Thanks for helping.
Bizarrely - I wrote the simple code you suggested and it worked - but the only thing that I effectively changed was the directory where the jpg was (I copied the code from the main() of the out_of_focus_deblur_filter and simplified it with hardcoded path, imread and imwrite).
In the case where it doesn’t work it is in ~/Pictures. In the case where it does work, it is in the same dir where the built executable is (~/git/opencv/build/bin). The jpg in each dir has the same user:group and permissions (744). The out_of_focus_deblur_filter code also works where the jpg is in the same directory as the executable.
This is in my fresh ubuntu install. In Manjaro this test does not work at all.
Ubuntu is
Linux hostname 5.8.0-33-generic #36-Ubuntu SMP Wed Dec 9 09:14:40 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Manjaro is
Linux hostname 5.9.16-1-MANJARO #1 SMP PREEMPT Mon Dec 21 22:00:46 UTC 2020 x86_64 GNU/Linux
In any case, this just proves that my java code should work and its not a platform or library thing (I can read jpgs with imread in java OK, its the wiener code Ive written thats messing things up).
Happy New Year by the way, I hope I don’t get any responses from you tonight or tomorrow, and I appreciate your attention today!