QT QImage Read From Error

[mvsri]

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
)
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.

[JonB]

@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?

[mvsri]

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

[Christian Ehrlicher]

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.

[JonB]

@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.

[mvsri]

@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).

[JonB]

@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?

[Christian Ehrlicher]

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

[JonB]

@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?

[mvsri]

@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?

[Christian Ehrlicher]

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?

[mvsri]

@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

[Christian Ehrlicher]

@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.

[J.Hilk]

@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.

[mvsri]

@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

[mvsri]

@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.

[Christian Ehrlicher]

@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.

[mvsri]

@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

[Christian Ehrlicher]

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

[Bonnie]

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