Does anyone know if this works on hardware and is used by any game? I'm just curious.
Mirroring is done at the cartridge level, it switches where the address lines go to the PPU's RAM. So it can be changed at any time. Battletoads switches from one nametable to another after it draws the status bar, and so do a lot of other games which use single-screen mirroring.
Crystalis simulates single screen mirroring by using timed vertical scroll changes so that the top screen acts like it wraps.
As for changed from horizontal to vertical mirroring mid screen, I don't know about that. It wouldn't be very practical.
Thanks, that's interesting.
Yes it's possible to change mirroring and/or switch nametables mid-screen.
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As for changed from horizontal to vertical mirroring mid screen, I don't know about that. It wouldn't be very practical.
Rad Racer does that.
Yep, Rad Racer totally does. Use horizonal mirroring for the narrow but tall track, then switch to Vertical mirroring when you need to display the wide section.