[FFmpeg-user] How to deinterlace with simple blend or add

[MI]

I have a video which was orginally shot progressive, then encoded as interlaced. How 
can I most efficiently get back to the original progressive frames?

For real deinterlacing, I usually use  -vf "yadif=0:-1:0". That works very well with 
material shot as interlaced, but requires a lot of processing.

In this particular case, I don't need any sort of smart processing. Fields 1 and 2 of 
my "pseudo-interlaced" movie only need to be added together into a progressive frame. 
But I cannot find the correct filter to do this very simple task. And I obviously 
want to make sure I'm not just dropping every second field, and losing resolution.

Or is there  a summary somewhere on the various deinterlace options, with a 
description of what they do exactly? I couldn't find it.