Since quite a few years back, it's been quite commonplace in fantasy anime and manga to represent casting magic as some kinds of 2D circles or geometric figures with patterns, possibly multiple ones, as in the image on the right (Frieren). I see this idea used in almost all – if not all – anime and manga now.

It hasn't always been that way though. Older anime and manga used other and different kinds of representations, say from blinking stars (image on the left, Majokko Megu-chan) to smoke, or even just sound effects.

I'm just curious: does anyone have any info or links to share about how this "2D representation" came about and why it's become so popular? Cheers!