Parity

AssumeFieldBased / AssumeFrameBased

AssumeFieldBased(clip clip)
AssumeFrameBased(clip clip)

AviSynth keeps track of whether a given clip is field-based or frame-based. If the clip is field-based it 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 are 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.

AssumeBFF / AssumeTFF

AssumeTFF(clip clip)
AssumeBFF(clip clip)

Introduced in v2.0.4.

AviSynth keeps track of whether a given clip is field-based or frame-based. If the clip is field-based it 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 normally default to assuming bottom field first (with the exception of the MPEG2Source plugin which gets it right!). AssumeTFF and AssumeBFF let you explicity tell AviSynth what you believe the field order type a clip is.

AssumeTFF and AssumeBFF do not change the actual field order, just the internal state flags in AviSynth relating to the source clip used.

Examples:

AviSource("test.avi")
AssumeTFF()
SeparateFields()

will always return the top field of the first frame followed by the bottom field of the first frame and so on.

AviSource("test.avi")
AssumeBFF()
SeparateFields()

will always return the bottom field of the first frame followed by the top field of the first frame and so on.

ComplementParity

ComplementParity(clip clip)

If the input clip is field-based, ComplementParity changes top fields to bottom fields and vice-versa. If the input clip is frame-based, it changes each frame's dominant field (bottom-dominant to top-dominant and vice-versa).

Like AssumeBFF and AssumeTFF, ComplementParity does not change the actual field order, just the internal state flags in AviSynth relating to the source clip used.