A Mathematical Theory for Why People Hallucinate

I've held this theory on my own after experiencing geometric shapes while hallucinating after smoking a lot of marijuana years back. I sat down closed my eyes and experienced an intense feeling of flying through what I can only explain as the starfield screen saver from Windows but with colored 2D shapes heavily dominated by hexagons of various sizes. During that experience I realized that I might be experiencing the inner workings of my brain in real time. I didn't give it much more thought but that ideas stuck in my mind for years. Recently I began fitness training with a trainer who believes in the stoned ape theory which reminded me of my own experience and I again read into it a bit. I'm not sold on the idea but the overall theory is solid in my book. Still to this day if I get really high I experience geometric hallucinations.

How do we know that reality itself isn't a mass hallucination?

I had a similar experience when I forgot to take my SNRI antidepressant. I woke up in the middle of the night, and when I tried to continue sleeping I had these ultra-detailed geometric patterns dancing before me. The visuals were nice, but then my ears started ringing too which started bothering me, so I opened my eyes and woke up completely, which stopped the visuals and the ringing.

Is this 'hallucination' though? Or just some weird stuff happening in visual processing?

I underwent some very serious periods of sleep deprivation (Army) and our 'hallucinations' were of a totally different kind: having conversations with people that were not there, seeing things that were not there, misidentifying people whom you know really well who are right on front of your face, taking on different personalities etc..