[FFmpeg-user] sntsc

[Mark Filipak]

On 1/8/24 01:46, Andrew Randrianasulu wrote:
> пн, 8 янв. 2024 г., 09:13 Mark Filipak <markfilipak.imdb at gmail.com>:
> 
>> On 1/8/24 00:26, Andrew Randrianasulu wrote:
>>> пн, 8 янв. 2024 г., 05:14 Mark Filipak <markfilipak.imdb at gmail.com>:
>>>> What is it that you want to know?
>>>
>>> I think I like to try and see bigger picture behind acronyms.
>>
>> What acronyms?
> 
> SAR, DAR, PAR .....

DAR is a physical aspect (i.e., a W/H). It is the dots-by-lines aspect of the view. Note that I 
don't use the word "display" because that makes many people think of a whole screen. I use the word 
"view", instead, as in "lines per view", not "lines per display".

PAR is a physical aspect (i.e., a W/H). It is the pixels-by-rows aspect of the 'picture' matrix. 
Note that I use the word 'picture', an alias, because data matrices are not really pictures. But 
MPEG and the rest of the world call them "pictures" so I call them 'pictures'.

SAR is not a physical aspect, it's an aspect transform. It transforms pixels-by-rows to 
dots-by-lines. SAR = DAR/PAR means view(W/H)/'picture'(W/H). For example, (1024/576)/(720/576).

Details.

"DAR" means "display aspect ratio" and you probably know all about that.
It's dpl/lpv, i.e., (dots per line)/(lines per view).
    DAR
  dpl/lpv
1024/576 = 16/9 for 16:9 'PAL',
  768/576 =  4/3 for  4:3 'PAL',
  853/480 = 16/9 for 16:9 'NTSC',
  720/540 =  4/3 for  4:3 'NTSC'.

"PAR' means "picture aspect ratio".
It's ppr/rpp, i.e., (pixels per row)/(rows per 'picture').
   PAR
ppr/rpp
720/576 = 5/4 for 16:9 'PAL',
720/576 = 5/4 for  4:3 'PAL',
720/480 = 3/2 for 16:9 'NTSC',
720/480 = 3/2 for  4:3 'NTSC'.

"SAR" means "sample aspect ratio". That term is from the days of flying spot film scanners. Modern 
SAR is actually not an aspect at all. It's a transform.
SAR transforms pixels-by-rows into dots-by-lines [note1].
It's dpp/lpr, i.e., (dots per pixel)/(lines per row).
         'SAR'
        dpp/lpr
  (dpl/ppr)/(lpv/rpp) [note2]
(1024/720)/(576/576) =  1024/720   =  64/45  for 16:9 'PAL'  -- 64 dots per 45 pixels,
  (768/720)/(576/576) =   768/720   =  16/15  for  4:3 'PAL'  -- 16 dots per 15 pixels,
  (853/720)/(480/480) =   853/720   =  32/27  for 16:9 'NTSC' -- 32 dots per 27 pixels,
  (720/720)/(540/480) = 1/(540/480) = 1/(9/8) for  4:3 'NTSC' -- 9 lines per 8 rows [note3].

[note1] Though modern SAR is actually a transform -- a 'picture'-to-view transform (PVT) -- the 
"SAR" acronym has become so entrenched in the art that it would be impossible to get people to 
change to "PVT". I try to get around the issue by calling SAR-as-transform, 'SAR' (meaning it's an 
alias, not a real aspect ratio).

[note2] The following is not a 'stupid math trick', it is real.
'SAR' = dpp/lpr =  (dpl/ppr)/(lpv/rpp) =  (dpl/lpv)/(ppr/rpp) = DAR/PAR. For example,
'SAR' = dpp/lpr = (1024/720)/(576/576) = (1024/576)/(720/576) = (16/9)/(5/4) = DAR/PAR.

[note3] Why is 4:3 'NTSC' so different? Well, there's two ways to look at it:
1, for 4:3 'NTSC', 'SAR' is less than one while for the others, 'SAR' is greater than one, and
2, turning 4:3 'NTSC' into 640x480 would lose resolution whereas 720x540 preserves resolution.
The difference is moot because TVs don't use 'SAR' anyway. What they do is look at the size of their 
screen (e.g., 1920x1080), look at DAR in the PES header (e.g. 4/3), compute the size of the view 
based on that DAR (e.g. 1440x1080), and scale directly (e.g., turn 4-by-4 pixel blocks into 8-by-9 
dot blocks). That makes 'SAR' a kind of fiction (which it is in a way).

If you're unhappy with 'SAR', then just forget it. Know DAR and PAR though. Know them well.

-- Mark.