Pulldown

From Avisynth

Pulldown(clip, int a, int b)

[edit] a:b Pulldown

In general, a:b pulldown (also called telecine) means that the first frame is represented by "a" fields of video, the second frame is represented by "b" fields of video, which is repeated till the end of the clip. inverse telecine: means undoing the telecine.

Example:

2:3 or 3:2 pulldown: 23.976 -> 29.97 by adding duplicate fields in the following way: Thus the first frame is represented by two fields of video, the second frame by three fields of video, which is repeated to the end of the clip:

source: AtAb BtBb CtCb DtDb (four frames) 2:3 pulldown: AtAb BtBb BtCb CtDb DtDb (five frames) 3:2 pulldown: AtAb AtBb BtCb CtCb DtDb (five frames) etc ...

[edit] Pulldown as AviSynth filter

Pulldown(a,b) (with a < b) selects frames a, b, (a+b)+a, (a+b)+b, 2*(a+b)+a, 2*(a+b)+b, ...

Example:

DoubleWeave()
Pulldown(0,3)

is the same as undoing a 2:3 (or 3:2) pulldown (ie 29.97 -> 23.976).

Suppose you have material on which 2:3 pulldown is applied, that is

AtAb BtBb CtCb DtDb => AtAb BtBb BtCb CtDb DtDb (five frames)

After DoubleWeave:

AtAb BtAb BtBb BtBb BtCb CtCb CtDb DtDb DtDb

Pulldown(0,3) selects frames 0, 3, 5+0, 5+3, 10+0, 10+3, ...

AtAb BtAb BtBb BtBb BtCb CtCb CtDb DtDb DtDb
 *              *         *              *

thus getting

AtAb BtBb CtCb DtDb

The reason you need to use DoubleWeave first is that many capture cards sometimes recombine fields in the wrong way. In terms of fields, the 3:2 pulldown sequence is simply "At Ab Bt Bb Bt Cb Ct Db Dt Db ...", where "A" through "D" represent the original film frames (before the broadcaster telecined it). But many capture cards the fields into frames with no respect for the pulldown pattern, and you get this (fields swapped):

AbAt BbBt CbBt DbCt DbDt

After DoubleWeave:

AbAt AbBt BbBt BbBt CbBt CbCt DbCt DbDt DbDt

Pulldown(0,3) selects frames 0, 3, 5+0, 5+3, 10+0, 10+3, ...

AbAt AbBt BbBt BbBt CbBt CbCt DbCt DbDt DbDt
 *              *         *              *

thus getting

AbAt BbBt CbCt DbDt

For AVI streams it doesn't matter of course (which is always BFF for AviSynth), so the field-swap won't be corrected. I guess it holds for MPEG-2 caps (where the BFF/TFF info is written in the header). I will check that soon ...

This is all very complicated to describe, but in practice undoing the pulldown is just a matter of inserting some boilerplate code. See the example below under ShowFiveVersions.

Pulldown(a,b) is implemented internally as SelectEvery(5,a,b).AssumeFrameBased.