[OT] Why do so few people understand aspect ratios?

[Thomas Milne]

On Thu, May 27, 2010 at 1:49 PM, Lennart Sorensen
<lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> On Thu, May 27, 2010 at 01:34:36PM -0400, Thomas Milne wrote:
>> Unfortunately, it's getting worse lately, perhaps because a lot of
>> amateurs are getting involved, and people using crappy
>> equipment/software.
>
> Perhaps.  I still suspect encoding files recorded from TV that was
> broadcasting letter boxed.  I really hope that's all it is.
>
>> EXACTLY, thank you. I have to admit, I was kinda waiting for you to
>> weigh in on this ;)
>>
>> I can't complain _too much_, because the videos I'm talking about are
>> not 'legitimate'. I just didn't realize there were actually displays
>> out there that were such bad quality.
>
> I really hope this only happens where people encode the broadcast signal
> (or perhaps video camera image if in a theatre).  Since the video contains
> the aspect ratio of the broadcast or video recorder in that case, the
> black bars would be part of the video stream and were never actually
> added by the person encoding the file.  In any other case it would be
> just wrong.
>
> For example:
>
> $ ffprobe 5yrs-linux_1080p_loq.mp4
> FFprobe version SVN-r92, Copyright (c) 2007-2009 Stefano Sabatini
>  libavutil     49.15. 0 / 49.15. 0
>  libavcodec    52.20. 0 / 52.67. 0
>  libavformat   52.31. 0 / 52.62. 0
>  built on Jan 30 2010 11:08:30, gcc: 4.4.3
> Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '5yrs-linux_1080p_loq.mp4':
>  Metadata:
>    major_brand     : isom
>    minor_version   : 512
>    compatible_brands: isomiso2avc1mp41
>    encoder         : Lavf52.61.0
>  Duration: 01:20:05.05, start: 0.000000, bitrate: 15262 kb/s
>    Stream #0.0(und): Video: h264, yuv420p, 1920x1080 [PAR 1:1 DAR 16:9], 15002 kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc
>    Stream #0.1(und): Audio: ac3, 48000 Hz, 2 channels, s16, 256 kb/s
>
> So it says that's a 1920x1080 file, with a pixel aspect ratio of 1:1
> (square pixels are nice, although a few things like DVDs sometimes
> have retangular pixels).  The overall disaply aspect ratio is 16:9.
> So the player is responsible for deciding if the pixels needs scaling
> up or down and the player is responsible for making sure that on the
> display it outputs to the result of the video content is 16:9 aspect.
> It can cut the sides or add black or pink bars if it needs to, but the
> data in the file itself is to be treated as 16:9 and nothing else.
>
> If a file contained black bars in the video itself, then it would almost
> certainly be a 16:9 or similar video encoded as 4:3 with black bars in
> the data.  Now the player can only know that this is a 4:3 video, and
> it the display happens to be 16:9 or 16:10 the player has to add bars
> to the side of the video so now you end up with a video taking up 1/4 of
> the screen in the center with bars all around.  That's why a video file
> should NEVER have black bars in the content.  So if you add them, you
> are an idiot.  If they are already there because you are capturing letter
> boxed video from a TV broadcast, well then I guess you are stuck with them
> unless you use clever software to cut off the black bars before encoding.
>
> Letter boxing should always be added by the player, not the encoder
> because only the player knows what the display is actually capable of.
>
> TV broadcasts being the exception, because they only have a fixed set
> of resolutions they can broadcast at.
>

Okay, that makes sense. What I'm seeing, I guess, is people uploading
shows that are supposed to be 16:9, but wherever this show is coming
from they're broadcasting a 4:3 stream, therefore the black bars are
part of the stream to compensate. The people doing this are probably
completely unaware of what they're doing, because with their display
it looks 'normal'.

Still, it causes havoc with real 16:9 displays because I end up with a
16:9 image, with black bars 'squashing' it so it's more like 2.33:1.
Unwatchable, of course.


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