The same way you do with any core, open the OSD and go right to Video Processing to choose your pre-set filter.
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lightwo
Ah okay, the difference here is that ScummVM (which I was comparing it to) runs independently from MiSTer, whereas Sonic Mania is a MiSTer core, even if all it does on FPGA is video processing. Neat!
Edit: Ah! 3SX port ended up being even bigger than it initially seemed, as the FPGA logic used here was inherited from there, even if it was a vibed port.
Edit: Ah! 3SX port ended up being even bigger than it initially seemed, as the FPGA logic used here was inherited from there, even if it was a vibed port.
I'm very intrigued, I'd love to try this out!