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Aspect Ratios

Discussion in 'Other video questions' started by tenchars, May 24, 2004.

  1. tenchars

    tenchars Member

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    Hello everyone!

    The purpose of this thread is to hopefully kick off a discussion based on the correct use of aspect ratios based on some questions I have....

    Q1) How do I get out the actual stored video frame size IN PIXELS of any particular MPEG / AVI video?

    Q2) How is aspect ratio stored in an MPEG / AVI video? Is this basically the result of Q1) above?

    Q3) When I play a video in Media Player, the properties of an example video will state 768x576 (AR of 1.33:1 aka TV 4:3) but is played correctly with an AR of 2.55:1 (aka CinemaScope). How does this work?

    Q4) Using example in Q3) above, Flask reports that the size is 480x576, PAL, AR:4:3. Wierd, 480x576 = taller than it is wider!! In order to maintain the correct aspect ratio what video frame size should I use when converting to DivX?

    Q5) My best guess to Q4) above is 480x352 or should this be 768x576. Which should I use to produced the best quality output without unnessesary shrinking / enlarging of original?

    An info regarding Aspect Ratios greatly appreciated!

    (Apologies in advance if there is an obvious answer to these questions but I am finding Aspect Ratios a fascinating facet of Video Encoding that I would like to get right!)

    Many thanks,

    Andy.
     
  2. Minion

    Minion Senior member

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    Well I know enough about aspect ratios to get me By...
    I"ll try to answer the questions the best I can...

    Q1)To get the resolution of any AVI or Mpeg file you can Load the File into Virtual-Dub-Mpeg2 and then go to "File Information"....

    Q2)AVI files do not use Aspect ratios they are displayed useing there Resolution so in essence the Aspect ratio of every AVI file is 1:1....
    Mpeg files have the aspect Ratio stored in the Header so the Aspect ratio can be changed by simply Changeing the header which can be done with some tools Like "DVD Patcher"....

    Q3)??????????

    Q4)to maintain a 4:3 aspect ratio with a Mpeg file that is 480+576 when encodeing to DivX my calculations tell me that 480+352 is the Correct resolution, But if the Mpeg file has black Boarders you might want to crop them off and after cropping them off the Resolution should be close to 480+352..

    Q5)You should keep the Resolution at 480+352 because any inlargeing or the resolution will reduce the Quality of the Image....

    You should look for an Aspect ratio calculator which will help you convert Mpeg aspect ratios into the correct resolution for AVI files......

    I hope some of this helps...Cheers
     
  3. tenchars

    tenchars Member

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    Thanks Minion!

    ..Q3) Silly me, original was letterboxed @4:3, visible video frame 2.55:1. Not always obvious in Media Player! That unravels a lot of potential confustion!

    OK, VirtualDub v1.5.10 MPEG also says 480x576 so I guess that the player takes Hres of 480, AR of 4:3 & squishes Vres from 576 to 480/(4/3) = 360. I am now wondering WHY the Hres seems unnessecarily large.

    My only guess is PAL TV = 625 lines (less hidden lines: -25 top & bottom) = 575. Take Vres of 575*(4/3) & this gives Hres of 768 i.e. that reported by Media Player!

    So, converting to DivX with a target frame size of 480x352 means that I will loose video data from the original (approx loss of 40%). If I use 480x575 in order to maintain 0% loss of video data the damn thing looks pissed when played in Media Player!

    (And like you say, there is no way of embedding the aspect ratio in an AVI file.... I wonder why THAT is!)

    Facinating!

    PS
    Thinking about it, the film is letter-boxed and the black boarders constitute 40% of the displayed video frame......
     
  4. Minion

    Minion Senior member

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    The Original Video was Probably 480+352 and they just letterboex it so it would fit in the 480+576 Frame..
    This file was probably a SVCD File because SVCD is the only format that uses the 480+576 Resolution....
    That resolution doesn"t look right as an AVI file but as a Mpeg2 file and Displayed in the 4:3 aspect ratio it looks correct.....
    Media Player ignores Aspect Ratio flags in Mpeg files so it is not good to use Media Player to watch Mpeg files because it will allways display the file in it"s resolution and not in it"s aspect ratio, A software DVD Player like PowerDVD or WinDVD is best for Mpeg2 files......Cheers
     
  5. kumasan

    kumasan Member

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    Yes, because Media Player is a poor player, any good player like Zoom Player will allow you to display it correctly. Keeping a maximum number of pixels is a good practice, even it doesn't display well in WMP who cares :) ?
     
  6. tenchars

    tenchars Member

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    Many thanks for all your help, time & effort so far!

    So the upshot of this is to essentially ignore aspect ratios and just encode to an avi (using whatever codec to compress) setting horizontal and vertical resolutions to that reported by programs such as Flask.

    To then play the resulting avi, don't use Media Player rather use something that allows you to manually set the aspect ratio, such as DivX Player.

    Cheers,

    Andy.
     

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