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SelectEvery(clip clip, int step-size, int offset1 [, int offset2, ...])
SelectEvery is a generalization of filters like SelectEven and Pulldown. The easiest way to describe it is by example:
SelectEvery(clip, 2, 0) # identical to SelectEven(clip)
SelectEvery(clip, 2, 1) # identical to SelectOdd(clip)
SelectEvery(clip, 10, 3, 6, 7) # select frames 3, 6, 7, 13, 16, 17, 23, 26, 27, ... from source clip
SelectEvery(clip, 9, 0) # select frames 0, 9, 18, 27, ... from source clip
And how about this:
# Take a 24fps progressive input clip and apply 3:2 pulldown,
# yielding a 30fps interlaced output clip
AssumeFrameBased
SeparateFields
SelectEvery(8, 0,1, 2,3,2, 5,4, 7,6,7)
Weave
