QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?

[SGaist]

You need to add QT += widgets to your .pro file to use QApplication.

[kshegunov]

@R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

Changing it gives the error that it's not found.

Can you show the project file (*.pro)? I suspect you did not add QT = core gui widgets to it.

[R-P-H]

I will check the .pro file again and post back. However, I never used to modify anything in it before (besides linking the OpenCV library) and it was working fine like that at some stage.

[R-P-H]

@SGaist said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

You need to add QT += widgets to your .pro file to use QApplication.

@kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

@R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

Changing it gives the error that it's not found.

Can you show the project file (*.pro)? I suspect you did not add QT = core gui widgets to it.

QT -= gui

CONFIG += c++11 console
CONFIG -= app_bundle

QT += widgets

DEFINES += QT_DEPRECATED_WARNINGS

SOURCES += \
        main.cpp

If I add either QT += widgets or QT = core gui widgets to the .pro file and replace QCoreApplication with QApplication it works fine. It will not work with QCoreApplication however.

[kshegunov]

@R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

It will not work with QCoreApplication however.

QCoreApplication does not perform the needed steps to initialize the runtime for creating widgets. So yes it shouldn't be possible to create widgets with QCoreApplication; if it is, then you're using an undocumented behavior that's internal to the library, and it can stop working at any time without warning or crash. Warnings from the Qt library should be treated as errors in the user code.

[R-P-H]

@kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

@R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

It will not work with QCoreApplication however.

QCoreApplication does not perform the needed steps to initialize the runtime for creating widgets. So yes it shouldn't be possible to create widgets with QCoreApplication; if it is, then you're using an undocumented behavior that's internal to the library, and it can stop working at any time without warning or crash. Warnings from the Qt library should be treated as errors in the user code.

Alright thanks, at least I know how to get it to work now although I still find it strange that it was working before without having to use QApplication.

In terms of using QCoreApplication I was referring to @mranger90 post below:

@mranger90 said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

I've just confirmed that this will work on my system, Qt 5.10.0, MSVC 2017, OpenCV 3.1.0 (?), so it's not a matter of QCoreApplication vs QApplication.

where he confirmed that it does work with QCoreApplication. Strange.

[sierdzio]

@R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

Alright thanks, at least I know how to get it to work now although I still find it strange that it was working before without having to use QApplication.

You may want to rise this issue with OpenCV developers - looks like it is some change on their side. The documentation for imshow() does not mention any GUI config requirements.

And if it worked before it may be a regression.

[kshegunov]

@sierdzio said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

The documentation for imshow() does not mention any GUI config requirements.

It mentions that opencv will have a backend, and this is specified when building as well (--with-qt option), thus Qt requirements propagate through to opencv.

And if it worked before it may be a regression.

I doubt it. I continue to claim that creating a widget, which is what apparently imshow does, is not supported with QCoreApplication, never was and never will, regardless of whether it worked before, now or in the future.

where he confirmed that it does work with QCoreApplication.

Before applying the patch update 5.10.1.

[sierdzio]

@kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

It mentions that opencv will have a backend, and this is specified when building as well (--with-qt option), thus Qt requirements propagate through to opencv.

Is Qt the only supported backend? If yes, then indeed requirements do propagate. If no, then it may be the case that OP is using Qt for his console app, but OpenCV uses other framework to construct the window.

Anyway, I don't know OpenCV so I'm only speculating.

[kshegunov]

@sierdzio said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

Is Qt the only supported backend?

This I don't know, I don't know or use opencv as well. To be honest OpenCV's documentation's a bit vague and somewhat suspicious on the matter.

OpenCV uses other framework to construct the window.

This conflicts with the code snippet and the runtime error the OP gets ... and the fix. Apparently opencv in that case uses Qt to create the widget for display.

[sierdzio]

Yep, you're right.

[SGaist]

@sierdzio said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

Is Qt the only supported backend? If yes, then indeed requirements do propagate.

No, it has been added later on. It's maybe even the latest that has been implemented.

AFAIK, what the OP is hitting here, is that OpenCV doesn't have an implementation for imgshow for iOS. It would require to either use Qt to show the result of what he's doing or native stuff.