VOB aspect ratio

[Kangiten]

Greetings everyone

I have a question that many of you will probably find silly but here goes : is there a way/tool to know the exact display aspect ratio of a VOB file (not the generic "16/9") ? Is this information contained in the DAR flag and if so, how can I access this information ?

Thanks for any help.

[bugster]

AFAIK, no, not as such. the DAR flag will show either 16:9 or 4:3. DVD does not explicitly support other aspect ratios. If a different AR has been used it will have been achieved by encoding black bars top and bottom of the video.

[Kangiten]

Thanks for the reply.

Maybe I should've explained the context. I want to convert one of my dvds to xvid. So what I thought I'd do (maybe I'm already wrong there) is to crop the black bars in VirtualDubMod and then resize the movie so it keeps the original aspect ratio. The problem is : how am I supposed to achieve that when I don't even know the original aspect ratio in the first place ?

[bugster]

I thought you might want something like that. Take a look at doom9's site, they may have ways to help as its what they do on a regular basis.

[midnightsun]

well, you don't know but you can guess it very accurately. Crop if necessary and write down how much you have cropped/are left with. Multiply the resolution height (before you do any cropping!! i.e. 576/288 for PAL, 480/240 for NTSC) by the DAR, which for DVD's is either 16:9 (1.77777777....) or 4:3 (1.333333.....) and you have the display width. You can then resize to a frame proportional to the one you have just calculated.

Example: 16:9 PAL DVD, res:720x576
You crop 56 pixels top and bottom: "active" area is (576-2*56)=464pixels tall.
DAR=16:9 => the display width is 576*16/9=1024pixels

The active area therefore is 1024x464 (PAR=2.207). To preserve the proportions you have to resize keeping a PAR of 2.207 or thereabouts.

It should be added that many DVD->mpeg-4 tools do that for you automatically. (gordian knot for example)

[Kangiten]

Thank you very much for this explanation

[edit] Ok, I've tried GK and it works very well, thx for the pointer.

Just out of curiosity, sometimes you also see vertical black bars on the left and right side of the screen. How do you take that into account when calaculating the exact aspect ratio ?

Thanks for all the help.