Resizing vs Anamorphic for 4:3 DVD?

[nxmehta]

I am encoding a NTSC Non-ITU Fullscreen (4:3) DVD using x264 in MeGUI. I'm trying to figure out what resolution I should use in order to retain maximum quality.

From what I understand, the DVD storage aspect ratio (SAR) is 720x480. Because it's a NTSC Non-ITU Fullscreen DVD, the display aspect ratio (DAR) should be 4:3. That means the pixel aspect ratio (PAR) is 8:9.

After cropping, my resolution is 712x480 (cropped 4 pixels from both the left and right). I should note that I do not care about having any resolution as mod16. This doesn't matter in modern versions of x264.

It seems like I have 2 options:

1. Leave the resolution at 712x480. Maintain the PAR as 8:9 (encode anamorphically), let the DAR be 712/480 * 8/9 = 1.3185185 (as opposed to 4:3, which is 1.3333333). This will not stretch the image, but the DAR will not be exact.

2. Leave the resolution at 712x480. Set the PAR such that the DAR will be exactly 4:3 (should be 4/3 * 480/712, which is 80/89). This will stretch the image an imperceptible amount, but the DAR will be 4:3.

3. Resize to 640x480. This means the PAR will be 1:1, or square pixels. This will stretch the image an imperceptible amount (because we're going from 712x480 -> 640x480 instead of 720x480), but The DAR will be exactly 4:3.

4. Resize to 632x480. The PAR here will also be 1:1 square pixels. The DAR will be 1.3185185. This will not stretch the image, but the DAR will not be exact.

What do you guys think? I would lean to just avoiding options 2 and 3 because having the DAR be exactly 4:3 doesn't seem to matter. I don't have a 4:3 TV so who cares if the DAR isn't precise. I'm going to be watching most of this stuff on my PC anyways.

Between option 1 and 4, I'm not sure. It seems like resizing is not a good thing to do and I should just encode anamorphically. Who cares about having square pixels in an H.264 video when you can encode anamoprhically? But, because it's a fullscreen DVD, it's not like there is important information that's going to be lost by resizing, right? The DVD is already 720x480 which is supposed to be displayed at 640x480, so whatever hardware that would be playing the DVD is already scaling it down. It seems like storing the image at 712x480 is just wasting space since it's going to get scaled down anyways.

I don't know though. Would you resize or encode anamorphically, and why?

[hello_hello]

Yeah, I think you'll find the difference between 640x480 square pixels and 720x480 anamorphic is fairly close to zero. Whichever way you do it, it's probably going to be necessary to sacrifice a few extra pixels if you want a nice 4:3 DAR without distorting the picture. That's what I do, although if you don't care then it's probably simplest to just crop as necessary, use anamorphic encoding and and let the DAR be whatever it'll be.

I work out the required cropping to remove the crud from the sides, then I crop extra from the top and bottom (and sides if need be) until the display aspect ratio in the Script Creator's Script tab is back to 4:3. If you're only cropping 8 pixels from the sides then I think around a total of 6 pixels top and bottom will get you very close to 4:3. You can use the "acceptable aspect error" setting in MeGUI's options to let it fudge the aspect ratio a tiny bit if it's hard to get it exactly 4:3. If my calculations are correct though, increasing the cropping of the sides to a total of 12 pixels, then cropping a total of 8 pixels top and bottom will give you 4:3 without any aspect ratio distortion. I'm pretty sure if you then disable anamorphic encoding while using the same cropping you can resize to 640x480 square pixels without any aspect ratio distortion also. I'd definitely adjust/increase the cropping as necessary in order not to distort the picture if I was resizing to 640x480.

If I'm encoding 4:3 episodic DVDs, it's almost a certainty each episode will require a different amount of cropping, in which case I either use the "resize to mod16" option or just resize to square pixels. That way I can still achieve nice 4:3 display aspect ratios by adjusting the cropping as above, but each episode will be encoded using the same dimensions. There's probably no need, but while I've managed to get over the "all the same size" thing, I still seem to have OCD when it comes to the display dimensions.

