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Old 11th July 2005, 14:13   #1121  |  Link
SeeMoreDigital
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Quote:
Originally Posted by yaz
it worx right but ... it's not sar but (custom) par.
that's correct then (16/9)x(5/4)=(20/9) ~ 2.25:1
Agreed

I think it would make more sense if it read something like... "Custom PAR" and not "Sample Aspect Ratio": -




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Old 11th July 2005, 14:37   #1122  |  Link
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Custom PAR is misleading.
I modified it and it's "Sample AR" now...
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Old 11th July 2005, 14:49   #1123  |  Link
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SAR is pixel aspect ratio, no?

why not simply call it "Pixel AR" than?
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Old 11th July 2005, 15:01   #1124  |  Link
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Because "Pixel" and "Picture" are sooo similar sounding...

This should be the reason why at least since ISO/IEC 13818-2, MPEG uses "Sample AR" and "Display AR".
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Old 11th July 2005, 15:02   #1125  |  Link
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you guys make me cry can't you just come up with a unique and precise name for every single thing, nooo you have to make life diffcult.

here is my proposition, let's erase all text in the gui's so that no one knows what option does what, obscure but FUN FUN FUN!! you never know what you gonna get
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Old 11th July 2005, 16:14   #1126  |  Link
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Aspect Ratio Naming

And here was me thinking this sort of thing had been agreed ages ago

3ivx has been using the terms PAR (for Pixel Aspect Ratio) and DAR (for Display Aspect Ratio) for ages. XviD has also been using the term "Pixel Aspect Ratio" for ages - And the term "Picture Aspect Ratio" to describe the Display Aspect Ratio... but lets not go there!

In my opinion the term PAR (Pixel Aspect Ratio) is all we need because PAR offers the greatest flexibility when it comes to altering the shape of your encodes. Including the possibility of generating encodes to either, PAL 4:3 / 16:9 DAR or NTSC 4:3 / 16:9 DAR

That said, an incorrectly shaped encode with PAR signalling can be a bit of a pain to sort out "after the event".... Which is why I feel we could really do with an tool such as MPEG4 Modifier which x264 friendly.... but that's for another thread!


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Old 11th July 2005, 17:00   #1127  |  Link
bond
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Quote:
Originally Posted by LigH
Because "Pixel" and "Picture" are sooo similar sounding...

This should be the reason why at least since ISO/IEC 13818-2, MPEG uses "Sample AR" and "Display AR".
well "sample" can also mean the whole pic imho
i personally definitely wouldnt think of "pixel" when i read "sample"
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Old 11th July 2005, 17:26   #1128  |  Link
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Actually, i still think 'sample' is the most appropriate term. 'Sample' refers the smallest / undivisable unit of the signal, so it can't refer to the frame. And 'pixel' refers to what is shown on the screen ( meaning, after scaling ), so it would be inappropriate in that case. But that's bitching on the definition... ( /me thinks the mpeg4 guys spent at least a week in meetings just to decide that name )
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Old 11th July 2005, 17:28   #1129  |  Link
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Quote:
Originally Posted by bond
well "sample" can also mean the whole pic imho
i personally definitely wouldnt think of "pixel" when i read "sample"
Would anyone send me a sample video please
SCNR.

bis besser,
Tobias

P.S.: It would be too easy if we would understand each other right away, right?
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Old 11th July 2005, 17:30   #1130  |  Link
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BTW: Did someone already mention the "texel"? -- IMHO: "like in the specs" is okay.
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Old 11th July 2005, 18:21   #1131  |  Link
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As far as I'm aware SAR (Storage Aspect Ratio) refers to the pixel aspect ratio of the source as it stored digitally on electronic media, before correction...

In the case of DVD, a PAL 720x576 source, has a SAR of 1.25:1. And an NTSC 720x480 source, has a SAR of 1.50:1


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Last edited by SeeMoreDigital; 12th July 2005 at 07:28.
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Old 11th July 2005, 19:06   #1132  |  Link
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No

PAL 720x576 (4:3) has a SAR of 16:15
PAL 720x576 (16:9) has a SAR of 64:45
NTSC 720x480 (4:3) has a SAR of 8:9
NTSC 720x480 (16:9) has a SAR of 32:27
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Old 11th July 2005, 19:16   #1133  |  Link
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May depend on how exact you calculate: 702:{768|1024} or 704:{768|1024}. But such cancelled fractions shall be suitable as long as the differences between the ITU specs and the calculated fractions are below recognition...
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Old 11th July 2005, 21:33   #1134  |  Link
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Quote:
Originally Posted by Manao
No

PAL 720x576 (4:3) has a SAR of 16:15
PAL 720x576 (16:9) has a SAR of 64:45
NTSC 720x480 (4:3) has a SAR of 8:9
NTSC 720x480 (16:9) has a SAR of 32:27
Errrm... as far as I'm aware....

PAL 720x576 (4:3) has a DAR of 16:15
PAL 720x576 (16:9) has a DAR of 64:45
NTSC 720x480 (4:3) has a DAR of 8:9
NTSC 720x480 (16:9) has a DAR of 32:27


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Old 11th July 2005, 21:36   #1135  |  Link
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@ SMD:

No. 4:3 video has a DAR of 4:3, 16:9 video has a DAR of 16:9 (DAR = "Display ASpect Ratio" is the whole picture, after resizing: width:height). At least it is documented this way in the MPEG-2 specification ISO/IEC 13838 Part 2 in chapter 6.3.3.

MPEG-2 video with aspect ratio flags "DAR 4:3" or "DAR 16:9" have to be resized so that the ratio between the width and the height of the decoded frame has to become 4:3 or 16:9.

