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Old 5th July 2008, 16:43   #1  |  Link
shammah
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Custom DAR setting

Hello, I'm backing up my favorite move from a DVD. It says on the back the ratio is 1.66:1, and it's actually encoded as 720x480 at 4:3 (which of course means black bars along the top and bottom which looks stupid on my widescreen monitor).

I used to back up my movies just by resizing to the correct frame size, but in the last couple of weeks I've tried using AR's instead.

Anyway, after cropping, I get an actual frame of 712x380 that should, according to my calculations, have an (exact) AR of 5696:3420 (1424:855), or 1.665497:1.

When I punch these numbers into the XViD encoder as a DAR, though, what I get is a movie with an AR of 979:570, or 1.7175439 (as reported by MPC).

So, I loaded it into MPEG4Modifier and punched the same numbers into that program's DAR. It resulted in a movie with an MPC-reported AR of 3331:2001, or 1.664668, much closer to what I was wanting.

Is there some restriction on XViD's custom DARs? More importantly, is there a reason I should go with XViD's results?

Thanks in advance for your help,
-Shammah
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Old 6th July 2008, 00:03   #2  |  Link
CWR03
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You're tlaking about a difference that's so small that no one would ever tell, less that 0.1%. Regardless, I'd use the one that gave the overall better results.
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Old 6th July 2008, 04:43   #3  |  Link
shammah
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Er... Actually, XViD is about 3% off, prompting my concern. I was just asking because it looks like either:
1) I'm not understanding something advanced about XViD's custom DARs.
2) XViD's custom DAR code has a bug in it.

-Shammah
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Old 6th July 2008, 08:09   #4  |  Link
Sharc
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I Xvid encoder select <Pixel Aspect ratio> instead of <Display Aspect Ratio>
I assume your source is NTSC, so you should probably select 16:9 NTSC in XviDs pulldown menu.
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Old 7th July 2008, 05:27   #5  |  Link
shammah
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Um, OK...

I don't think anyone is understanding my question here... I've based all my calculations off of DAR, so setting PAR, especially to 16:9 on a 4:3 video would look quite strange.

Does XViD's custom DAR setting have a bug in it or not?

-Shammah
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Old 7th July 2008, 06:57   #6  |  Link
Sharc
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I assume your source is anamorphic, NTSC (?). In order to display it correctly without resizing one has to signal the Pixel Aspect Ratio and the playback device must stretch the picture accordingly provided that the player does not ignore the AR signalling. When one selects 16:9 NTSC The PAR is signalled as 40:33, when one selects 4:3 NTSC the PAR is signalled as 10:11 for example. The PAR is independent of any cropping one applies to the source. Have you tried setting the PAR instead of the DAR in XviD?
I am not aware of a bug in XviD, but as far as I know it scales any entered x:y pair internally to the 1 .....255 range which may result in minor rounding errors. Not sure though.
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Old 7th July 2008, 09:14   #7  |  Link
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These sorts of discussions are much easier to resolve when you have some "uncorrected" still images of the source to look at....
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Old 7th July 2008, 22:11   #8  |  Link
shammah
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Sharc -

I see what you're saying now. Using PAR would be much easier, mathematically, than DAR, because of the cropping.

I'm attaching a still shot of the source. This is without the 4:3 AR.

-Steve
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Old 8th July 2008, 01:18   #9  |  Link
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Quote:
Using PAR would be much easier, mathematically, than DAR, because of the cropping.
Extremely easier since no math would be involved at all.

– When indexing DGIndex tells you (status window) if the video is PAL or NTSC and 16:9 or 4:3. Or have a look at the DVD box if for some weird reason you don’t use DGIndex.
– Crop as much as needed. Don’t resize.
– In Xvid set the »Pixel Aspect Ratio« to whatever DGIndex told you. No need to mess around with DAR.
– Don’t worry if your DAR ends up not to be one of those nice »even« numbers. That’s perfectly normal.

Your screenshot indeed seems to be from a NTSC 4:3 DVD. So »4:3 NTSC« would be the right »Pixel Aspect Ratio«.
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Old 9th July 2008, 06:59   #10  |  Link
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Would it be easier to for XviD's encode if we crop in a multiple of 8 (or even 16) to align with the MPEG2 macroblocks / block artifacts?
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Old 9th July 2008, 13:29   #11  |  Link
shammah
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Thank you, Brother John!

I have indeed been doing it the hard way...
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Old 14th July 2008, 16:15   #12  |  Link
Sharc
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Quote:
Originally Posted by henryho_hk View Post
Would it be easier to for XviD's encode if we crop in a multiple of 8 (or even 16) to align with the MPEG2 macroblocks / block artifacts?
Coding with Xvid is most efficient when the picture is cropped to a multiple of 16 horizontally and vertically.
Worst case is to leave black borders which fall somewhere in between a 16x16 macroblock, because the sharp transition between the useful picture and the black border consumes extra bits for encoding.

Last edited by Sharc; 14th July 2008 at 16:34. Reason: Typo
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