Technical Discussion: Interlace

Help/How To
1

Everyone makes various assumptions based on prior experience. My particular assumption in this case was, “If the source is interlaced video then the exported result should also be interlaced.” I have been experimenting and I am thinking perhaps I need to update my assumptions. I would like your opinion and experience in replies here.

I have only three sources for my video: DV, HDV and iPhone video. DV and HDV are interlaced source while the iPhone records in full HD progressive.

Unlike just a few years ago, I don’t know anyone who might view my videos who owns a CRT based TV with 4:3 aspect ratio. Therefore, all my new videos ought to be exported in 16:9 aspect. The question has been, “Should I now export progressive?” So, I have been experimenting, exporting using the libx264 codec with High profile option and the .mp4 container. I exported the same file in progressive and then interlaced and, quite frankly, I could not tell the difference on the 47 inch living room HDTV. What makes evaluating the results so complicated is the fact that playing either file with VLC on the PC monitor DOES show artifacts on BOTH files!

Extra credit: As I said previously, I have been exporting my DV (actually analog video imported through A/D converter) as 16:9 aspect, 720x480 but the Properties exposed in Shotcut show what is called Rec. BT.709 ATSC color standard vs the Rec. BT.601 standard for NTSC television. Professional video production would require a standards conversion but, I can’t see any difference for not doing any conversion so I guess it doesn’t really matter. -shrug-

I was curious after watching a YouTube video which the author explained that the export properties for .mp4 video progressive - the Field order, Deinterlacer and Interpolation values don’t matter. The salient part is one minute into the video.


I suppose this should have been a “duh” moment for me but since the options were still there I thought I had to select them - and then I fussed about which ones were best! Since they don’t apply for progressive export should they be removed from the video properties once progressive is selected?

Also, this guy claims that “Constant bitrate” gives him the best quality video. What is your preference?

Well, that ought to provoke some discussion. See you down the page.

-=Ken=-

2

The output scan type has less to do with the source material, and more to do with what you intend to do with the video. If the web is your final destination, the progressive will be required. If broadcast TV is your destination, then interlaced might be required. If you are going to give the video to another company who will distribute it, then they will have a number of video format requirements (including scan type). You never mentioned your intention for your project, so it is difficult to advice you on the scan type.

For myself, I converted/edited a bunch of DV home videos some time ago. I chose to convert them all to progressive because I am confident that I will only ever view them on progressive systems.

One important item that you ignored is the project “Video Mode” (under the “Settings” menu). That includes parameters such as resolution and scan type. What you select there will dictate how the scan type is handled while editing. It is best if you match the export scan mode setting to the video mode you have selected for your project. Otherwise, you may end up with artifacts.

Regarding bitrate, I personally prefer “Constant” because I get more predictable file sizes and I know my file will be more compatible with “device” players like Blue-ray and Roku players. Some devices have trouble with files if they have a highly variable bitrate. If you know you will only ever use your file on a computer, then “Variable” will probably give better quality/bitrate ratio.

3

Thanks, Brian, for the reply. I guess I didn’t make it clear that I am attempting to preserve family memories from video tape, some of it now 20 years old. I have pretty well determined that even preservation on DVD or Blu-ray is now aging tech and so deduced that the MP4 container is likely to be the best archive method. Instead of creating disks with chapters I can just create an mp4 file of each grandchild growing up - on an mp4 file and pass along the collection on a set of archive flash drives.

Being an aging geek, I tend to fret about “archive quality preservation of digital assets.” I am begrudgingly admitting to myself that few others appreciate the endless tweaking that can be done to coax a little more detail, color accuracy and so on from the originals. Shotcut makes the selection process easy (mostly) in that the target display is the living room HDTV. Under Settings/Video I set it to the North America standard HD1080P 29.97 fps. So I will convert the old 4:3 footage to 16:9 and trim the analog head switching lines off the top and bottom of the frame. One way or the other we are going to see the black bars on each side - it’s old video.

I am not sure why there is a Progressive check box under Settings/Player. And, I am still not sure why Interlaced options are still viewable after Progressive scan mode is selected for Export.

