I walk head up but look around and if I see someone I know, I do look at them, usually a glance but keep walking unless it's obvious a chinwag is wanted. (Was in town a month or so ago, 'are you going to ignore me?) I was asked. I stopped and realised I'd walked straight past a lady I speak to once a month in the pharmacy, she said I was miles away. We had a good gossip for about 10 minutes.
Many years ago, I was crossing a road, an unpleasant relative was also crossing, we met in the middle, she took one look at me, turned around and walked quickly away. "Good morning *** " I said. She stopped, turned around, fake smile, and said she didn't see me!
A few weeks later, saw her again at a wake, I was talking to another relative, she came, stood between us with her back at me. Fortunately the other relative had a few more manners and told her she was speaking to me.
Even now she's obnoxious! My dad calls her Veruca after Veruca Salt due to the "I want". It's the attitude of her, she's also about 58 though she tells those who don't know, she's late 40's! (Meaning she'd be younger than me!)
I put it down to manners. Acknowledgement won't cost anything.