Myself086 wrote: ↑Mon Jul 11, 2022 1:49 amIt's even used in Puzzle Bobble, I think it's to change to a mode with offset-per-tile and one layer had to be moved to a different BG. I haven't fully looked into it.
The game basically has two layers. The background is 2bpp. The foreground is divided into regions. The fixed part at the top/bottom is mode 1 with 4bpp, and the playing area is mode 4 with 8bpp and OPT.
So, because the 2bpp background is: BG3 of mode 1 = BG2 of mode 4, they have to swap out the address when they switch modes. (Also the OPT data has to go at the BG3 address in mode 4.)
The reason they use mode 4 is to have 8bpp for the bubbles on the foreground. Each bubble is only 8 colours, but because they stick together in a hexagonal packing, 16-colour palettes can't cover all the possibilities where you get a two-bubble combination tile.
OPT lets them shift the bubbles down in the middle while keeping the frame at the sides steady.
There's other ways they could have done this, but switching from mode 1 to mode 4 necessitates the BG address switch.
(Interestingly, they probably could have used a colour math window for the "transparent" looking panel behind the playfield, but that's actually just a darkened palette on the background layer. This also means that levels with a scrolling background don't have this transparency effect.)