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QT QImage Read From Error



  • Hey there,
    I am developing a GUI, where i read a Qimage from a stored location and display it.
    I have used QThread for image grabbing and used signal and slot mechanism to display image in mainwindow.
    The code is working as expected but the problem is when the frequency of image grabbing increase, image does not show fully it has some blank portion covered.
    i'm attaching the image below
    ![alt text](![image url](error_img.png image url))
    as you can see bottom part of the image is completely blank.
    Just wanted to know what could be the reason for it and how to approach a better solution.

            bool check = doesFilexExist(image_path);
            if(check){
                bool result = image.load(image_path);
                QThread::msleep(20);
                if(!image.isNull()){
                    qDebug() << "==> Timestamp for image grabbing: " << QTime::currentTime();
    
                    emit sendImage(image);
                    QFile::remove(image_path);           
                    QThread::msleep(50);
                }
    

    I have attached code snippet here.



  • @mvsri
    What is the scope of your image variable? Does it remain in scope throughout the time the main UI thread accesses it? Does this code get re-entered and re-load the same image variable while it is still being accessed in the main UI thread?



  • @JonB
    it get re-entered and re-load the same image variable.


  • Qt Champions 2019

    Why do you need a thread for this at all?
    Simply create a new QImage object instead using a local / member var would be a first step.



  • @mvsri said in QT QImage Read From Error:

    @JonB
    it get re-entered and re-load the same image variable.

    Then I'm thinking that is not a good idea?

    Start by looking at @Christian-Ehrlicher's advice.



  • @JonB @Christian-Ehrlicher reason for using thread was the entire code snippet will be in a while loop, continuously grabbing image. and main GUI has other functions on it.
    and also i have tried without using qthread but there was no real time image capturing happening there (i also used QTimer).



  • @mvsri
    Just for now: instead of a single image declaration somewhere, in your code try:

    QImage *image = new QImage;
    bool result = image->load(image_path);
    ...
    emit sendImage(*image);
    // or, with corresponding change to signal/slot parameter
    emit sendImage(image);
    

    I know this will leak the image! Assuming it's not too big, and you don't do it too often, so that memory is not an issue, does that make a difference to the "black" you see?


  • Qt Champions 2019

    @JonB No need to create it with new, a local instance on the stack will work fine



  • @Christian-Ehrlicher
    I don't see that as safe? If the stack variable in the thread goes out of scope while the signal has been emit()ed passing the image for the main UI thread to use, won't that release the image while it is still needed by the main thread?



  • @JonB @Christian-Ehrlicher
    i tried both methods using new and local instance.
    still it doesn't make any difference.
    will it be advisable to use QMutex for this?


  • Qt Champions 2019

    No need for a mutex, please post code with stack variable.

    will be in a while loop, continuously grabbing image. and main GUI has other functions on it.

    So you say the image reading take too long?



  • @Christian-Ehrlicher
    This is the code

    while(true)
       {
           bool check = doesFilexExist(image_path);
           if(check){
               QImage image;
               bool result = image.load(image_path);
               QThread::msleep(20);
               if(!image.isNull()){
                   qDebug() << "==> Timestamp for image grabbing: " << QTime::currentTime();
    
                   emit sendImage(image);
                   QFile::remove(image_path);
                   QThread::msleep(50);
               }
           }
           else {
               // Do nothing
           }
       }
    

    @Christian-Ehrlicher said in QT QImage Read From Error:

    So you say the image reading take too long?

    I'm not so sure about that.

    The logic is very simple :
    1. Check if image exists in folder.
    2.if exists, load image
    3.emit the image
    4. delete image from the folder

    Note: 3rd party camera stores a single image in that folder


  • Qt Champions 2019

    @mvsri So are you sure the camera does not override the image while you try to load it? Please try without the camera with a static image.


  • Moderators

    @mvsri Have you thought about using QFileSystemWatcher to listen to changes in the folder,?

    No need to reinvent the wheel, you can simply react to the directoryChanged signal


    If @Christian-Ehrlicher is right, and the camera overwrites the image, than the fileChanged signal will be much handier to use.



  • @Christian-Ehrlicher
    I think the camera does override the image when i try to load it because when i tested it with static image the code worked perfectly



  • @J-Hilk said in QT QImage Read From Error:

    @mvsri Have you thought about using QFileSystemWatcher to listen to changes in the folder,?

    I haven't thought about that will try to use it now.


  • Qt Champions 2019

    @mvsri said in QT QImage Read From Error:

    I think the camera does override the image when i try to load it because when i tested it with static image the code worked perfectly

    So you've your explanation of the issue - Qt can't do anything against this.



  • @Christian-Ehrlicher
    yeah i got that, but the thing is if you see the code,
    after reading the image i'm deleting the image from the folder, so the chances for image override is very less


  • Qt Champions 2019

    @mvsri Maybe try to rename it before you try to read it.



  • Maybe try to find out if that 3rd party camera has other API than save the image to the disk.



  • @mvsri
    I really don't understand what is going on! What is it that produces that image_path file? Something external? How do you know that whatever is creating it has finished writing the file and flushed it before your thread decides to read it in, such that it might read in something incomplete? :confused:



  • @JonB said in QT QImage Read From Error:

    @mvsri
    I really don't understand what is going on! What is it that produces that image_path file? Something external? How do you know that whatever is creating it has finished writing the file and flushed it before your thread decides to read it in, such that it might read in something incomplete? :confused:

    image_path is nothing but a QString which stores the path to a bmp image in a folder. the path is static it doesn't change.
    I don't know whether the image writing is finished or not. that's why i used QFile exists to check if the file is created or not and read the image if the path exists.
    I also have the same doubt whether i'm reading the image in an incomplete mode.

    @Christian-Ehrlicher said in QT QImage Read From Error:

    @mvsri Maybe try to rename it before you try to read it.

    I'm trying it now.

    Thank you everyone for all the help. will try to update if i found something useful.



  • @mvsri said in QT QImage Read From Error:

    image_path is nothing but a QString which stores the path to a bmp image in a folder. the path is static it doesn't change.

    I know that! My question is about the content of that file.

    I don't know whether the image writing is finished or not. that's why i used QFile exists to check if the file is created or not and read the image if the path exists.

    But that doesn't tell you anything about whether it has started but not finished writing to that file, does it? (Unless you are relying on Windows or something not allowing a file to satisfy "exists" until it has been closed, which I would see as dodgy in the extreme.) In which case, you will read in an incomplete image file, maybe that's why you have "black" at the bottom? At least put in a qDebug() << image.sizeInBytes() after loading it (though I'm not sure if that's reliable)....

    QFile::remove(image_path);

    It gets worse! This, or renaming: how do you know that at the instant you execute this the camera has not re-started writing to that file for the next capture, and you are (trying to) removing/renaming a file while it is being written anew?

    Is your camera-image-capture-write-to-file a separate process from your code? How do you know when the capture has started/finished writing to the file?