Flip

From Avisynth

FlipHorizontal(clip clip)
FlipVertical(clip clip)

FlipVertical flips the video upside-down. It is useful for dealing with some video codecs which output everything upside down. FlipVertical doesn't modify the parity flags for interlaced clips.

Likewise FlipHorizontal (which is present in AviSynth v2.5) flips the video from left to right.