I guess I’m not doing it right. I opened up a mode I see an offset in, I went to horizontal geometry, changed the H CENTER from 12/15 to 10/15, 7/15 etc and test, and it looks the same no matter what I choose. Am I doing this right? I did see a change in another mode however when doing this same process on it. But it didn’t seem to affect my game (it’s a dosbox game actually, not mame)
You're not allowed to be using the mode you want to adjust. Set your desktop to a different mode, then go into the mode you want in ArcadeOSD and play with it. Sometimes it seems to get stuck, so try restarting if you have problems. You also can't modify the EDID-emulated 480i mode, or anyway I can't. I'm not sure about other modes set in EDID, so maybe Calamity can weigh in.
Nice, I want to center some resolutions too. Because if I have TMNT centered Street Fighter II is displayed to the right.
So, how can I set this range with selected resolutions? Because last time I test it, image was displayed to the left in all games, so Street Fighter was 100% center es but TMNT don't.
You can use ini files to do this. In the ini folder under your main GroovyMAME folder, make a txt file and rename it tmnt.ini, for example. Copy any lines you want from mame.ini into it. In this case all you want is the crt_range0 line. Then you adjust the horizontal timings in the tmnt.ini file only, to adjust the horizontal position for that game alone.
You can use something like neogeo.ini to affect all neo-geo games, and then i think add mslug3.ini on top. So mame.ini gives way to values in neogeo.ini which in turn give way to values in mslug3.ini. Keeping everything in the mame.ini and only using the bits you want to change per machine or per game keeps the ini files simple, and means you don't have to wade through mountains of configs to fault find - trust me, i've made that mistake.