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Old 03-12-2014, 09:07 PM   #1
dougcollins
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Default Understanding Square Pixels

I'm having a minor issue exporting to mp4. My problem is likely due to my lack of understanding of square pixels vs non-square pixels.

The project is 1440x1080. When I export with the Edius' H.264 exporter plugin, I get a satisfactory mp4 which displays properly in 16x9. However, I know that Handbrake will give me a much smaller file size with equal quality.

I am exporting to an uncompressed AVI for Handbrake to convert it to an mp4. This works fine, except the output is now a squeezed 4x3 video.

The properties of both mp4 files indicate a 1440x1080 frame size.

I am guessing this is a square/ non square pixel issue. Can anyone enlighten me?

Thanks,

Doug Collins
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Old 03-12-2014, 09:57 PM   #2
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Hi Doug!

Yes your file is anamorphic with a 1.33333 PAR (non-square pixel). If you can't set the pixel aspect ratio in handbrake (don't know it well enough to tell you) then simply export from Edius as 1920x1080.

Best
Dave.
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Old 03-12-2014, 11:32 PM   #3
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Thanks Dave.

Changing the initial encoding to 1920x1080 fixed it. I kind of thought setting the anamorphic option in Handbrake to "none" would have worked too, but it didn't seem to make a difference.

Thanks,

Doug
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Old 03-13-2014, 02:40 AM   #4
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Quote:
Originally Posted by dougcollins View Post
Thanks Dave.

Changing the initial encoding to 1920x1080 fixed it. I kind of thought setting the anamorphic option in Handbrake to "none" would have worked too, but it didn't seem to make a difference.

Thanks,

Doug
Hi Doug.

You may find that it is down to the decoder/player as to wether or not it identifies the encoding as anamorphic or not.

To be safe, just do as Dave is saying. This will sort out any discrepancies with the decoder/player recognising SAR and PAR, and will do so without wrecking the geometry or resolution.

Cheers,
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Old 03-13-2014, 07:50 PM   #5
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Originally Posted by LTVHD View Post
Hi Doug.

You may find that it is down to the decoder/player as to wether or not it identifies the encoding as anamorphic or not.

To be safe, just do as Dave is saying. This will sort out any discrepancies with the decoder/player recognising SAR and PAR, and will do so without wrecking the geometry or resolution.

Cheers,
Dave.
Hi Dave,

I will do that (and I did), but even though the problem is solved, I'm still wondering why one file (exported by Edius H.264) is reported as a having a 1440x1080 frame size, and the second (exported as an uncompressed AVI) also shows as 1440x1080, yet one plays as expected (H.264), but the other (AVI) plays squeezed.

It doesn't matter, I know how to correct it now, I'm just curious.

Thanks,

Doug
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Old 03-14-2014, 03:39 AM   #6
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Originally Posted by More4K View Post
It's because mp4 (or even h264 elementary stream inside mp4 container) has stored information about pixel aspect ratio. This information 'tells' player to apply this 'correction' (resize), so your final displayed frame has proper aspect ratio.
AVI is an old container which can hold this information, but there is no standard for it, so when you play your AVI player simply displays frame size as is (assuming square pixels). If files use square pixels than this still will be OK, but in case of anamorphic files you get wrong aspect. Different companies stores additional metadata in AVI container in different ways, so not many of them work between each other. If you load your file to Edius it will interpret it properly, as GV does put pixel aspect ration inside AVI. Same file probably won't load properly in Premiere as Adobe uses different way of storing metadata inside AVI. This is the reason why Edius and Premiere allow for manual interpretation per each clip. This is mainly a problem with AVIs- MOV, MXF are much better in terms of metadata, so they are more reliable in this area. They still relay on fact that exporting app sets headers correctly and the same that reading application reads them correctly. In most cases this is OK, but sometimes you still have problems, so manual correction is useful. This is not only the aspect ratio info, but also field order, time code, color space etc.
In your case file is 1440x1080 with 4x3 pixel aspect ratio, which gives final frame ratio (display aspect ratio) 16x9. The whole trick is done to save bandwidth/bitrate, so you store less pixels, but give end user 'bigger frame'. It's a very common technique used in cameras, eg HDV format.
When you do encoding for web the best is to simply resize it to 1920x1080 (square pixels), because most players use simple resizing methods during playback, so final quality can be affected. In the same time when you do it you need higher bitrate as frame is bigger. There also issues with some players not reading aspect ratio flags in MP4 files, so it's safer to use square pixels.
An excellent explanation (I think). I will have to read it a few more times to get my head around it, but in the meantime, I will be sure to encode 1920x1080.

