Qt Error on cv2.imshow - Specific to version 4.2.0.32

[NeilGraham]

Issue

Calling cv2.imshow with opencv-python 4.2.0.32 results in the following error:

qt.qpa.plugin: Could not find the Qt platform plugin "cocoa" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Solution

I am able to fix the issue by:

• uninstalling the current version: 4.2.0.32
• reinstalling the previous version: 4.1.2.30

pip uninstall opencv-python
pip install opencv-python==4.1.2.30

Repoduce

import cv2 
img = cv2.imread('./path/to/picture.png', cv2.IMREAD_GRAYSCALE)
cv2.imshow('image', img)

• macOS 10.15.2 x86
• Python 3.8.1
• opencv-python 4.2.0.32

[skvark]

Qt version was upgraded from 4 to 5 in the latest update (Qt 4 dependencies were too old and caused major issues during builds). The issue is most likely related to that upgrade. I don't own a Mac so it's quite hard for me to debug the issue. However, I'll try to look into it.

[skvark]

See: https://forum.qt.io/topic/98816/qt-could-not-find-the-platform-plugin-cocoa

Set QT_DEBUG_PLUGINS to 1 before running your script. It will tell where Qt is looking for the cocoa plugin.

[dlech]

This is with opencv-contrib-python 4.2.0.32 in a virtual environment.

QFactoryLoader::QFactoryLoader() checking directory path "/usr/local/Cellar/python@3.8/3.8.1/Frameworks/Python.framework/Versions/3.8/Resources/Python.app/Contents/MacOS/platforms" ...
qt.qpa.plugin: Could not find the Qt platform plugin "cocoa" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

zsh: abort      QT_DEBUG_PLUGINS=1 python __main__.py

I made some progress setting QT_QPA_PLATFORM_PLUGIN_PATH=/usr/local/Cellar/qt/5.13.1/plugins. It gives errors about not being compatible though since it is not the same build as the Qt library in the wheel.

There is /usr/local/Cellar/qt/5.13.1/plugins/platforms/libqcocoa.dylib installed by homebrew, but no such plugin in the opencv-contrib-python wheel.

So, I think the first step is to get libqcocoa.dylib in the binary wheel.

[NeilGraham]

I am getting a similar error message:

export QT_DEBUG_PLUGINS=1
python3 run.py

QFactoryLoader::QFactoryLoader() checking directory path "/Library/Frameworks/Python.framework/Versions/3.8/Resources/Python.app/Contents/MacOS/platforms" ...
qt.qpa.plugin: Could not find the Qt platform plugin "cocoa" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

zsh: abort      python3 run.py

• For what it's worth, I did have an independent version of Qt 5 installed on my computer when I encountered this issue. I attempted to reinstall Qt to fix the issue but had no luck (same error message). At the moment, it is not installed on my system as of executing the command above.

[skvark]

@dlech Yeah, it looks like libqcocoa.dylib needs to be embedded to the wheel. Also qt.conf file is maybe needed to set the plugin search path relative to the wheel binaries folder: https://doc.qt.io/qt-5/qt-conf.html

I don't own a Mac so it would be helpful if you can test if the Qt which is embedded to the wheels can be configured to look for the dylib from the wheel install directory.

[dlech]

The QT_QPA_PLATFORM_PLUGIN_PATH environment variable can be used to set the plugin search path.

So if we can make it so that QT_QPA_PLATFORM_PLUGIN_PATH is set when the cv2 module is imported, that could be a solution.

[skvark]

I'm pretty sure that won't work because the environment variable must be set outside / before the process.

[dlech]

It should be fine as long as it is set before we try to use Qt.

[skvark]

It's not very convenient to require every user to export that environment variable before use.

[dlech]

Yes. I am suggesting that we do it automatically when the cv2 module is imported so that users don't have to do it themselves.

[dlech]

I spend quite a few hours last night trying to get this repo to build so I can actually try it out. I never could get it to build with python3, but finally got it to build with python2. So making some progress.

[skvark]

It's quite hard to do the export because it must be done outside the Python process. You can't just set it with Python because the process has already started and afaik setting env variables during the process does not have any effect.

[dlech]

Please try this simple example to see that it is in fact possible to modify environment variables after a process starts. It is true that processes cannot modify the environment variables of another process, but the process itself can modify its own environment variables.

import os

os.environ['PYTHONINSPECT'] = 'on'

It has the same effect as PYTHONINSPECT=on python