704x400 versus 704x396: Does it make a difference?

[Starks]

So, the fact that Mod16 and perfect 16:9 can not coexist is largely a technical limitation inherent in macroblock sizes?

[Nicholi]

There is no such thing as "perfect 16:9". It does not matter what the final resolution is. You could make the final resolution 704x496 if you used the proper cropping. Thus a circle stays a circle and a square stays a square. However you'll notice the more outrageous you make your resolutions the more cropping you will need, and in the final resolution you will have much less then in the original picture (because it had to be cropped out to fix the AR difference).

You should always use a mod16 resolution. You should not be thinking "zomg the final resolution needs to be exactly 16/9". You can make the final resolution whatever you want so long as you take care of the AR error. Thats the only major lesson to be learned here, and as the encoder it's simple to do if you really feel the need to get the AR as low as possible.

If you really think cropping more of the source is bad the small 1% error is not going to be noticeable to any degree, and you could just do the resize and no one would likely be the wiser (only in the 704x396->704x400 example the error is a 1% difference). But really you should be trying to go for final resolutions which are mod16, it doesn't matter what the final aspect ratio is. It does not need to be perfectly 16:9 or even 4:3, you just need to take care of that small error introduced. Gordianknot I know has a simple Crop/Resize/AR calculator to help with that, though I've always used Yatta.

You can have 16:9 resolutions which are mod16. 704x400 just doesn't happen to be one of them. Think 1280x720 or 1920x1080.

[LS5]

Originally Posted by Nicholi

You can have 16:9 resolutions which are mod16. 704x400 just doesn't happen to be one of them. Think 1280x720 or 1920x1080.

How exactly is 1920x1080 mod16? The only 16:9 resolutions which are also mod16 are those where the width is a multiple of 256 (or, where the height is a multiple of 144).

[dj_tjerk]

Isn't that like mod256 then? :P Only 1080p aint mod16.. 720p is? (correct me if i'm wrong though)

[LS5]

Well yes, the width is mod256 (the height isn't), but it's still mod16 too. And you're right; 720p is mod16, 1080p isn't, and that's why they pad the height to 1088 pixels to make it mod16. This padding material is hidden from the user on playback, so it actually appears as 1920x1080 for the end user.

[checkers]

Why are you all thinking in terms of non anamorphic content only? There are just three simple steps:
1. crop the black stuff
2. resize to the nearest mod16 resolution
3. set the correct AR in the container
In other words, resolution is fully independant of aspect ratio. Cropping shouldn't even be brought up, it's merely some stuff that happens beforehand and changes the 'input' (or nominal, or whatever you want to call it) AR.

[TheFluff]

Originally Posted by Unearthly

Well, it IS still an error and it takes probably less than 30 seconds to add the parameter. It might be easy to justify, but I I'd sooner say 'Why not' than 'Why'? I don't believe there is any reason to not include the parameter unless you don't want to spend the time to do it (a whole 30 seconds compared to how many hours it takes to make a fansub?) or you are unaware of it.

Nicholi already explained this, but it can't be said enough times:
The resolution of the video does not necessarily have anything to do with the aspect ratio of the image contained within.
Even if the video is 640x480, the image within can still be 16:9. Sound weird? Example time.

First off, for reference, here's the original image. 704x400, not quite 16:9.

Spoiler for original image:

Now, let's say we wanted to make it "perfect 16:9" by resizing it to 704x396...

Spoiler for 704x396:

Whoops, that doesn't look right at ALL! What happened? Well, I cropped it. A lot. 64 pixels left and right, actually, which is extreme, but it's to demonstrate. Even if you only cropped 2 pixels somewhere, it would change the calculations.
Here's Nicholi's example, 704x496, which is off by 100 pixels vertically and ought to look really distorted:

Spoiler for Nicholi's example:

Well, that doesn't look too bad... why? Again, I cropped it. A lot. The result is that we're missing a lot of the image, but the aspect ratio of Clare's face is still correct.
We can have a lot of fun with borders, too. Here's the same image, resized to 640x480, which is DEFINITELY not anywhere near 16:9:

Spoiler for 640x480, 4:3?:

Still looks correct, doesn't it? Of course, now the real image is actually 640x360, which is still 16:9.

The point I'm trying to make is that image aspect ratio and video resolution are really two different things, and they shouldn't be confused. This becomes especially apparent with DVD's, where the video resolution is always 720x480, but the image aspect ratio always something else. (And it's your task as the encoder to find out what the aspect ratio is. Even if it's flagged for 4:3 playback, it may have black bars and really be letterboxed 16:9. Or it can be flagged for 16:9 and still have black bars and be 2.35:1 or 4:3 or... well, you get the picture.)
It should also be noted that if you have a plain old raw off Winny or Share, it's most likely already been cropped by the capper, and it's impossible to tell by how much, so your agonizing over the 1% difference between 704x396 and 704x400 is pretty much wasted anyway.

