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AssumeFrameBased(clip clip)
AssumeFieldBased(clip clip)
AviSynth keeps track of whether a given clip is field-based or frame-based. If the clip is field-based is also keeps track of the parity of each field (that is, whether it's the top or the bottom field of a frame). If the clip is frame-based it keeps track of the dominant field in each frame (that is, which field in the frame comes first when they're separated).
However, this information isn't necessarily correct, because field information usually isn't stored in video files and AviSynth's source filters just guess at it. AssumeFrameBased and AssumeFieldBased let you tell AviSynth the correct type of a clip.
AssumeFrameBased throws away the existing information and assumes that the clip is frame-based, with the bottom (even) field dominant in each frame. (This happens to be what the source filters guess.) If you want the top field dominant, use ComplementParity afterwards.
AssumeFieldBased throws away the existing information and assumes that the clip is field-based, with the even-numbered fields being bottom fields and the odd-numbered fields being top fields. If you want it the other way around, use ComplementParity afterwards.
