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Field order confusion in VirtualDub

Why are the "even field first" and "odd field first" labels in VirtualDub reversed?

Well, actually, they're not, depending on how you label scan lines.

The even field in VirtualDub corresponds to the field that has the upper set of scan lines, and the odd field is the one with the lower set. The reason is that internally VirtualDub numbers scan lines so that the top scan line is scan line 0. That means the even field consists of 0, 2, 4... and the odd field is 1, 3, 5..., meaning that the even field is positioned higher than the odd field. If you consider scan lines to be numbered starting from 1, then this would be backwards and thus confusing. When this issue was raised, I did some searching around and didn't see a clear consensus on scan line numbering, so the plan is to abandon even/odd terminology in UI and just use top/bottom instead across the board.

If you're confused about the field order of a clip, the best way to check it is to use bob deinterlacing to double it to field rate and then check if you get motion juddering. The mode that gives you the smooth output is the correct one. You can do this in VirtualDub via the "bob doubler" filter in 1.8.0+, or the "deinterlace" filter in 1.9.2+. Unfortunately, there are a few places where I've goofed the field order settings at times; the bob doubler had this backwards until 1.8.2, and I've just been informed that it's currently backwards in the new IVTC filter in 1.9.x. I'm working on making sure everything's correct for 1.9.3.


This blog was originally open for comments when this entry was first posted, but was later closed and then removed due to spam and after a migration away from the original blog software. Unfortunately, it would have been a lot of work to reformat the comments to republish them. The author thanks everyone who posted comments and added to the discussion.