[nxmehta]

Thanks for the reply! I suppose I could crop everything down to 708x472 and then resize down. I'll do a test run to see if I can notice any difference.

If there is a difference between resizing vs anamorphic, I guess it would be due to the difference in the resizing function I use in MeGUI (Spline36) vs the scaler in my player (Media Player Classic, which I have set to Bucubic a=-0.60)? I guess the spline resizer is better than bicubic, so maybe it is worth scaling down. I guess I'll see :)

Cropping can be kind of a pain though. I am encoding a lot of episodes so cropping isn't uniform. I might just let my OCD about the DAR go so that I don't loose too much from the cropping. A lot of the shots are really zoomed in so if I overcrop I might regret it. In that case, I might just have to stick with anamorphic with a DAR != 4:3.

[hello_hello]

The last lot of episode 4:3 DVDs I encoded needed vastly different cropping. At best they needed 8 pixels from the sides, but some of them had "external" shots which required a lot more cropping. I think 30 pixels from one side was the worst. So anyway, I just took the "average" width after cropping and used it with the "resize to mod16" method to resize them all to the same dimensions. Naturally some need a lot more of the picture cropped from the top and bottom to keep them all 4:3, but I've not regretted it. In theory if you play the DVDs using a DVD player and an overscanning TV you don't see the crud at the sides due to the overscanning, but for the same reason you don't see the extra picture you're cropping from the top and bottom either. Each to their own though.... when it comes to 4:3 DVDs you'd probably have to do some serious resizing to have a noticeable effect on quality.

[sneaker_ger]

But, because it's a fullscreen DVD, it's not like there is important information that's going to be lost by resizing, right? The DVD is already 720x480 which is supposed to be displayed at 640x480, so whatever hardware that would be playing the DVD is already scaling it down. It seems like storing the image at 712x480 is just wasting space since it's going to get scaled down anyways.

No. Displays/players will typically upscale. If players would downscale to 640x480 on the common 1920x1080 pixel displays, the picture would be tiny. So you will lose information if you downscale to 640x480 instead of encoding anamorphically.

[nxmehta]

No. Displays/players will typically upscale. If players would downscale to 640x480 on the common 1920x1080 pixel displays, the picture would be tiny. So you will lose information if you downscale to 640x480 instead of encoding anamorphically.

For the sake of argument, if I was watching a SD DVD on a SD TV, wouldn't the player be downscaling?

[TheSkiller]

No, not at all. It works totally different. As soon as the DVD-Player converts the digital MPEG2 data to an analog video signal for the (SD) TV there are no pixels anymore! It is video bandwidth at this point. And the "odd" resolution of 720x480 derives from the way this conversion is carried out.

So no, there will be no scaling - that wouldn't make any sense anyway, would it. ;)
You have to consider that DVD is designed to store SD TV, obeying the technical parameters of analog SD TV and storing it digitally in a way so that it can be converted to analog again – that's the whole point of DVD, interlacing and pixels that are not square.


As soon as you downscale you lose something, period. Whether it is very visible or not depends on other factors.

[nxmehta]

No, not at all. It works totally different. As soon as the DVD-Player converts the digital MPEG2 data to an analog video signal for the (SD) TV there are no pixels anymore! It is video bandwidth at this point.

So, no there will be no scaling from 720x480 to 640x480 - that wouldn't make any sense anyway, would it. ;)


As soon as you downscale you lose something, period. Whether it is very visible or not depends on other factors.

Oh... wait, so what exactly happens such that the 720x480 image gets displayed on the 640x480 TV? At the point where the DVD player is doing DAC, is the input to the DAC a 720x480 image or a 640x480 image? So you're saying the DAC is doing the conversion to square pixels?

The thing that is confusing is how can you NOT be losing information going from 720x480 -> 640x480 just playing the DVD in your DVD player, but you ARE losing information 720x480 -> 640x480 using a resizing algorithm?

