Something unusual about your lossless sample, At 720 pixels it should include some black pixels on the sides, if you have cropped them off how come it's still 720x480?

Anyway here is how it looks when it's uploaded to youtube:

No, I've already explained why if you read my entire post, Anything beyond 1080 is just line doubling and does not require complex mathematical approximations.

But when you're dealing with resizing, a 1.5x isn't substantially different than 2x. However, a 2.25x is. The main problem is, again, aliasing, with source being SD analog interlaced video. The act of deinterlace already creates aliasing, and upsize will amplify it. While 2.25x is not awful (being a quarter multiple), it's still less ideal. You're also granted more control over filters by two-stepping it.

This sort of discussion gets far into the weeds of math (Nyquist), where even I don't care to go. I'd much rather watch paint dry, cut the lawn with scissors, etc. It gives me a headache.

For Youtube, I doubly don't care.

If you want to upscale in a single pass, go for it. I'm more detailed when it comes to upscale work that I do for documentary filmmakers. When it comes to mere Youtube, I'll also one-pass it, as again, I don't care that much. It's just Youtube.

The audience I really care about are those who want to order the video(s) on disc, because most of them are likely going to be my former shipmates. Yes, I'll put it out there on YouTube for all and sundry, but I want any orders on disc to look as good as the source material (and my skill!) permits.

You can share that link with your shipmates as a demonstration sample if you want to, more convenient than having them download a file.

I'd rather give them a link to a completed video with titles, graphics, and voice-over narration.

-- merged --

After thinking over the very good advice provided here, I've come to a decision. I'm going to enlarge the video ONE time, either to 720p or 960p, and then include the VHS segments in my 1080p project in a "letterbox" to minimize any additional artifacts being introduced. So which option would you recommend (720 or 960), and how best to make it look good in a single Avisynth script?

Doesn't the Blu-ray spec permit 480i video? Wouldn't it be possible to upscale the video for YouTube and then leave the video as-is for the Blu-ray?

Huh? :question:

You mail or personally deliver discs.
Anything done online ("link") isn't a disc. While you can provide disc images as a download link, odds are they won't know what to do with it, nor have the ability to burn a BD-R.

Uh ... what? :question:
Once to 720p (960p?) ... then :unsure: magically to 1080p?
Letterbox? ... to minimize artifacts? :huh1:

You confusing me, and I think you're confusing yourself. :laugh:
Back up, start over....

You moved the goal posts a lot in these posts.

First figure this out: What is the highest quality distribution going to be. That's what matters, everything else is downconverted later to lower qualities.

Mixed-content SD+HD never assumes you crop into SD material. You're always supposed to pillarbox it. So is that where this odd "960p" comes from?

BD spec provides narrow 480i options. Good, better than DVD, but narrow.

But he mentioned mixed content (documentary style), so not an option.

And SD on a BD is honestly a waste of a BD, unless you need SD 15mbps MPEG-2 interlaced. For an SD archive, great. But for distribution, not what I'd opt for. Only if they needed/wanted the archival 15k SD quality, which is usually collectors of movies/TV, not mom/pop and Joe Sixpack watching a disc of something.

I want to produce this project with disc output in mind. "Snippets" of the project will be posted for streaming...say five minutes on after steering, ten minutes on the engine room, anchor windlass, ship's laundry, etc. But eventual disc output is what I want to plan for.

Well, I'm still planning the project. You've helped. Thanks.

Yes, this will be documentary style with mixed content...video, titles, still images, graphics. The videos will all be produced at 1080. When I talk about "letterboxing", my intent is to enlarge the SD portions of the video to either 960 x 720 or 1280 x 960, and then to "overlay" that video within the 1920 x 1080 window (with borders around the content). Actually, as part of the deinterlacing process I am already cropping and adding borders to remove switching noise, so I'll need to either shrink the overlay size slightly or else pick a background that will blend with my added borders. Clear as mud?

When you put 4:3 file into a 16:9/1080P project, you'll get pillarboxing ie black sides. Letterboxing is when you put a 16:9 file into a 4:3 project: you get black tops and bottom (like a letterbox).

Assuming you have a video editor (you mentioned Resolve earlier), I wouldn't even bother resizing your VHS. I don't use Resolve, but in my editor (and I'm sure Resolve is the same), I would simply create a 1920x1080 project and drop the SD VHS capture files into and resize them to fit vertically into the project frame. They will have black side areas (pillarboxing). Do the same for your images. Of course, if there is no valuable video on the top or bottom of the VHS files, you could crop it all off to create a clip that fully fits the HD window ie removing the black pillarbox sides.