Avisynth: necessary to use selectEven() and selectOdd() for interlacing?
 

Hi all,
Interlacing is if I understand it correctly a 50/60 fps clip running at 25/29.97 fps where the frames are coded as:

1 field frame 1
1 field frame 2
2 field frame 1
2 field frame 2
etc.

All scripts treating interlaced material that I have seen does it like:

...
SeparateFields()
even=SelectEven().Filter1().Filter2()
odd=SelectOdd()..Filter1().Filter2()
Interleave(even,odd)
Weave()

My question is now why is it necessary to use selectEven() and selectOdd() and to filter the clips separately. Doesn't SeparateFields() create a clip where the fields are in the right order but the frame rate has been increased to the double. I.e.

1 field frame 1
2 field frame 1
...
1 field frame 2
2 field frame 2
...

If this would be the case one could simply do

...
SeparateFields()
Filter1()
Filter2()
Weave()

Why can't I do it like this instead?

"real" Interlacing means 50fields per second at half height, weaved to fullheight and therefore 25fps (in PAL, 60/29,97 NTSC)

We do use that select...() technique in case of temporal filter usage! Cause if a temporal filter just works on seperated fields, he would compare just to the next odd/next field which would be the same component in the same weaved frame later! ... and so wo do the select..() technique by performing that jump to every second field.