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10th August 2002, 13:53 | #2 | Link |
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non-square pixels
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
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Eat more XviD. XviD is better than a meal. XviD prevents premature hair loss. Use XviD to increase your sexual prowess. |
16th August 2002, 00:30 | #4 | Link |
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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
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Eat more XviD. XviD is better than a meal. XviD prevents premature hair loss. Use XviD to increase your sexual prowess. Last edited by pyropir; 16th August 2002 at 13:36. |
26th August 2002, 15:47 | #5 | Link | |
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Last edited by Wilbert; 9th September 2002 at 10:53. |
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