[TheSkiller]

No, no. There are no pixels anymore after the DAC, not in the signal and not in the TV in case of a traditional CRT one.
A CRT does not have distinct pixels, it just locks on any signal within it's capable scanrate and displays it natively. No scaling.
The TV does not have 640x480. It has anything so to say.

Yes, it may be hard to understand coming from a totally digital point of view.

Have you ever wondered why SD video looks so good on a CRT?

[nxmehta]

Hmm... reading some things I think I'm seeing what you're saying. Analog TV do not have pixels for sure, but they do have lines, right? So I guess you're going from 720x480 pixels -> 480 lines for a SD TV. So, the MPEG-2 DAC must be somehow converting each row of 720 pixels into an analog scan line.

The question is then I guess do the original 720 pixels per line encoded at a PAR of 8:9 contain more information than the resized, 640 square pixels per line that result from a resizing operation after that DAC conversion? I guess I could see the argument that you lose something in that conversion. Kinda weird though :)

[Asmodian]

The question is then I guess do the original 720 pixels per line encoded at a PAR of 8:9 contain more information than the resized, 640 square pixels per line that result from a resizing operation after that DAC conversion?

Yes they do, 720/640 = 1.125 so 12.5% more information in the scan line derived from 720 vs 640, weather you can notice this loss depends on the display and content of course. This is counting the information from the black/overscan pixels which I do not think is worth saving. Though it might be nice to keep it if displaying on an SD display so as to not lose active picture area to overscan. I would set overscan areas to full black to increase compressability though and so it wouldn't count as extra information.

[LocalH]

The question is then I guess do the original 720 pixels per line encoded at a PAR of 8:9 contain more information than the resized, 640 square pixels per line that result from a resizing operation after that DAC conversion? I guess I could see the argument that you lose something in that conversion. Kinda weird though :)
I'm unsure why you're referring to "resized, 640 square pixels". Neither NTSC nor PAL generally have 640px width when referring to the standards. 640x480 is just a nice, commonly-supported resolution that is close enough to what the proper square-pixel resolution would be for NTSC such that the error wouldn't matter to 99% of viewers. D1 resolution NTSC is 720x480, period (well, that or 720x486, but this doesn't change PAR or DAR, and in most instances those 6 lines can be cropped from the bottom with absolutely no ill effects). D1 resolution PAL is 720x576, period. When connecting digital video sources (or computers in general) to an NTSC CRT set, the number of pixels that can be displayed is a function of how much time each pixel takes up, the quality of the interconnect, and the sharpness of the CRT's shadow mask or aperture grille. For example, the Amiga running the Enhanced Chip Set can output 35ns pixels and with RGB or S-Video on a high-quality studio monitor, it's possible to resolve most of the pixels on a 1280px wide+overscan screenmode. But, I digress.

If you take an NTSC DVD player, connect it to an NTSC TV, and played an NTSC DVD on it, it's going to be outputting a 720px wide image on that analog signal, period. Nowhere is there a resampling to 640px width done, before or after DAC. If anything, the DVD player might crop to 704px width as that's the true 4:3 picture area, the extra 16 pixels are there for headroom in case there is a delay introduced during digitization (even just 70ns delay will shift the image roughly one pixel to the right).

Basically, if I'm understanding your question correctly, the only instance in which resolution is dropped is when outputting 16:9 material on a 4:3 display, and that's vertical detail, not horizontal.

Personally, I always preferred to just keep 720x480 material at 720x480 and slap a 4:3 DAR on the file. Especially in the case of interlaced content, messing with the vertical dimension is just asking for trouble (either causing the field order to flip if you crop an odd number of pixels off the top, or causing distortion of the interlaced field structure if you resize vertically at all). If you insist on keeping 712x480, I'd suggest option 1, since an exact 4:3 DAR doesn't matter so much as the metadata being accurate to the image content.

[ramicio]

Leave the resolution as is (cropped 712x480) and encode with a 80:89 SAR. Then when you mux, just set the aspect ratio to 4:3 and all will be fine.