>There are two kinds of aspect ratios in the wiki.
>
>,-----
>| pixel aspect ratio or PAR (also known as "sample aspect ratio" or SAR), is
>| the ratio of the width to the height of a single pixel in the video image.
>`-----
>
>This seems contradict with the pixel aspect ratio definition used in the
>transcode manual:
>
>______________________________________________________________
>--export_par C[,N] set export pixel aspect ratio to C[,N]
>
>To encode anamorphic material, transcode can encode the target pixel aspect
>ratio into the file. This is NOT the actual aspect ratio of the image, but
>only the amount by which every single pixel is stretched when played with an
>aspect ratio aware application, like mplayer.
>______________________________________________________________
These are saying the same thing; if anything, the transcode manual page
definition is rather misleading, but both refer to the same ratio of pixel
width to pixel height.
>The way PAR is defined in transcode manual is more widely accepted. Most
>video converting tools stick to such definition. E.g., mplayer, tovid,
>ffmpeg, etc.
I'm not aware that any of those define PAR differently from its actual,
er, definition. Can you provide examples?
>,-----
>| One is display aspect ratio or DAR. this is the ratio most commonly referred
>| to by the term "aspect ratio", and is the ratio of the video frame's
>| physical (displayed) width to its height, regardless of the number of pixels
>| used to represent the video image.
>`-----
>
>Ok, here is another aspect ratio, DAR. What I can't make out is that neither
>of above 2 aspect ratios are the commonly used 4:3 or 16:9 video aspect
>ratio according to the wiki. seems to me there are 3 aspect ratios implied
>in wiki, the most commonly used is unnamed.
Um, what? DAR is in fact the "4:3" or "16:9" video aspect ratio you refer
to. What makes you think otherwise?
>,-----
>| DAR width
>| --- = ------
>| PAR height
>`-----
>
>hmm... any reason behind this? I can't figure why there is such equation.
The four terms are defined as:
physical video width
DAR = ---------------------
physical video height
physical pixel width
PAR = ---------------------
physical pixel height
physical video width
width = -------------------- (i.e. number of pixels across one line)
physical pixel width
physical video height
height = --------------------- (i.e. number of lines in the video image)
physical pixel height
Basic mathematics then gives us:
DAR 1
--- = DAR * ---
PAR PAR
physical video width physical pixel height
= --------------------- * ---------------------
physical video height physical pixel width
physical video width * physical pixel height
= --------------------------------------------
physical video height * physical pixel width
physical video width * physical pixel height
= --------------------------------------------
physical pixel width * physical video height
physical video width physical pixel height
= -------------------- * ---------------------
physical pixel width physical video height
1
= width * ------
height
width
= ------
height
>Further, since most LCD televisions and monitors have a PAR of 1:1, so
>mostly DAR = DAR / 1:1 = width / height. but this will contradict with the DAR
>definition above.
Okay, I see what's confusing you. PAR is being used to refer to two kinds
of aspect ratio: the PAR of the video data (the size of one pixel in each
video frame) and the PAR of hardware display devices (the actual dots that
light up on your screen). I'll try to clarify that.
>,-----
>| As another example, suppose you have a 672x272 video file on your computer
>| that you want to transcode to an NTSC (720x480) DVD.
>`-----
>
>The "common-sense" aspect ratio, ie 4:3 or 16:9, for this 672x272 video file
>is not mentioned in the wiki. Would the calculation that follows still
>hold, if the video file is of 4:3 or 16:9, or change between them?
This was taken directly from a question on the mailing list. The person
who asked didn't (to my recollection) specify the aspect ratio, but my
impression was that it was a standard AVI file with a PAR of 1:1, so the
DAR would have been simply 672:272 (or 42:17 in reduced form).
I think it may be clearer if I add more examples and move them to a
separate page, so I'll do that.
>I have two suggestions.
>
>- would it be possible to base your wiki according to transcode manual?
> if the wiki is entirely different than the transcode manual, people
> would have a hard time trying to follow both.
As mentioned above, it's the transcode manual that's unclear and needs to
be rewritten. This will be accomplished at some point in the future, but
the wiki should be considered the primary source of documentation.
--Andrew Church
[EMAIL PROTECTED]
http://achurch.org/