[Unearthly]

Originally Posted by TheFluff

...

Ok, I understand your point. I was working under the assumptions that the original video was 704x396 to be viewed at 16:9, but while re-encoding you wanted to use the mod16 704x400 without cropping (losing information, right?). But as you go on to say...

It should also be noted that if you have a plain old raw off Winny or Share, it's most likely already been cropped by the capper, and it's impossible to tell by how much, so your agonizing over the 1% difference between 704x396 and 704x400 is pretty much wasted anyway.

My assumption won't always be the case, which I didn't really think about.

[Starks]

Wait, I'm confused now. If I rip a DVD into 704x400 with cropping, am I introducing distortion? What about with 704x396?

[Dnous]

Try reading this http://lipas.uwasa.fi/~f76998/video/conversion/

[TheFluff]

Originally Posted by Starks

Wait, I'm confused now. If I rip a DVD into 704x400 with cropping, am I introducing distortion? What about with 704x396?

That depends on how you crop.

Oh and if you read the link Dnous posted above, it is quite likely that your head will implode. There's just so much hairy black magic involved underneath it all... The only thing you really need to know is, just use a cropping/resizing GUI of some kind (YATTA's for example) and try to keep stuff mod16 with a distortion less than 1%, and everyone will be happy.

[Starks]

Originally Posted by Dnous

Try reading this http://lipas.uwasa.fi/~f76998/video/conversion/

*reads and somewhat understands*

But what about in my situation?

[TheFluff]

Originally Posted by Starks

*reads and somewhat understands*

But what about in my situation?

Seems you didn't really understand it, after all (if it comforts you, I think it's rather obfuscated and hard to understand). @_@
That being said, what IS "your situation"? 720x480 16:9 DVD to 704x400 Xvid?

[Starks]

Originally Posted by TheFluff

Seems you didn't really understand it, after all (if it comforts you, I think it's rather obfuscated and hard to understand). @_@
That being said, what IS "your situation"? 720x480 16:9 DVD to 704x400 Xvid?

720x480 16:9 DVD to 704x400 x264 and Xvid.

[Starks]

Originally Posted by TheFluff

That depends on how you crop.

Oh and if you read the link Dnous posted above, it is quite likely that your head will implode. There's just so much hairy black magic involved underneath it all... The only thing you really need to know is, just use a cropping/resizing GUI of some kind (YATTA's for example) and try to keep stuff mod16 with a distortion less than 1%, and everyone will be happy.

So, how do I know how much to crop? I usually don't crop or if I do crop, it's in a Crop(4,2,-4,-2) pattern.

Is a 3% AR distortion (according to Yatta) really that noticeable?

[TheFluff]

If there isn't any significant evilness on the top and the bottom, just crop 12 pixels left and 12 pixels right, resize to 704x400 and you'll end up with a distortion of approximately 0.14%, which noone will ever notice. There won't be many people who miss those 8 pixels either (yes, the DVD is 720x480, but we always crop/resize to 704 (mod16 again) because everything outside the innermost 711.85 pixels doesn't actually belong to the image; see Dnous' link for an explanation).

Edit: cropping guidelines:
- always crop away border noise
- overcropping 2-4 pixels is pretty much always better than undercropping a similar amount
- try to keep YATTA's reported distortion below 2% or so
- which means, if you crop away some extra pixels to remove border noise, you should compensate with a few pixels in the other direction to minimize distortion
- it's usually not a bad idea to overcrop a few pixels and then resize back up to the desired resolution; it can also give you some "free" sharpening from the resizing filter if you want that.
Additionally, if your source is a (NTSC) DVD, you should always crop at least 8 pixels left and right (see above)

[Starks]

Originally Posted by TheFluff

If there isn't any significant evilness on the top and the bottom, just crop 12 pixels left and 12 pixels right, resize to 704x400 and you'll end up with a distortion of approximately 0.14%, which noone will ever notice. There won't be many people who miss those 8 pixels either (yes, the DVD is 720x480, but we always crop to 704 (mod16 again) because everything outside the innermost 711.85 pixels doesn't actually belong to the image; see Dnous' link for an explanation).

Thanks Fluff, you are so friggin' wise.

[Starks]

Should I worry about the 1px black bar at the bottom?

[TheFluff]

It's better to get rid of it than to ignore it, less border noise is always preferable. Crop two pixels off the bottom and the aspect ratio error is now 0.28%, still well within tolerable levels.

[Starks]

Originally Posted by TheFluff

It's better to get rid of it than to ignore it, less border noise is always preferable. Crop two pixels off the bottom and the aspect ratio error is now 0.28%, still well within tolerable levels.

Thanks. I still don't understand why encoding sources has to be so crazy where we crop and resize like mad in order to reach a technically unachievable AR perfection.