Last edited by LigH; 11th July 2005 at 21:38.
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Old 11th July 2005, 21:54   #1136  |  Link
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In the case of say, an PAL 16:9 anamorphic DVD...

An 720x576 (1:1) source image without correction = SAR
An 720x576 with 64:45 PAR correction = DAR


EDIT: Or as SteveG put it..........

Quote:
Originally Posted by stegre
==================================================================================
Assuming

DAR = display aspect ratio - x/y ratio of physical dimensions (e.g inches or cm - NOT pixels) of display screen
PAR = pixel aspect ratio - x/y ratio of the physical dimensions of a single pixel
SAR = storage aspect ratio - x/y ratio of the #of pixels being encoded, always known from the file

==================================================================================
MPEG-4

DAR = PAR x SAR, where

PAR is based on "aspect_ratio_info" in the Video Object Layer

For aspect_ratio_info values 1 - 5,
PAR = 1:1, 12:11, 10:11, 16:11, and 40:33 respectively.

If aspect_ratio_info = 15, PAR is the custom value:
"par_width" / "par_height"

where par_width & par_height are any two relatively prime integers
between 1 and 255. Those two values are stored in the VOL as well.

==================================================================================
MPEG-2

Based on "aspect_ratio_information", stored in the Sequence Header

If aspect_ratio_information is "1", PAR = 1:1, and therefore DAR = SAR

If aspect_ratio_information is "2", "3", or "4", DAR = 4:3, 16:9 or 2.21:1 respectively,
in which case PAR = DAR/SAR

However (this is relatively rare):

If Sequence Display Extension is present and display_horizontal_size
and display_vertical_size are present and differ from the SAR,
then their ratio will replace SAR in the above equation.

==================================================================================
MPEG-1

Based on "pel_aspect_ratio" located in the Sequence Header

DAR = PAR x SAR

where PAR = the reciprocal of 1.0000, 0.6735, 0.7031, 0.7615, 0.8055,
0.8437, 0.8935, 0.9157, 0.9815, 1.0255, 1.0695, 1.0950, 1.1575 or
1.2015 for pel_aspect_ratio values of "1" thru "14" respectively.

However:

0.9157 is commonly accepted to mean 11/12 or .9166, the reciprocal of 12/11 (1.091) and,
1.0950 is commonly accepted to mean 11/10 or 1.1, the reciprocal of 10/11 (0.909)

==================================================================================

more or less, anyway
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Last edited by SeeMoreDigital; 11th July 2005 at 22:03.
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Old 11th July 2005, 22:07   #1137  |  Link
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Well, his definition of SAR is his own, but not the standard's one, that's all. Once and for all, SAR means SAMPLE aspect ratio, ie, what he calls pixel aspect ratio.
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Old 11th July 2005, 22:31   #1138  |  Link
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Okay think of SAR x PAR = DAR this way...

An SAR of 1.25 (ie: 720x576) x an PAR of 1.0666666 (ie: 16:15) = an DAR of 1.3333333 (aka 4:3)
An SAR of 1.25 (ie: 720x576) x an PAR of 1.4222222 (ie: 64:45) = an DAR of 1.7777777 (aka 16:9)

An SAR of 1.50 (ie: 720x480) x an PAR of 0.8888888 (ie: 8:9) = an DAR of 1.3333333 (aka 4:3)
An SAR of 1.50 (ie: 720x480) x an PAR of 1.1851851 (ie: 32:27) = an DAR of 1.7777777 (aka 16:9)


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Old 11th July 2005, 22:48   #1139  |  Link
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@SeeMoreDigital,

You are both right, but the problem is the naming of things. The MPEG-2 specs (page 47) says the following

SAR = DAR * horizontal_size/vertical_size

with horizontal_size the width of the displayable part of the luma component of pictures in samples (vertical_size idem ...). I think this just means the number of horizontal pixels.

So, what they call 'horizontal_size/vertical_size' is called 'storage aspect ratio' by Stegre. What they call SAR (or sample aspect ratio) is called '1 / PAR' by Stegre.

So, for example

SAR = DAR * horizontal_size/vertical_size = 3/4 * 720/576 = 0.9375

Interestingly, the MPEG-2 doesn't mention PAR at all. So, a question for Manao, where is the term PAR coming from (if it isn't mentioned in any specs)?

edit: i didn't know the DAR values in the MPEG-2 specs are actually 3/4, 9/16, etc ...

Last edited by Wilbert; 14th July 2005 at 16:36.
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Old 11th July 2005, 22:57   #1140  |  Link
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Quote:
Originally Posted by Wilbert
Interestingly, the MPEG-2 doesn't mention PAR at all. So, a question for Manao, where is the term PAR coming from (if it isn't mentioned in any specs)?
MPEG-1 specs, ISO/IEC 11172 Part 2, Chapter 2.4.3.2 "Sequence header":

pel_aspect_ratio -- This is a four-bit integer defined in the following table.

pel_aspect_ratio - height/width - example
0000 - forbidden
0001 - 1.0000 - VGA etc.
0010 - 0.6735
0011 - 0.7031 - 16:9, 625line
0100 - 0.7615
0101 - 0.8055
0110 - 0.8437 - 16:9, 525line
0111 - 0.8935
1000 - 0.9375 - CCIR601, 625line
1001 - 0.9815
1010 - 1.0255
1011 - 1.0695
1100 - 1.1250 - CCIR601, 525line
1101 - 1.1575
1110 - 1.2015
1111 - reserved
__

Unfortunately, I do not yet know where to find MPEG-4 specifications...
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