Thanks for taking the time to explain this for me.

Doug
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Old 03-14-2014, 07:35 AM   #7
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Doug.

You may also want to be a little careful when setting up your project settings. Inside of the Aspect Ratio tab on project settings, Edius refers to both PAR, pixel aspect ratio, and SAR, screen aspect ratio or GV's terminology is Display Aspect. The inclusion of both types of aspect ratio, SAR and PAR, under one tab simply marked as Aspect Ratio, can lead to a lot of confusion to people who may not fully understand the differences. Both PAR and SAR settings should really be under two separate tabs.

Like I said before the issue you've been having will be down to the decoder/player not understanding the contents of the container.

If you are interested. The Anamorphic process originated in the military, as a two stage optical process. You can "almost" think of this as the first codec. The encoder was a piece of glass that squeezed and compressed the light horizontally. So the picture was stored as what looked like a vertically stretched image, although it is actually horizontally squeezed. Then the decoder in the process is another piece of glass that un-squeezes the light back to its original horizontal position.

The initial use of the Anamorphic process was not to save on anything, but was to get a wider aspect and relative field of view out of the film equipment without changing or re-manufacturing new equipment and film to the measurements of the new aspect/relative field of view.

As it happens, the process apparently is also helped by the fact that humans are not as susceptible to horizontal resolution as they are to vertical resolution. This must be something that applies to a five year old, as at my age I am not susceptible to either :)

The first use of this process electronically, at least for most of us. Would have been the introduction of wide screen TVs for standard definition. Because it was impossible to create another SD transmission and equipment standard. The new TVs either zoomed into a letterboxed picture inside a 4:3 frame to fill the screen, and lose a shed loud of resolution. Or, as it was for DVD and certain forward thinking broadcasters, usually the Japanese. The anamorphic process was applied to the picture electronically so that it fitted 16:9 content into a standard SD frame. And then either the 16:9 TV or the DVD or a combination of both would apply the horizontal pull and fit it into the 16:9 space.

Up to this point the Anamorphic process wasn't really used to save bandwidth, although its effects were the same. Actually, once the film industry realised the possibilities of this optical process, it meant that they could shoot with less perforations per frame and still get a great wide picture, but also save on film length in the process and therefore bandwidth. More4K's reference to bandwidth saving probably really came in with HDV, as the process helped with many things such as the sensor, and the ability to keep bandwidths down, although keeping the bandwidths down may not have been an exorcise in bit budgeting per se, but rather to help with balancing the technical limitation for the speed of the tape and how much was needed for a certain shooting time.

Cheers,
Dave.
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Old 03-14-2014, 09:07 AM   #8
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Wow Dave! That was far more information I ever expected to get from this question, but I truly appreciate it and find it very interesting.

Now I'd be very interested in hearing your perspective on the newest (and most annoying) aspect ratio - vertical video, brought on by the iphone!

Thanks again for your wonderful insight,

Doug
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Old 03-14-2014, 10:05 AM   #9
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Doug.

I love the iPhone and think it is one of the most underrated cameras out there. Although I personally can't stand the way people shoot portrait with them or any phone. But this is a taste thing and those who shoot like that are not cinematographers but people who know what they want and don't care for a smart *** like me :)

There are a few apps for the iPhone that allow you to jack up the data rate to around 50mbit with uncompressed audio and a fixed frame speed and manual iris and focus.

Here is a video shot on an iPhone and edited in Edius.

http://www.youtube.com/watch?v=4_XTYbMNLeM

You can also edit video and compose music on iPhones. This video has got music that was made on an ipad, it's an incomplete piece but will give you an idea. It's based on work by John Carpenter.

http://m.youtube.com/watch?v=e_7YH-0Hwe0

Cheers,
Dave.
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Old 03-14-2014, 12:02 PM   #10
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Please take any further Iphone discussion to the lounge.
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