I appreciate your advice. My best,

-=Ken=-

4

If you use a video mode setting that matches your interlace source material, you can disable the Progressive player setting to actually see the comb artifacts (due to intentional inter-field motion, of course) of interlace material. This is easiest to see in a fresh app session with Video Mode set to Automatic and not using the Timeline or Playlist (to avoid further confusion for now) - just working on a clip in the Source player.

FYI. When you use Automatic, then the first clip you add to the Playlist or Timeline sets the mode. This is not the first-most clip in the Playlist or Timeline, but literally the first time you add something it gets “locked in” and everything added afterwards is conformed to its mode. (You can see the mode in use by clicking the top, left cornerstone cell of the Timeline and viewing Properties.)

I chose not to hide/show fields to prevent UI elements from shifting around and instead to disable them. The dark theme does a very poor job of showing disabled items. However, if you try to use or change Field Order when Scan Mode is Progressive, you will notice it is disabled. Similar for Deinterlace when Scan Mode is Interlaced. Maybe I should add some “None” entries here upon disabling them to reduce confusion.

Interpolation does affect the algorithms used for scaling and sub-pixel rendering of things like Rotate filter. So, it does matter. But, if you upload interlaced to YouTube they will deinterlace it. When you see interlace artifacts on YouTube, it is because interlace was not signaled properly in the file, or the artifacts got “baked in” during some processing step prior to upload.

5

For your application, I think that 1080p29.97 is a good choice. In DV, pixels are not square. So scaling up to a higher resolution with square pixels will help you preserve the fidelity of the original content with minimal luma/chroma distortion.

The next important thing to think about will be the scaling quality and bitrate. Make sure that you select “Hyper/Lazcos(best)” in your Interpolation setting so that minimal quality is lost when scaling. And then choose a sufficiently high bitrate when you export so that no quality is lost during compression. I would suggest 12Mbps or higher. Also, use 2-pass encoding to eek out even a little bit more encoding quality.

6

I think you have me steered in the right direction now. Thanks again for the advice. I am running 17.08.01 on Linux Mint 17.3 Cinnamon.

[quote=“shotcut, post:4, topic:2449”]
When you use Automatic, then the first clip you add to the Playlist or Timeline sets the mode. This is not the first-most clip in the Playlist or Timeline, but literally the first time you add something it gets “locked in” and everything added afterwards is conformed to its mode. (You can see the mode in use by clicking the top, left cornerstone cell of the Timeline and viewing Properties.) [/quote] Following your steps I see that my DV video is identified properly in Properties. I use Plus in Timeline to Append the video there. If I click on the track cornerstone cell of the Timeline I get Properties for Track:V1. Then (and only then) if I select Settings/Video Mode I see that Automatic is selected. IF I CLICK ON AUTOMATIC (even though it is already selected) the Properties changes to Timeline with lines for Resolution, Aspect ratio, Frame rate, Scan mode and Colorspace. This was kind of “tricky” to find again - but the colorspace was one of the items I mentioned in my original post. Pro editors have a filter for making the conversion between NTSC and ATSC. Perhaps I am pushing my luck asking about what may be a nit picky detail. I am impressed enough that Shotcut identifies the colorspace. Even more, Shotcut is one of only two video editors on Linux that identifies HDV properly as 16:9.

[quote=“shotcut, post:4, topic:2449”]
Interpolation does affect the algorithms used for scaling and sub-pixel rendering of things like Rotate filter. So, it does matter. [/quote] [quote=“brian, post:5, topic:2449”]
Make sure that you select “Hyper/Lazcos(best)” in your Interpolation setting so that minimal quality is lost when scaling. [/quote] Thanks for setting this straight. So many options - for so many differing needs. The casual user often doesn’t know any better. I know the history and the reason for interlacing but actually selecting the right options is not always obvious, at least to me.[quote=“brian, post:5, topic:2449”]
And then choose a sufficiently high bitrate when you export so that no quality is lost during compression. I would suggest 12Mbps or higher. Also, use 2-pass encoding to eek out even a little bit more encoding quality. [/quote] Good to know. More options!

Thank you for your generous contributions here.
Blessings,

-=Ken=-