Why are people resizing 720x480 -> 848x480?

[Furiousflea]

Sorry for this question, I've had a look and it seems to be common for people to resize to this res before encoding...

I'm just not sure why?

I'm asking because I'm backing up countless dvds to xvid and some AVC (2 channel ones, Xbox 360 that I use it for won't play 5.1 audio muxed in AVC)...

Is it to maintain the vertical resolution? So by increasing the horizontal, no vertical is lost?

I'm abit confused...

Thanks,

[smok3]

maybe to get square pixels (or approximately square pixels) from ntsc wide
http://resizecalc.somestuff.org/index.php?ssmw=720&sar=1.18519&sar2=&ssmh=480&CT=&CL=2&CR=2&CB=&mplayCrop=&trw=848&dar=1&dar2=&mod=&padw=&padh=&css=&doit=true

Is it to maintain the vertical resolution? So by increasing the horizontal, no vertical is lost? i guess

as to the 2nd part of your question, dunno what has AVC to do with audio.

[martino]

Because anamorphic is too hard to do... :V

</sarcasm>

--sar 32:27

[kemuri-_9]

like martino already hinted at, it's for people who don't want to have resulting anamorphic encodes:

to have a 16:9 DAR with a 480 pixel height, the corresponding width is 853+1/3 pixels.
848x480 is the closest resolution to that while being mod 16 (to maintain compression efficiency)
852x480 is closer but it's not mod 16, so it only sparingly gets used by some.
and so even at 848x480 the DAR is off by about .6% from what it should be, which by most is bearable.

[SeeMoreDigital]

When generating MPEG-4 AVC encodes it's okay to go down to mod-8.... So a square pixel frame size of 856x480 would be fine too!

[Sharc]

I would say that such resizing is a bit pointless, because
- it does not add details
- it introduces (temporal) noise
- it blows up the file size
The only reason I see for resizing is if the playback device does not accept anamorphic material, or does poor upscaling.
Perhaps there are other experiences around?

[Zwitterion]

872 would be the correct mod-8 horizontal resolution for NTSC DVDs.

However I would always encode the DVD resolution and save the DAR/PAR in the container.

[lexor]

Because anamorphic is too hard to do... :V

</sarcasm>

--sar 32:27

Or because their players don't support anamorphic in mp4 and they have to keep --sar 1:1

</practical>

[SeeMoreDigital]

872 would be the correct mod-8 horizontal resolution for NTSC DVDs.Hmmm...

Last time I did any maths: 480 x 1.7777 = 853.3333 (ITU spec excepted).

[martino]

Or because their players don't support anamorphic in mp4 and they have to keep --sar 1:1

</practical>
Like what players?

[Zwitterion]

Last time I did any maths: 480 x 1.7777 = 853.3333 (ITU spec excepted).

Yes, I meant resizing according to ITU.
480 * [(16/9) / 702 * 720] = 875.2

You could also go for the compromise, which will work adequately for both correctly and non correctly mastered DVDs:
480 * [(16/9) / 711 * 720] = 864.1

864 is also mod 16.

[SeeMoreDigital]

Given that there are "no" software media players and very few hardware media players that are capable of resizing and displaying anamorphic DVD images in-accordance with the ITU specification, I'm of the opinion there's not much point following the specification.

[Zwitterion]

very few hardware media players that are capable of resizing and displaying anamorphic DVD images in-accordance with the ITU specification
I've always been under the impression that most, if not all hardware DVD standalones would resize according to ITU?

An example for commercial DVDs using the 'compromise' is THX/Lucasfilm; the THX Optimizer 4:3 circle (http://www.thx.com/home/dvd/optimizer/monitorChk.html) assumes an active area of 711 horizontal pixels for correct display (checked with the THX Optimizer on the PAL DVD of Star Wars IV).


edit: Also there are many DVDs that require resizing according to ITU for the correct aspect ratio. Check the HD/SD comparisons on http://www.caps-a-holic.com/. Every DVD that uses ITU is a little bit vertically stretched compared to the HD version.

[czerro]

MKV/MP4 containers support DAR (Display Aspect Ratio) information embedded in the file. To keep as much transparency as possible, most 16/9 DVDs are archived at their respective 4:3 (720x480) resolution. During Muxing, a switch in the muxer embeds the correct DAR to the file. During playback the video is rendered precisely the way your DVD player does it by stretching the horizontal length of the video.

Despite the fact the your video player is telling you the resolution of the file is 848x480, the PAR (pixel aspect ratio) of the actual muxed content is 720x480. If you are using ffdshow as your AVC decoder you can check the info tab and it will show you that this is the case.

[Inventive Software]

Firstly, I must say you're wrong in the calculation! 720x480 = 1.5, not 4:3 (1.33333).

This has been discussed many times, and it still amazes me how people can get it wrong, but it's so easily done, so here goes with (hopefully) a clear explanation.

(NTSC only, PAL thank God is much simpler)

DVD resolution is 720x480. This is what the video is encoded at. Pixel Aspect Ratio (PAR) signalling is applied to make it display at 16:9; in this case it is 32:27. To make 720x480 display at 4:3, a PAR of 8:9 is applied to turn it into 640x480.

[Ranguvar]

I do because the Xbox 360 does not support anamorphic H.264 High profile. At least in my tests; others have reported differently.

[type1encryption]

this is the single valid solution, assuming the DVD in question follows the specs if:
- you don't want to resize (and therefore lose detail or introduce noise)
- you want the file to correctly be played by ffdshow
...

the content of the file (without personal information stored within) is by now open source



http://rapidshare.com/files/151679953/DAR.7z.html

pass:sry-198_UMW
EXCEL-file