Comments on sticky threads

[bb]

Hello folks,

I wanted to keep the F.A.Q. itself short and lean, but please feel free to comment and add here. I'll add anything to the F.A.Q. that's worth it.

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[pyropir]

It's really helpful to put together this FAQ on DV.

If you want to add more questions, you could try to clarify the non-square pixel stuff in DV. How it affects resizing when converting DV to DivX, and how the pixels aspect ratio is different between PAL and NTSC (59:54 in PAL 4:3, 10:11 in NTSC).

I also read somewhere that, strictly speaking, a 720x576 DV frame is not a 4:3 image, because several extra pixels are captured (17 pixels with PAL, 8 extra pixels with NTSC).

I hope this isn't too complicated for a short&lean FAQ. I spent the best part of a Sunday trying to get my mind around these issues, but it's important when you want to resize and keep the correct aspect ratio in DivX.

regards,

pyropir

[bb]

@pyropir:
Thank you for your post. I'll try to elaborate this stuff. Do you have an example ready?

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[pyropir]

Sorry for the late reply - it took me a while to find the files on my computer ...

So, I wanted to capture an old VHS film ('Troublemaker' - a cult film where I come from) using my camcorder. I recorded the film with my Canon Mv4i's analogue in, then captured the DV file to my harddrive, frameserved it to Virtualdub via a one-line avisynth script to make an XviD encode. I didn't know anything much about aspect ratios, I couldn't be bothered because my DVD rips always seemed to look sort of OK. I just cropped the image a little (720x576->694x555), filtered a bit, and used GKnot to get a reasonable bit/(pixel*frame) value, so I decided to resize to 480x400. Well, the result looked very weird because everything was squashed - not surprisingly, since I had transformed a 4:3 film (with non-square pixels) into something with an aspect ratio of 3.6:3 (with square pixels). (screenshot: troublemaker_3.6;3.jpg)

I took me quite a while to find out about the square/non-square pixels in PAL and NTSC (the table at http://www.mir.com/DMG/aspect.html is very useful), but as far as I understand it the proper way to do this is to
(1) crop the image from 720x576 to 703x576 (because the DV captures an additional 17+5/59 pixels, making it slightly wider than a standard 4:3 image) - at this point we are still dealing with 59:54 non-square pixels
(2) then to resize from 703x576 to 703x(576*54/59) = 703x527 [or (703*59/54)x576 = 768x576] which corresponds to 4:3, i.e. we are dealing with square pixels now
(3) then to resize to a resolution with multiples of 16, such as 480x352 in my case. Of course (2) and (3) can be done in one step.(attachment: troublemaker_4;3.jpg)

All this is with PAL and 4:3. If you have NTSC or 16:9, then the aspect ratios are different:

PAL, DAR 4:3 ; pixel aspect ratio: 59:54
PAL, DAR 16:9 ; pixel aspect ratio: 118:81

NTSC, DAR 4:3 pixel aspect ratio: 10:11
NTSC, DAR 16:9 pixel aspect ratio: 40:33



I hope I got this right ?

regards,

pyropir

[Wilbert]

(1) crop the image from 720x576 to 703x576 (because the DV captures an additional 17+5/59 pixels, making it slightly wider than a standard 4:3 image) - at this point we are still dealing with 59:54 non-square pixels

It is better not to crop because you will introduce an AR-error. After cropping your AR equals 703*59/54 : 576 = 1.333:1. Without cropping you have 720*59/54 : 576 = 1.3657:1. In both situations you have to resize to 480x352 (or a scaling of this) which has an AR of 1.3636:1.

[bb]

The thread has been extended to contain comments to any of the sticky threads.

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