weird field rendering puzzler
by Andrew Shanks
on
Jun 10, 2005 at 8:06:29 pm
Hi Guys,
a friend of mine had a very curious puzzler of a problem the other day, she showed it to me and I cannot get my head around it, so thought I might post it here for your musings. Basically she bought in some interlaced PAL footage into AE, rendered out frame rendered targa's, worked on those in Commotion (good old superclone), then brought them back into AE for final comping and rendered output. The issue was that when she went to render upper field first out of After Effects, it just rendered footage that to all intents and purposes was still progressive (i.e. non-interlaced), she tried with lower frame too, same deal. We then tried doing it through combustion and same result. It has me scratching my head as I've brought in animated footage (3D) before that has been frame rendered, comped that with 2D video elements, and rendered that out with fields, never having a problem, ...just for some reason her project seemed a bit heywire. Any ideas? Another interesting quirk was I checked the field order of her source footage, ...the facility that provided it said it was upper field, yet After Effects thought it was lower field (i think due to aspect ratio and frame size), ...weird thing is, you could interpret it either way, ...and you wouldn't get the usual stepping forward, stepping back, issue, in either interpretation mode, that you always get if you interpret something wrong. I was wondering if maybe footage that was captured in one field dominance and then convereted to another field dominance when exported (i.e. captured on an upper field avid then exported lower field) might cause this to happen? Its no biggy as she just exported the job in progressive at the end of the day, but I'm just curious for my own interest.
Re: weird field rendering puzzler by Dave LaRonde on Jun 10, 2005 at 8:32:28 pm
It sounds like the footage in question was shot in 25P.
There are still two fields in 25P, but instead of scanning them 1/50 of a second apart, which is what happens in normal interlaced footage, the camera scans both fields at the same time. So you don't get any of that annoying backwards-forwards stuff, no matter which field order you choose.
Re: weird field rendering puzzler by Andrew Shanks on Jun 10, 2005 at 11:43:37 pm
Thanks Dave!
the footage was definitely interlaced, I work on progressive (25P HD) footage most of the time (man, I love working with whole frames rather than interlaced). With the footage in question, each field is distinctly different (in that there is a difference in the frame depending on the interpretation, ...i.e. a ball rolling through screen might be further on with lower than with upper field, ...which you expect with 50 fields per second, ...but i still don't know why the motion wasn't doing the little step forward step back dance under one interpretation, ....very strange.... :-)
