I understand about the (no) scanlines effect very well. Agreed it is just a monitor display solution to get around different resolutions by missing out lines. I thought we all knew this so didn't elaborate. You need to look at the actual game .jpg files I have supplied. I have many more shots but a pain to stick them on this forum (only one at a time per post). My goal was to compare actual arcade shots against MAME masks and to replicate the effect as closely as possible via experimentation.
I understand that different monitors have different ways of displaying things so different masks will have different effects. I don't have access to loads of monitors, sadly, so can only tell you the effects using mine. Essentially, I have rotated the slot_mask by 90 degrees and removed the darker intersection lines. Comparing my actual arcade shots against a MAME setup (on CRT or panel) IMHO it looks better (not perfect, better) and certainly way better than using no mask at all. That just looks plain wrong.
People can choose to use these or not - it's not a problem to me. However, I'm much happier with my set up that vertical3 more closely resembles an actual arcade monitor running the original h/w on both CRT's and panels under MAME. So give it a try and use it or not dependent on the effect you desire.
God, I miss those dark, musty arcades ....