Fix Qt platform plugin "xcb" error by setting the plugin path and importing cv2

[dvida]

Description of the fix supplied by Ed Harman:

Ever since we started building RMS using the opencv-python package I have been aware of a conflict due to the fact that -

Traditionally when we build opencv from source we compile with Gstreamer support so modules like ShowLiveStream use that to render their output

If we omit that lengthy opencv build process and use instead the pre-compiled version that pip supports we get -

opencv with Qt5 as it's rendering module and no Gstreamer support.

The impact of this is varied depending upon which distro and version you use and how you install it, however what it is masking is this -

opencv-python installs a QT5 python-plugin version under cv2 called libqxcb.so

SkyFit2 utilises QT5 as its rendering framework, which is installed by pip - PyQt5 and it loads its own version of libqxcb.so under the QT5 plugin path.

The issue is that SkyFit2 imports opencv implicitly via other RMS modules and so when it comes to load QT it finds it has already got the plugin it requires (but which is incompatible) and promptly crashes -

(vRMS) eharman@rms-ubuntu-lts:~/source/RMS$ python -m Utils.SkyFit2 ~/RMS_data/UK001B/ArchivedFiles/UK001B_20240224_180023_923809 running build_ext skipping 'RMS.Astrometry.CyFunctions' extension (up-to-date) running build_ext skipping 'RMS.Routines.MorphCy' extension (up-to-date) QObject::moveToThread: Current thread (0x5642ad6b9b70) is not the object's thread (0x5642ae405bf0). Cannot move to target thread (0x5642ad6b9b70)

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/eharman/vRMS/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.

Aborted (core dumped) (vRMS) eharman@rms-ubuntu-lts:~/source/RMS$

The web is littered with much noise and misinformation about this and the QT folk are particularly unhelpful and merely state 'you are only supposed to have one version of a plugin library installed..'

There is however a rather simple workaround - If in SkyFit2 we initially pre-load opencv and then reset the QT plugin path then it all works ok, and the beauty is that say for RPi3 users using stock RMS images, they are unaffected by this change because they are likely using a custom compiled version of opencv and so don't have multiple QT libraries installed, i.e. it's transparent to them.

I've tested this on my RPi4 with Bookworm, Ubuntu 20.04/22.04 and stock Buster RMS image 64bit only and it works fine, grateful if others can test that this is backward compatible on all supported distros - I don't have a RPi3 to play with.

I appreciate that some folk only use SkyFit2 from their Windows systems

• I cannot test this since last time I attempted to install Anaconda it could not resolve some dependencies however am confident it should work. Also in my experience even on an RPi4 SkyFit2 is perfectly useable, pretty fast etc even when running headless over the network...