Resizing HD to SD 720 or 1080

[Chris Harding]

Hi All

I have often seen comments that 1280x720 footage on the timeline resizes better to SD than 1920x1080

Does any bright spark out there know exactly why??? Is it applicable to both NTSC and PAL SD too???

Both HD formats (assuming it's progressive) have identical aspect ratios so is there any real reason why Vegas might do a better job downsizing 720 ???? or has this got something to do with only interlaced footage where the file is now 1440x1080 which has a closer aspect to SD interlaced as one is 1.333 and the other is 1.365???

Some mystery solving would be appreciated

Chris

[Jay West]

What I remember from a couple of years ago is that some folks think full rate progressive --- 50p in PAL and 60p in NTSC --- downconverts better to SD because you are working with full frames to make each corresponding interlaced field. They mostly discussed 1280x720 (aka 720p) because we have only recently started seeing a lot of quality cameras that shoot full rate 1920x1080 progressive (1080/50p and 1080/60p). The idea makes intuitive sense since a full-rate progressive frame gives you a lot more image to work with than coming from an interlaced one. However, not everybody sees a significant difference in their work on their computers and projects. I played around with with comparisons with some footage from dance recitals, comparing some 720p from an NX5 with 1080i (some from an NX5 and some from the essentially similar HDV sibling, the FX1000). It seemed to me that the 720p footage was slightly cleaner when viewing the very-high-motion-very-high contrast footage from the cameras on a calibrated 30-inch HDTV, but I personally could not see any significant differences when everything was transcoded to either SD (for timeline playback) or DVD (max render quality, etc.) Mileage seems to vary, as they say.

[David Jimerson]

I don't find that to be the case any more in V10. It was certainly true in previous versions.

The problem doesn't have anything to do with the aspect ratio or interlacing; it was most likely an issue with the scaling algorithm.

[Eric Olson]

Repeating what was said above, many NLEs do a lousy job rescaling interlaced footage. A reasonable algorithm to rescale 1080i60 to 480i60 is to first use a frame-rate doubling deinterlacer to obtain 1080p60, rescale this to 480p60 with a high quality anti-aliased scaling algorithm and then weave the resulting frames back to interlaced 480i60. I don't know why the engineers who designed the NLEs chose lower quality algorithms for rescaling interlaced video. However, an amusing result of this decision is that blu-ray disks look even better than they would otherwise when compared to DVD.

[David Jimerson]

But the scaling problem wasn't limited to interlaced footage; it was also apparent in progressive footage.

[David Rice]

Pro 10 is doing a much better job at resizing than Pro 8 did.

[David Jimerson]

Yes. No longer a problem.

[Chris Harding]

Thanks Guys

Much appreciated...that's what I figured too but I keep seeing posts around saying that 720 is a better "fit"

Yeah, V10 certainly does a good enough job with 1080 down to SD (We are PAL of course) and I have no complaints!

The input is much appreciated

Chris

[Peter Manojlovic]

Originally Posted by Eric Olson

Repeating what was said above, many NLEs do a lousy job rescaling interlaced footage. A reasonable algorithm to rescale 1080i60 to 480i60 is to first use a frame-rate doubling deinterlacer to obtain 1080p60, rescale this to 480p60 with a high quality anti-aliased scaling algorithm and then weave the resulting frames back to interlaced 480i60..

Bingo!!!
We have a winner....
This needs to be a sticky for HD>SD downrezzing..I've got a homemade diagram explaining exactly what you've stated....
A picture speaks a thousand words..

[Chris Harding]

Hi Peter and Eric

That sounds like perfect sense...I assume it would also work well for PAL where we are resizing to 576/50i???
Based on your assumption then guys, would it be an advantage to NOT shoot interlaced 1080 but in fact shoot (in my case) 1080 50P which should mean that the NLE already has de-interlaced double frame rate footage to resize???

If I look at project properties with 720 50P Vegas does show it at double frame rate. Sadly my cams don't shoot 1080 50P only 25P and according to the specs the 25P is over a 50i wrapper. If the NLE doesn't have the first step to worry about then this is probably why people are saying that 720 does a better resize as plenty of cameras can shoot 720 50 or 60P without any issues.

Chris

[Peter Manojlovic]

Well firstly, to be quite honest, I'm not a Vegas user, but have stumbled here because of the thread title.
It was something i spent days on end figuring out, and hoping to save other people the same headaches got.

The thing with 720 footage, is that it can be scaled down correctly..
It starts of as progressive, gets resized as progressive, and nothing gets hurt. Think of it as bicubic resizing your 2000x2000 photos...

With interlaced footage, since they are on two different fields for every frame, if you resize this, you will ruin the picture. You can't resize every 4th line...

Therefore split the fields, doubleframe, resize do SD (via interpolation), and reweave..
This is at the essence of proper resizing interlaced frames..

I do not no of any program, except for AVISynth (or within VirtualDub), that can do this.

As far as using 25FPS footage inside a 50i wrapper, the important thing is to have your NLE interperate the footage correctly.
But rule of thumb is, if it's progressive, don't deinterlace..As far as DVD and BluRays go, never deinterlace unless absolutely necessary. These are mediums which were intended for interlaced output..

Update:
Hey Chris...
Just read your post a bit closer.
Advantage of shooting 1080P (wrapper or not), your NLE should be able to rescale better. But i don't understand the whole deinterlace, doubleframe withing Vegas. I'm assuming that you think that Vegas is doing some extra work in the background?
Another advantage is for web export (the obvious)..
Unless shooting sporting events, or broadcast, or fast action cars, stick with progressive. Especially if you want to avoid headaches at the rescaling end..

Good luck..

[Adam Stanislav]

Originally Posted by Peter Manojlovic

But rule of thumb is, if it's progressive, don't deinterlace..

Uh, sorry? If it’s progressive, you can’t de-interlace.

[Jeremy Dallek]

Originally Posted by Adam Stanislav

Uh, sorry? If it’s progressive, you can’t de-interlace.

I believe if you have for example a 1080p30 clip (which is in a 60i wrapper) and then run a deinterlace on it using interpolate, then you will loose half your lines of resolution. De-interlacing progressive footage will make it worse.

[Adam Stanislav]

If you have 30p wrapped in 60i, then it still is progressive, so what is there to de-interlace? De-interlacing only makes sense with true interlaced footage.

[Eric Olson]

Vegas will automatically deinterlace 60i footage in a 30p project. This results in quality degradation of 30p progressive video wrapped as 60i unless you either set deinterlacing mode to none or reset the properties of the 60i wrapped clip to progressive. This is most important when editing for blu-ray.

Note that when scaling from 1920x1080 to 720x480 it is important to low-pass filter the image to avoid aliasing. The sharper the original image the more important the filtering. In particular, a blurry HD image may look better than a sharp HD image after both have been scaled to standard definition using some editing systems. An amusing comparison of anti-aliasing filters is given at

http://bvdwolf.home.xs4all.nl/main/f...own_sample.htm

In seems possible that the image blur introduced by accidentally applying blend fields deinterlacing to 30p progressive video wrapped as 60i could actually improve the quality of subsequent downscaling by the editing system.