[jpsdr]

Ok, i think i understand the differences between the 3.
MT_NICE_FILTER : Not possible if you have to use global internal buffer. Personnaly, i'll never malloc/free on each get frame, and don't want to use MT specific features making the filter working only on MT versions. But on something so simple as brigthness/contrast (for exemple), it will work.
MT_MULTI_INSTANCE : You can use global internal buffer, no several getframe from the same instance, but input frames are "random", so works only if, basicaly, the filter needs only the actual frame. I think it's this case for the actual nnedi3.
MT_SERIALIZED : The filter needs the frames to be entered in the correct order, and probably also have buffer. A simple example would be something like an RII filter (like y[n]=0.5*x[n]+0.25*x[n-1]+0.25*y[n-1]). A such filter is not compatible with the others two modes.

Thanks, things are clear now. I think...