elianda wrote:The framerate reduction YT does is another problem. What tool would you use to reduce a video featuring a 70 fps (usual DOS mode) video with fluid animation to 30 fps?
Any tool will do. Virtualdub, AviSynth are good ones. The x264 binary can do it directly. It might (not sure about this) be better to encode only every third frame, reducing the framerate to 23.33 fps.
elianda wrote:I think if you use identical algorithms for up- and downscaling the result is more or less reversible.
No, they're not reversible. Only a point upscale (nearest neighbor) is lossless. Every time you scale using bilinear, bicubic or some other lossy algorithm, you lose some information. Take any screenshot of a game, scale it up and then back down, and you'll see what I mean. Using integer scaling factors reduces the effect but doesn't avoid it.
It might not matter all that much, though. It really depends on the circumstances and the quality of your input material. But personally I prefer not to upscale content except with point upscale (because it allows you to see nice and crisp pixel art in the HD encodes).
edit: (I'd only recommend point upscale when your content is lossless to begin with, though.)