A way to solve "Could not load the Qt platform plugin "xcb" in xxxx" in MMagic.

[zhongqiu1245]

I want to add this solution into BUG Report, but when I fix this problem, I found this is not mmagic fault, it should blame PyQt and opencv (lol)
I found this ERROR when I played demo(after I installed mmagic successfully):

got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/home/dan/anaconda3/envs/mmws/bin/platforms" ...
loaded library "/home/dan/anaconda3/envs/mmws/lib/python3.10/site-packages/cv2/qt/plugins/platforms/libqxcb.so"
QObject::moveToThread: Current thread (0x555d0d8ce440) is not the object's thread (0x555d0d7e8310).
Cannot move to target thread (0x555d0d8ce440)

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/dan/anaconda3/envs/mmws/lib/python3.10/site-packages/cv2/qt/plugins" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: xcb, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, webgl.

已放弃 (核心已转储)

I thought it might be the way I installed at first (pip install -v -e .). But it was OK.
Then I thought it might be the Qt's thing (according to the ERROR message). I tried everything I found, such as uninstall-install, version matching, pip-install/conda-install, but it didn't work.
Unfortunately, this ERROR would spread to other mmcodes which rely on PyQt and opencv, for examlpe, mmseg. My mmseg didn't work after this error came out.
After debugging painfully, I found opencv was the real troublemaker.

Here is the solution, please follow this step by step:
(Make sure you PyQt is installed by pip install. If you install it by conda install, please uninstall it and pip install it)
Step 1: install your mmagic successfully(pip install -v -e . or pip install mmagic or something else)
Step 2: pip uninstall or conda uninstall everything about opencv. In my env, I had opencv-python, opecv-python-headless and opencv-contrib-python, I uninstalled them all.
Step 3: choose you version of opencv (for example, opencv-python==4.5.4.60. Don't choose the latest version)
Step 4: pip install the opencv things. BUT follow this order:

pip install opencv-contrib-python==your version
pip install opencv-python==your version
pip install opencv-python-headless==your version

Make sure your versions of opencv things are in same. for example:

pip install opencv-contrib-python==4.5.4.60
pip install opencv-python==4.5.4.60
pip install opencv-python-headless==4.5.4.60

AND, the most important thing, do not mess this order.
Step 5: Done

The reason why this error appear is the compatibility between opencv and PyQt is not very well. And it bug still not be fixed in the latest version, both of them.
I hope this report can help other people. If the authors of mmagic are interest this, you can add this into doc.
My English is not very well. If I write something wrong, please point it out. I can improve my English in this way (smiling face). Thank you in advance.

[zhongqiu1245]

My env:

xxxxxxxxxxx                       xxxxxxxx
opencv-contrib-python             4.5.4.60  
opencv-python                     4.5.4.60      
opencv-python-headless            4.5.4.60
pyqt5                             5.15.9                  
pyqt5-qt5                         5.15.2                   
pyqt5-sip                         12.12.1                 
qt-main                           5.15.8 
qtconsole                         5.4.3
qtpy                              2.3.1
xxxxxxxxxxxx                      xxxxxx