Couple of years ago I talk about this on the Snes9x Mushroom post and Extrem posted a link to the code for Scale2xEx.If you find the differences between the original scale2x and this scale2xEX, maybe I can derive a version in C. I have never seen a description of scale2xEX.
"The relevant code is available here:
- https://github.com/extremscorner/emgba/blob/main/source/gx.c#L65-L92 (required setup)
- https://github.com/extremscorner/emgba/blob/main/source/gx_packed.c#L174-L221 + https://github.com/extremscorner/emgba/blob/main/source/gx_packed.c#L61-L106 (first pass)
- https://github.com/extremscorner/emgba/blob/main/source/gx_planar.c#L282-L413 (second pass)