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Pixel Aspect Ratio

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Scott Bush
Pixel Aspect Ratio
on Jul 31, 2009 at 8:38:28 pm

When converting footage with non-sqaure pixels (like, say, DV or D1) for viewing on a computer monitor (so to 640x480) how do I handle pixel aspect ratio?

In Episode, for example, I can use the "resize" filter and have options to maintain proportions by "Cut", "None (Distort)", or "Letterbox(Pad)". It seems to me that none(Distort) would make the most sens in this case (going non-square to sqaure) but I wanted to check. Thanks for any info.


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Brian Alexander
Re: Pixel Aspect Ratio
on Aug 1, 2009 at 4:22:40 am

You definitely want to choose None (Distort) when converting to square pixels for computer playback.

Converting 720 x 480 NTSC with the resize filter in Episode will give you the following results:

Cut: This crops the sides of your image. Result will be 640 x 480 but your pixels will not be corrected for aspect ratio.

Letterbox (Pad): This scales your 720 x 480 to fit inside a 640 x 480 window leaving you with a letterboxed image. Again, this does not correct the pixel aspect ratio.

None (Distort): This converts your rectangle NTSC pixels to square pixels for your computer display leaving you with a 640 x 480 image.



--
Brian Alexander
Sr Video Engineer
Freeman AVS


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Scott Bush
Re: Pixel Aspect Ratio
on Aug 1, 2009 at 3:16:08 pm

Thanks, Brian. That was what I thought but the documentation was a little confusing - mostly just because they mention specifically anamorphic MPEG-2 but nothing else. I was trusting my eye but wanted make sure I was technically correct.

Could someone also explain formats that have a "display" aspect ratio? Like for example, if I have an MPEG-2 file at 720x480 for DVD - so non-square pixels - when play it in Quicktime player it shows up at the proper computer display of 640x480. Is this a function of quicktime being smart and knowing that 720x480 almost always means 640x480 for a computer display, or is it built in to the file somewhere as a "display" aspect ratio for use on computer?

Thx,
Scott



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Daniel Low
Re: Pixel Aspect Ratio
on Aug 1, 2009 at 9:45:12 pm

Agreed, Telestream need to improve the explanations for this filter.

[Scott Bush] " when play it in Quicktime player it shows up at the proper computer display of 640x480. Is this a function of quicktime being smart and knowing that 720x480 almost always means 640x480 for a computer display, or is it built in to the file somewhere as a "display" aspect ratio for use on computer? "

Quicktime is smart, but not all the time.


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Brian Alexander
Re: Pixel Aspect Ratio
on Aug 2, 2009 at 4:11:19 am

Not all pixels (picture elements) are created equal. Most modern computer and TV displays, DLP, Plasma, LCD, and LCoS, use square pixels to re-create images from an analog or digital video signal.

Video imagers, CCD and CMOS, do not always the same square pixel formats as displays. Sometimes they use odd (not normal) pixel counts on the imaging chips then process the image to create a standard size video frame. When this happens the video is tagged with a 'flag' to tell the computer, monitor, or equipment how to display the image.

The aspect ratio flag is part of the ancillary data and is represented by a 4 bit code word. Unfortunately a lot of times this flag is lost in the processing of the video signal. Quicktime does not always recognize this flag (probably because it is not present).

It's best to output assuming square pixels. If your video is distorted in QuickTime you may have to manually type in the correct pixel count (based on square pixels) to view your video distortion free. This is especially true when looking at 720 x 480 16:9 video in quicktime. Sony DVCAM products do not add this flag so the video will look squeezed horizontally or stretched vertically so manual input is a necessity to view the video distortion free.

--
Brian Alexander
Sr Video Engineer
Freeman AVS


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Scott Bush
Re: Pixel Aspect Ratio
on Aug 2, 2009 at 7:49:42 pm

Thanks, again.

This all makes sense to me, but of course I can't tell my client to "just type in the proper aspect ratio" it just needs to work for them, distortion free. I never send them DV footage anyway so I'll be making sure the pixels are correct in what I send them - but this helps me understand it better. Many thanks.



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