[MPlayer-users] Transcoding from one aspect ratio to another

Rolf Ernst rolf.ernst at silverlightning.org
Wed Nov 3 20:43:58 CET 2010

On 11/3/2010 2:05 PM, VDR User wrote:
> On Wed, Nov 3, 2010 at 11:45 AM, Rolf Ernst
> <rolf.ernst at silverlightning.org>  wrote:
>> 1:2.35 is standard anamorphic widescreen.
> I've worked with video filmed in many different ratios so while a lot
> of widescreen material actually is 1:2.35, to say it's standard is
> misleading.  Also in an earlier post you mentioned that reducing
> 1920x1080 to 720x480 gives you an identical image.  This is misleading
> as well.  I understand wanting to keep the conversation simple I guess
> but it doesn't hurt to give people accurate information to learn from
> either.  It's like calling something black when it's actually grey.
While you are, of course, correct I sort of wanted to give the time 
instead of explaining how the clock is built. Widescreen is anything 
that has a width > height. Not sure that is helpful. 

Common formats are 16:9, 1.85:1, 2.35:1. But really anything can be 
widescreen. Of course, an image where height >= width but has black 
orders on top and bottom is also often referred to as widescreen. Of 
course, resizing the image to a completely different pixel count and 
aspect ratio does not give 'the same image' but I think it is more 
ingenuous than delving into

a.) If you start out with something like a BluRay 1080p which is really 
represented by 1920x1080 pixels and watch a widescreen movie which was 
recorded at a 2.25:1 resolution a portion of the pixes at the bottom of 
the top of the image are actually represented as black pixels as the 
format does not allow for an anamorphic representation which would 
compensate for the difference in shape of the pixels used.

b.) If you transcode to a format which carries pixel shape information 
it is recommended to instead trim those black borders which for a 2.35:1 
would be represented by roughly 140 pixels at top and bottom.

c.) If the target format supported a descriptor for the shape of the 
individual pixels, often referred to as SAR and DAR you could use any 
pixel count you wished as target picture size. The MP4 container 
supports aspect ratio information.

d.) If you subsequently transcoded to a common DVD resolution (U.S.) 
such as 720:480 and included the proper pixel shape information the 
picture would not appear skewed. However, the pixel shape would not be 
square but the 480 pixels represent more picture information relative to 
the 720 for a display aspect ratio of 2.35:1

e.) However, sine 1080-(140*2) = 800 the 'crispness' or information 
carried in the picture information is different, especially considering 
that the width of the source = 1920 whereas that of the target is 720.

All nice and good but I believe this would confuse the author of the 
question a lot more than saying that you can let mencoder resize to any 
target and 'it will look identical' (which is technically incorrect but 
understandable to someone that asked the question considering the 
triviality of the question in the first place).

Anyhow, just giving the time and not building a clock.

"My friends, watch out for the little fellow with an idea"

Tommy Douglass

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