to it.

    @JonB said in Resource path for non-qt functions:

    Write that to a physical, external file (also probably via a QFile) to export to file for outside world.

    Seriously, i still don't get it, what exactly should i do with that? As far i understand that i should copy file from resources, paste it's copy somewhere in app's folder "buffered files or smth", then get absolutePath using QFileInfo and use that copy in the runtime of app?

    @JonB said in Resource path for non-qt functions:

    But a while ago I thought you were saying you were giving up on resource files and switching over to physical files anyway?

    I did such stuff before, and i understand the simplicity of dumb folder creation, but never used QFiles and started to be interested, plus, tired of seeking answer what is correct way to write relative path in qt.

    Ah, and QFileInfo gives zero info about the file path, same QString what i provided in the code(res directory), it gives in the runtime as a result :/myFiles/files/lego.jpg when this function absoluteFilePath() should return actual place of the file on the computer, but it does not.



  • @Engelard
    I have explained, I don't know how to explain it any clearer.

    If you ship a resource, it isn't an external file, you can't pass it to OpenCV anything.

    A resource is addressed via :/... path. That is not a real file. The only thing that can open it is QFile.

    Use that to open its content for read. Read its whole content, writing that to a real, external file. [EDIT @SimonSchroeder shows later on that you can simply use QFile::copy() to do this in one line.] Pass the path of that extracted file to your external OpenCV.

    If you don't understand this, you can't use a resource file for your purpose of passing it to OpenCV. In that case give up on resources and supply the file as an external file, not a resource.



  • @JonB said in Resource path for non-qt functions:

    If you ship a resource, it isn't an external file, you can't pass it to OpenCV anything.

    But how can it be? It exists somewhere in app folder... at least i was expecting that i could reach it. Thanks for explaining all this.

    @JonB said in Resource path for non-qt functions:

    In that case give up on resources and supply the file as an external file, not a resource.

    seems it will be easier, but can you please clarify what is the right syntax for writing relative directories in qt? "/files/lego.jpg" is not working, and bunch of other ways i found in google.



  • @Engelard
    /files/lego.jpg is not a relative path. It starts with a /, so it's absolute.

    Relative paths do not start with /. lego.jpg, files/lego.jpg and ../lego.jpg are all relative paths. They are relative to whatever the current directory is.

    These relative/absolutes have nothing to do with Qt. The only "special" path in Qt is :/..., which Qt alone treats as referring to a file in its resources, not accessible to the outside world.



  • @JonB said in Resource path for non-qt functions:

    They are relative to whatever the current directory is.

    okay, so folder "files" is relative to my .exe and other stuff in the debug folder of my project, but none

    "../files/lego.jpg"
    

    or

    "files/lego.jpg"
    

    is not working...

    P.S. how do you highlight some text with red in your posts?)



  • @JonB said in Resource path for non-qt functions:

    Read its whole content, writing that to a real, external file.

    you mean - make a copy of that file from resources? Even if i'll do that, i should create folder for that .jpg file, so it would'nt be messy, and then i should delete that copy when my program is about to be closed. Am i right?



  • Problem "resolved". From this topic i understand that i can't acces .res files of my qt app, it's okay. But then i found that i'm not able to simply use relative path's in my app for some strange qt reason.

    So i found such wonderful static function - QString QCoreApplication::applicationDirPath(); which returns directory of running app, then simply add desired "relative" path QString and here we go!



  • What OpenCV functions are you planning to use with the image files?
    If you only use cv::imread you can try to replace that with QFile + cv::imdecode or create a cv::Mat directly from a QImage.



  • @Bonnie said in Resource path for non-qt functions:

    or create a cv::Mat directly from a QImage.

    Tnx. I just started with OpenCV, by now need exactly Mat creation. Which one is better for performance? From QFile or from QImage?



  • Bonnie, you just saved the day. Because of your hint i found this golden answer work of art: https://stackoverflow.com/a/33923407 Everything tested and well explained, conversion from QFile as he mentioned 10 times faster then from QImage.



  • @Engelard said in Resource path for non-qt functions:

    So i found such wonderful static function - QString QCoreApplication::applicationDirPath(); which returns directory of running app, then simply add desired "relative" path QString and here we go!

    Which is why I wrote to you earlier on:

    When you open files at runtime you would be best making a full path to them. If you plan to pass this path to an external program, you may be advised to pass the full path. There are Qt methods to provide strings for the path, e.g. https://doc.qt.io/qt-5/qcoreapplication.html#applicationDirPath.



  • @JonB said in Resource path for non-qt functions:

    Which is why I wrote to you earlier on:

    Yeah, back then i did'nt give it serious attention, was thinking that using relative path's it is the only right way for program to use it's own files.



  • @Engelard said in Resource path for non-qt functions:

    Yeah, back then i did'nt give it serious attention

    Yeah, that's nice to hear ;-) I'll bear it mind for my future replies to you :D :D [<-- For the avoidance of any misinterpretations: winks/smileys!]



  • @Engelard One thing to consider – as you have mentioned that OpenCV might also be writing (to) files – is that if you distribute your app and people install it properly they might not be able to write into the applications folder. Usually, these folders are only writeable for administrators/root. Have a look at https://doc.qt.io/qt-5/qstandardpaths.html to figure out where to put which kind of files. Furthermore, there is QTemporaryDir and QTemporaryFile if the data is supposed to only exist until you close your software.

    BTW: If you provide something inside your resources and need to have a copy inside the real filesystem, a simple QFile::copy(":files/lego.jpg", "/some/tmp/path/files/lego.jpg") is sufficient. You don't need to open the file and read it in. (Though your solution you found on StackOverflow is also valid.)


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