On 1/8/24 01:46, Andrew Randrianasulu wrote:
пн, 8 янв. 2024 г., 09:13 Mark Filipak <[email protected]>:
On 1/8/24 00:26, Andrew Randrianasulu wrote:
пн, 8 янв. 2024 г., 05:14 Mark Filipak <[email protected]>:
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.
_______________________________________________
ffmpeg-user mailing list
[email protected]
https://ffmpeg.org/mailman/listinfo/ffmpeg-user
To unsubscribe, visit link above, or email
[email protected] with subject "unsubscribe".
