Thank you both! Argh... I think I've found why I couldn't see OpenGL shaders in action:
1. I thought OpenGL shaders would add special adjustment options in the TAB panel (like HLSL does) but I was wrong: it's all defined by the shader files. No additional sliders.
2. Since I redefined many games' screen aspect ratio (based on my perception of perfect squares and circles), this caused many .ini files to be already written in ini\ subdir, with OpenGL options = 0 and thus overwriting default game options
In short my mistakes sorry! I've learned something more about MAME advanced options...
Uhm... just deleted dirs ini and cfg to reset everything and again GL Shaders don't work anymore... What's the exact dependency of GL Shaders with cfg/ini etc. files??? Mame.ini, gamename.ini are the files to check-out?
Grande mamesick! È un piacere vederti ancora in giro nei forum MAME. MAMEUI-FX era sempre stato il mio preferito, anche grazie all'incredibile velocità di popolamento/auditing dei giochi (coi miei 600 giochi gli altri ci mettono 5-6minuti a confronto di MAMEUI-FX/ARCADE64 che impiega 30 secondi!)