Vegas deinterlace: my new understanding

Laurence wrote on 9/2/2009, 2:16 PM
Man I've been preaching a version of Vegas deinterlace that is off a little. I did extensive tests a while back and thought I had it down, but that must have been a version or two back of Vegas. In any case, after testing a bunch today, here is what I now realize:

1/ Vegas project properties, deinterlace method, and clip properties (interlaced or deinterlaced) all interact in a way that is extremely smart and well designed.

2/ You should always select a deinterlace method (interpolate or blend fields depending upon your preference) even if you are working with progressive footage and don't think you need it or are working with an interlaced final render and don't want it to be deinterlaced.

What is going on is this. If you select no deinterlace method and resize interlaced footage, what will happen is that the interlace comb will be resized and no longer correspond to even and odd lines (just like I've been preaching). Select a deinterlace method and resize to another interlaced size and Vegas will separate even and odd lines into separate fields, resize the separated fields, then refold them by alternating lines into an interlaced image at the new size.

The mistake I've been making the last couple of years is thinking that Vegas resized progressive footage the same way if a deinterlace method is chosen. It does not. Instead what happens is that Vegas will check the media properties and if these show that a clip is progressive, it will resize the image without field separation just like it should. You can confirm this pretty easily by taking a progressive shot, going into the clip properties and changing it from progressive to interlaced, then resizing. What you'll see is a loss of sharpness that doesn't occur if the clip properties are set to progressive. This is exactly the way it should be.

Anyway, the point is that I now realize that you should always set a deinterlace method regardless of whether you are working on a progressive or an interlaced project. If you are working in progressive, Vegas will ignore your interlace method selection on any clip that has it's properties set to progressive. Interlaced footage inserted on the same timeline will be separated into fields before resizing and deinterlaced if they are rendered to a progressive format.

As I now see it, always select a deinterlace method. Vegas will ignore it if it doesn't need it, and even if your rendering to an interlaced format and don't think you need a deinterlace, Vegas needs it to be selected in order to resize interlaced footage correctly.

Edit: If you want to use a deinterlace plugin like the excellent one Dawdle has done or the Mike Crash smart deinterlacer, you need to add the effect as "Media FX" rather than as "Event FX". This is important because adding the effect here does it at the media resolution whereas if you do it on the timeline you do it at the project resolution.


Hulk wrote on 9/2/2009, 7:31 PM
Thanks for taking the time to post your findings.

Good info.

- Mark
dreamlx wrote on 9/2/2009, 9:31 PM
As I do a lot of processing, I want to eliminate multiple deinterlacing/reinterlacing generations. Therefore I now deinterlace everything to 1080p50 files, do all the editing and afterwards reinterlace. I also render at 1080p50. So if the final customer wants 50p, I give the files like they are. If he wants 25p, I throw every second frame. If he wants interlaced, I reinterlace. You need to have more space and convert source footage to 50p, but you have one workflow for all delivery formats.
wikksmith wrote on 9/3/2009, 9:59 AM
Thanks for the information, but I'm still confused. I'm working with AVCHD clips (1920x1080, 60i) and large jpgs. Playback will be on standard DVD player and 42" LCD TV. I've been advised to make the project setting to match the source files but set to "progressive" to avoid deinterlacing issues. Are you saying I still need to select blend or interpolate? What about when I render?
Thanks, wikksmith
Marco. wrote on 9/3/2009, 10:07 AM
The selection of field order (upper field/lower field/progressive) in the Project Properties is for preview adjustment only.

Deinterlace method determines the way the video data will be processed - which is not equal to the way the video will be rendered at last. In almost any case you should select either Blend or Interpolate as Deinterlace method whereas Blend is the recommended setting.

After you selected the proper Deinterlace method it's up to you if your project will be rendered interlaced or progressive by selecting that kind of property within the render dialog.

So you need to distinguish between field order (for preview) and Deinterlace method (signal processing) in the Project Properties as well as between the field order selected in the render dialog (for rendering).

Laurence wrote on 9/3/2009, 10:19 AM
Are you saying I still need to select blend or interpolate? What about when I render?

What I'm saying is you may as well select a deinterlace method (I use blend fields) in your project properties, because Vegas will only use it when it needs to. For a photo montage like you are describing the deinterlace method will have no effect. If you stuck some interlaced footage on the timeline, it would kick in when it was needed if you have a deinterlace method selected.