Vegas has a strange way of reversing the field dominance

erratic wrote on 6/14/2004, 12:25 PM
I used Vegas to change the field order of a clip from upper to lower field first. It seems to work, but I'm baffled by the way Vegas does it. Click here to see the result of my experiment. I can do a better job with Avisynth. I don't understand what Vegas does exactly to reverse the field dominance, so if someone can explain it to me, please do.

Comments

Spot|DSE wrote on 6/14/2004, 12:44 PM
I can't comment on how Vegas processes the shift in terms of coding, (because I don't know the specific answer. Dither, you out there?) but why are you shifting dominance? All DV, most MPEG, and virtually all forms of uncompressed media are lower field first. If you are converting 486 down to 480, you'll need to either shift the dominance, but you can also crop it to the same effect.
Occasionally we'll need to crop stock media, or remove 4 upper pix and 2 lower pix to keep interlacing intact and correct field dominance, but it's pretty rare that we do this any longer, because the pan/crop tool nearly always nails this correctly.
erratic wrote on 6/14/2004, 1:04 PM
It was just an experiment. I don't really need to change the field order, so it's no problem. (I think Vegas is a truly excellent NLE, by the way.)

But I'm confused about the way Vegas shifts dominance if your source file is tff and you set the project properties to bff. I didn't expect to suddenly end up with two white lines (head switching noise) when there's only one line in the original video.

Anyway, never mind, and thank you for responding.
Spot|DSE wrote on 6/14/2004, 1:06 PM
You shouldn't be seeing two white lines. I just tried this with PAL and NTSC both, because you got my curiosity up. It's looking correct to me. If I can find it, I have an interlace chart here that lines everything with different colors every 8 pixels, so you can identify exactly where/what is happening. I'm searching now. Experimenting is what makes us all smarter, or at least more experienced. I just wondered why you were doing it, if you had a technical reason. I wanna know if it's weird too.
erratic wrote on 6/14/2004, 1:37 PM
Well, my source is PAL MJPEG. Vegas knows my source file is tff, because I've obviously configured it that way by right-clicking on the source file in the media pool and setting the field order to upper field first. In my project properties the deinterlace method is set to 'blend fields'. Vegas effectively changes the field order to bff when I render to PAL DV, but I get two white lines of head switching noise and there appears to be some blurring.

If I change the deinterlace method to 'None' I no longer get two white lines, but that's because in that case Vegas doesn't change the field order anymore, so my newly rendered DV AVI file is still tff.
Spot|DSE wrote on 6/14/2004, 1:44 PM
What's going on in the Media Properties? Just curious to see if although the project sees it that way, does the properties window?
erratic wrote on 6/14/2004, 1:54 PM
Media properties:

Attributes: 704x576x24
Format: PICVideo MJPEG Codec
Frame rate: PAL
Field Order: Upper field first
Pixel Aspect Ratio: PAL DV
Alpha channel: None

Project Properties: PAL DV with deinterlace method set to blend.
Vegas correctly adds two black bars left and right to change the resolution to 720x576.
Spot|DSE wrote on 6/14/2004, 2:01 PM
I'm stumped. I only have DV to test this with, and it's fine...No PAL MJPEG here, only NTSC MJPEG and that's fine too. Sorry I can't be more help, even if it's just an exercise in chasing knowledge.
GlenL wrote on 6/14/2004, 3:15 PM
Field dominance is used in two places in Vegas. You're looking at the "Project Properties/Deinterlace method" which only affects transitions with interlaced source (use the "?" to bring up the help for that field). The field dominance you want to look at is "Field Order" as part of the "Render As" option. I did a couple of experiments before burning a big project and there was a difference when I switch between TFF and BFF.
erratic wrote on 6/14/2004, 3:56 PM
My source file and media properties are TFF.

My project properties are PAL DV (BFF) and I render as PAL DV (BFF). I want Vegas to change the field order and it does so, but it seems to use a rather strange method.

Obviously if I keep the TFF field order from start to finish all goes well, no problems whatsoever. But my experiment was to see how Vegas changes the field order, and I get better results by shifting the video 1 line up or down, or by using Avisynth.

With Avisynth I can change the field order without a line shift. Some users claim that's better, but I have no idea if it really matters.
In Doom9's CCE FAQ I read this (for BFF to TFF):

If for some reason you want to encode bff video without that line shift, there are two ways to do this:

Frameserve bff video into CCE via AVISynth and convert it to tff "the right way". Here is a basic AVISynth script that does this:

AVISource("E:\Video\holiday.avi")
DoubleWeave().SelectOdd()
erratic wrote on 6/15/2004, 9:42 AM
By the way, I just noticed that if I use the pan/crop tool to shift the video 1 line up or down, it works fine if deinterlace method is 'none'. If it's blend or interpolate, shifting this way causes the same blurring as changing the field order. Weird stuff... :-)
erratic wrote on 6/17/2004, 5:15 AM
I mentioned this problem in Doom9's forum and an Avisynth expert figured out how Vegas reverses the field dominance. This Avisynth script causes just about the same unnecessary blurring and the double head switching noise line:

avisource("tff.avi")
assumetff()
bob()
separatefields()
selectevery(4,0,3)
weave()
wdormann wrote on 6/17/2004, 6:40 AM
I know you've already seen it Erratic, but for anybody else who might have missed it:

I have a page set up describing the Field Order Anomalies with Vegas 5
This page describes both issues with field order, and an issue with resizing a video in the horizontal direction causing the original interlacing to be destroyed.

Vegas 4 appears to behave as expected.
joelaff wrote on 6/17/2004, 7:44 AM
Did you check the pixel aspect ratio in the media and exvent settings to be sure it matches that of the project?

Strange results, no doubt.
erratic wrote on 6/17/2004, 8:14 AM
The resizing problem Will Dormann mentions is definitely an aspect ratio 'thing'. As we discussed elsewhere it can be avoided by using the pan/crop tool with the 4:3 Standard TV aspect ratio preset. This will crop from 720x480 to 704x480 and then Vegas resizes to 352x480 just fine. That is the correct way to resize 720 to 352 anyway. Resizing 720 to 352 without cropping changes the aspect ratio and apparently Vegas 5 tries to maintain the correct aspect ratio by also resizing vertically. I guess Vegas 4 didn't do that.