Is it possible to include a filter by writing it into a .def file
No. Not possible. It was a suggestion.
What I suggest is that, at the player variables section (which can go within the .def if you want), besides of [Size] xscale and yscale, you could have a new parameter for that section like this:
xscale = 2
yscale = 2
I know those scaling parameters are rarely used for chars, but most of my chars happen to use it. and MUGEN 1.0 uses nearest-neighbor scaling filter (which is basically not filtering a thing, just upscaling it) to do it. now if we could choose other filters to be used (even if xscale and yscale are 1) it would be great for low res chars.
No. Filters like that one have nothing to do with Mugen, they're done on each sprite individually outside Mugen.
Like which? You mean like HQnx? http://en.wikipedia.org/wiki/Image_scaling
The VBA, SNES and other emulators can use most of those filters on the fly. In fact old WinMUGEN could use some filters like bilinear on the fly, but applied to the whole screen. MUGEN 1.0 has lost it all! While some people in this thread say it was ugly, others (like the thread's creator and myself) still miss the capability.
filtering might be back in mugen 1.1 once it's finished and goes public (24/32 bit sprites are already being filtered in the test version).
There's currently no way to disable bilinear filtering on RGBA sprites. We might add an option in the future.
The filter is applied to all RGB or RGBA sprites. We may add options later to suit the users' preferences.
Now this is great news for those who love good filtering...
Now that Elecbyte has devised a per object filter that filters selectively, I hope they add support for HQnX filtering for low res chars.
That will do it! This makes me want MUGEN 1.1 even more now!
Now, Serio has confirmed Elecbyte has achieved a "per object" filter for MUGEN 1.1, which if I understand correctly means it will be possible for chars to be filtered individually and independently of what filter is applied to lifebars, stages, etc. Sadly that means we are still stuck waiting for Elecbyte to release all the good stuff 1.1. will bring.