Why we need aspect ratio?

[farjami]

for me it was a question that Why we need aspect ratio? and all of its reasons.
thanks

[mod]

Could you be more clear? Aspect Ratio is just a.. ratio. There's "no need" for it, it just exists..

[farjami]

i dont mean aspect ratio of a frame but i mean pixel aspect ratio feature that for example turnes a 352*288 video to 16:9 at decoding time (i.e scales the actual pixels to a larger Area)

to you see it in MPC go to view>video frame> check\uncheck keep Aspect Ratio tick, and then if you see change press alt+2 to see correct mode

and also found in xvid encoding options

[check]

so the aspect ratio of the final image is correct?

[Caroliano]

I know why YOU can need aspect ratio: because all DVDs use that, and if you don't want to loose resolution resizing it to the correct aspect ratio, you can encode in the native resolution and set an pixel aspect ratio as in the source. Or use the display aspect ratio if the container that you use suport it.

But the history of DVD aspect ratio I don't know... maybe because they needed all DVDs with an unique resolution for some reason...

[communist]

IIRC it stems from the standartisation of how to perform analog -> digital conversion and use same values for NTSC / PAL. And pixel aspect ration was/is appied to correct the aspect ration. Most digital video systems (DVD / digital video on tape etc.) are derived from this and hence have to make use of it.

/EDiT read more about why 720px as width was choosen here http://www.ebu.ch/en/technical/trev/trev_304-rec601_wood.pdf

[farjami]

thanks