Interlace question in VV4

farss wrote on 5/21/2003, 7:51 AM
I've just tried using track motion to move a white mask accross a black background and find I'm getting interlace artifacts on the leading and trailing edges. Now this make perfect sense according to the view I used to have of how interlacing works however only a few days ago several pros told me I was wrong.

According to their explanation each field is the alternate lines of the same image, there's no temporal shift between them and to achieve this most video cameras still us mechanical shutters.

Now if they are right (and I'm starting to doubt that) then VV4 is rendering the fields incorrectly. By that I mean it should render both fields at the same point in time, not calculate a new track position for each field.

Anyone able to shed some definative light on this?

Comments

mikkie wrote on 5/21/2003, 1:17 PM
IMO, Interlace is as you've been told, technically the same image without shifting. However, to make things smoother appearing on a TV (which relies on fast scanning to compensate for poor visual quality), these fields are normally shifted a bit. Also, as most of the time interlacing goes hand in hand with the telicine to 29.97, it's expected.
John_Cline wrote on 5/21/2003, 2:25 PM
Yes, there is most definitely a temporal shift between fields and Vegas is indeed handling it correctly. NTSC video is "sampled" 59.94 times per second. The first field of a frame is sampled at a moment in time and the second field is sampled 1/60th of a second later, so there are actually 59.94 discrete images in a seconds worth of video. You get twice the temporal resolution at the expense of only getting half the spatial resolution.

Virtually NO video cameras use mechanical shutters. It appears that ALL information you got from the "pros" was 100% wrong.

John
farss wrote on 5/21/2003, 6:09 PM
The confusion over shutters may have come from digital still cameras, they also use interlaced CCDs mainly to do with the time to scan the CCD I think. However they also use mechanical shutters so they don't get the temporal shift.

Thanks anyway guys for clearing that up. I live in PAL land so pulldown isn't an issue here.

I was pretty certain they had it wrong simply because I've looked at video field by field from a number of sources and you can definately see the temporal shift on moving objects.