[LocalH]

> You're not "throwing out half the motion".

Yes, you are. The odd and even lines from proper interlaced footage belong to two separate moments in time, and so when you deinterlace from 60i to 30p you are unavoidably losing half of those moments.

> Given that PC monitors these days don't have an interlaced mode, how would you display it?

You deinterlace it to 60fps. There exist several algorithms to do so without losing motion fluidity.

> Line doubling, so you're throwing out half the vertical resolution?

Losing half of the vertical resolution of a 60i video is losing half the motion.

[ErroneousBosh]

> You deinterlace it to 60fps. There exist several algorithms to do so without losing motion fluidity.

So what does "deinterlace it to 60fps" do? How does it work?

[LocalH]

The most basic method is to "bob" deinterlace, which is more of a form of "interlace simulation" - you take the original fields, place each one in the appropriate image lines of a separate frame of video, then perform some type of interpolation of each frame for the in-between lines. More advanced algorithms exist like yadif, that do some amount of motion detection to determine which parts of the image require deinterlacing, and which parts can be essentially done with a "weave", which is just taking part of the image data from a combined 30fps frame containing two fields.