KDE desktop effects no longer work (due to OpenGL not being detected)

Hi,

when I initially installed Kubuntu 14.04 (never reinstalled since then), my gfx card was an AMD one. Everything was working properly, including KDE desktop effects based on OpenGL, like the "gelatin" effect when dragging windows.

Recently, I replaced my AMD card for an NVidia one, to compare the performance of a game with each card. Despite of installing one of the NV92 drivers, I faced issues with my game (Minecraft with the Shaders Mod) and decided to revert back to my AMD card. With difficulties, I managed to get Minecraft work normally again.

I thought that the issues I had faced with MC could come from a mix of AMD and NV files on disk, so I listed all packages linked to NVidia (as now running with my AMD card again), found one and I decided to uninstall it + executed 'apt-get autoremove' as root to remove any unnecessary packages.

Some days after, I noticed that the "gelatin" effect was no longer working. Apparently due to OpenGL not being detected. Strange, since MC works normally with the Shaders Mod, I see no errors when running glxinfo (a package that I had previously installed - today displays "direct rendering: Yes", "49 GLX Visuals", etc.) or when running glxgears.

So I'm a bit confused that KDE no longer detects OpenGL... How may I fix this issue please?

I also don't feel comfortable switching from my AMD card to a brand new NV card, since I'm planning to replace my current card for a more recent model. Well, I won't feel comfortable as long as I'm not sure Kubuntu 14.04 can handle this kind of change correctly.

Thank you in advance for your advice.