digitalFAQ.com Forums [Archives] - Avisynth: Interlace artifacts or phase-shifting?

I've been reeding the excellent analogue capture guide using Avisynth posted by Jorel on the KVCD web site and I am particularly interested in cleaning up PAL fields prior to processing: In particular I was curious about the following:

Quote:

(to deal with)
PAL:
1) phase shifts and or fields in a reversed order (= swapped fields) [for more information, see readme of the Decomb plugin]
2) trully interlaced
3) bad NTSC to PAL conversions

To decide which case you are dealing adjust your avs file, such that the fields can be seen separately:

AviSource("d:\capture.avi") # assuming source is YUY2
Trim(begin-1, end-1)
SeparateFields()

and open it in VirtualdubMod. Right click on the left (source) clip and select "2x size" to enlarge the clip, and move the slider frame by frame (in a small range of frames, say 20). If you hardly see any horizontal lines in both fields, it means we are dealing with case 1. If you still see horizontal lines we are dealing with case 2. If you see blended frames (also called ghosting) it means you are dealing with case 3.

I might be missing something but if in case 2 if you separate fields (which become frames) and look at them in individually in Virtualdub, why would you see any artifacts since you are not viewing fields 1 and 2 jointly?
Cheers...ozjeff99