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Aspect ratio conversion

 
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SecShade



Joined: 17 Nov 2003
Posts: 4

PostPosted: Mon 17 Nov 2003, 19:50    Post subject: Aspect ratio conversion Reply with quote

Hi,

I'll appreciate if someone can guide me to convert the aspect ratio of a couple of MPEG-2 files.

Files are encoded in a 4:3 ratio and I want to transform them to 16:9 ratio.

I know that the picture will be distorted, but that's intended. Don't want to crop, or trim any piece of the actual video; and, will prefer the method that uses no re-compresion at all, if possible, or minimal re-compression instead. Maybe someway to fool the player and make it to "see" a 16:9 video.

I don't consider myself a newbie neither an expert in using TMPEGenc, but I just can't find the way to do it. Any help would be greatly appreciated.

Thanks
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RMN
Site Admin


Joined: 04 Feb 2003
Posts: 587
Location: Lisboa, Portugal

PostPosted: Mon 17 Nov 2003, 23:06    Post subject: Reply with quote

The output aspect ratio is determined by a simple "flag" (value) in the MPEG stream. I don't know any way to change it in TMPGEnc without recompressing, but I suspect there are some MPEG "toolkits" that will let you do it. It should also be possible to open the file with a hexadecimal editor and change only that flag, assuming you knew the address and the correct value.

BTW, I think the value is 0x31 for 16:9 and 0x21 for 4:3, but I don't know exactly how to determine the address. If someone has more information, or knows of a program that will change this flag automatically, please post.

RMN
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SecShade



Joined: 17 Nov 2003
Posts: 4

PostPosted: Fri 21 Nov 2003, 18:28    Post subject: Thanks for your answer Reply with quote

Very interesting answer

That was the kind of answer I've been looking for, unfortunately, I don't have the proficiency necesary to find the required address so will hope that somebody else can give us some lights on it.

Can you name any toolkit that would change aspect ratio without reencoding?

Thanks again!!!
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SecShade



Joined: 17 Nov 2003
Posts: 4

PostPosted: Fri 21 Nov 2003, 19:57    Post subject: Found a tool that claims to do qhat i want Reply with quote

Hi again rmn,

Searching the web, leaded by the ideas you posted, found a tool called pulldown, that claims to change the aspect ratio flag without re-encoding:

http://www.inwards.com/inwards/?id=36

Not only can change the aspect ratio flag, but some others flags that i really don't undestand clearly. Anyway, i've tried it and got a 16:9 video file with the very same file size. So I guess it works.

It says it doesn't re-encode anything... but Is there an EASY way to confirm if my video have been re-encoded or would be a lose of time to even try (as the resulting video is the same size)?

Anyway, thanks again for your help
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RMN
Site Admin


Joined: 04 Feb 2003
Posts: 587
Location: Lisboa, Portugal

PostPosted: Sat 6 Dec 2003, 1:31    Post subject: Reply with quote

How long does this program take to process the file? If it's very slow, it's possible it re-encoded the file (although I doubt that very much). If it's fast, you can be pretty much certain it didn't re-encode anything.

RMN
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Trulgor



Joined: 14 Dec 2004
Posts: 1

PostPosted: Tue 14 Dec 2004, 21:08    Post subject: Pulldown does not re-encode the source Reply with quote

I just went through the sourec code for pulldown (found at http://www.inwards.com/inwards/?id=36 ). It is safe to say that Pulldown does not re-encode the source independent of any option supplied to the program. It simply changes certain bits in the source before copying to the desired output.
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