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Old 1st February 2006, 04:52   #1  |  Link
kotrtim
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Sample Aspect Ratio calcutions

Source/Encoded Resolution ( X:Y )
Desired Display Resolution/Aspect Ratio ( H:V )

The formula to calculate PAR/SAR

Y X V x H = SAR fraction

for example a NTSC 16:9 with resolution 720:480

480 720 9 x 16 = 1.185185185... (32/27)
therefore the Sample Aspect Ratio that should be keyed in in MeGUI is 32x27

for NTSC 4:3 with resolution 704:480
480 704 3 x 4 = 0.88888...(10/11)
SAR = 10x11

CROPPING without resizing
won't affect the SAR

Any comments on the way I calculate SAR?
I took quite some time to understand that
SAR is actually the ratio of every single pixel..not
the ratio displayed by the video frame
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Old 1st February 2006, 10:28   #2  |  Link
yaz
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@kotrtim
the way u derive (s)ar is ok but why do u use different vertical resolutions for the 4/3 and for the 16/9 streams. maybe it's ok (i'm living in the pal world ) just ask.

however, the values calculated
Code:
- for ntsc 720 x 
4:3        64:72 (8:9)
16:9       64:54 (32:27)
- for ntsc 704 x
4:3        10:11
16:9       16:11
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Last edited by yaz; 1st February 2006 at 10:31.
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Old 2nd February 2006, 20:57   #3  |  Link
Raithmir
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Cropping will change the SAR, at least thats what I was told in the similar thread I posted. This comment might be of use, I saved it and use this calculation...

"The actual way meGUI calculates the SAR is as follows:
1) Apply the cropping and then determine the aspect ratio of the input video image (inputPixelCountRatio).
2) Scale the vertical resolution according to the horizontal resolution that's been specified (resizedVerticalResolution = horizontalResolution / inputPixelCountRatio)
3) Adjust the vertical resolution so that it matches the closest multiple of 16.
4) Set SARX to the horizontal resolution.
5) Set SARY to Round(HorizontalResolution^2 / (VerticalResolution * DAR)), this is then truncated to an integer."

So if you're not resizing you can ignore 1-3 and use the horizontal and vertical resolution after you've cropped.

Last edited by Raithmir; 2nd February 2006 at 21:04.
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Old 2nd February 2006, 21:03   #4  |  Link
Raithmir
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http://forum.doom9.org/showthread.php?t=104485

Some confusing information at first but read the whole thread.

No doubt someone else will come along and tell me it's all wrong and I'm doing it incorrectly again.
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Old 3rd February 2006, 05:06   #5  |  Link
kotrtim
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My opinion is SAR won't change after cropping as you crop off pixels, and SAR is the ratio of every pixel, so the value of SAR should not change whereas the DAR will change as DAR is the measurement of the whole frame, when you croped, the frame size definitely changes.

If we want to resize, why not just resize so that it is 1:1, of course this is just my opinion
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Old 3rd February 2006, 06:41   #6  |  Link
bkman
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Quote:
Originally Posted by kotrtim
My opinion is SAR won't change after cropping as you crop off pixels, and SAR is the ratio of every pixel, so the value of SAR should not change whereas the DAR will change as DAR is the measurement of the whole frame, when you croped, the frame size definitely changes.
For me this brings up the question of what is the true DAR of a DVD source. Ie. If you have what used to be a T.V. program, which as we know is 4:3 (right?), but it has black bars on the side and the resulting AR after cropping them off is not 4:3, what do you do then? Use the bar-less DAR when calculating the SAR, or use the DAR with bars included, or just use the 4:3 DAR that you know it must be as a T.V. source? Quite confusing. For that matter, how is 720x480 4:3??
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Old 3rd February 2006, 12:02   #7  |  Link
yaz
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cropping does not change the way of derivation (resizing). if u crop the stream (sar) u 'crop' also what u get on the display (dar). u'd better think of it as 'how to distort each pixel so as to get the right aspect ratio on the display'.

say, u got a pal 4:3 dvd. its stream size is 720x576 (by def) which is 5/4 ratio. it must be displayed 4/3 so as to get the correct ratio on the display, so, it must be resized to 768x576. it is the same as displaying each pixel in a 16x15 rectangle instead of a 15x15 square (measured in whatever unit).
if u crop it to 704x576 (for any reason) it must be resized the same way, so the display size must be ~751x576. otherwise the picture on the display will be distorted.

it's simple as that

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Old 3rd February 2006, 12:29   #8  |  Link
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But why is it that the "true" DAR includes black bars? Surely they are not part of the actual image, and it should be resized to 4:3 without them. Like if you cropped to 704x576 to give the full image, shouldn't that then be resized to 768x576 (instead of 751x as with counting the black bars) to give proper AR since the original T.V. show was 4:3 (and had no black bars originally)?
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Old 3rd February 2006, 13:33   #9  |  Link
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If you can guys, try and calculate "everything" as a fraction, because converting fractions to decimals and back to fractions again "can" produce conflicting Aspect Ratio Signalling (ARS) values.

If your NTSC movie has an aspect ratio of 1.33:1, then this can be represented as a fraction of 4/3. Meaning: -
Code:
4     480     1920                    8
-  x  ---  =  ----  lowest dominator  -  Giving an ARS value of 8:9
3     720     2160                    9
If your NTSC movie has an aspect ratio of 1.77:1, then this can be represented as a fraction of 16/9. Meaning: -
Code:
16     480     7680                    32
--  x  ---  =  ----  lowest dominator  --  Giving an ARS value of 32:27
 9     720     6480                    27
If your NTSC movie has an aspect ratio of 1.85:1, then this can be represented as a fraction of 37/20. Meaning: -
Code:
37     480     17760                    37
--  x  ---  =  -----  lowest dominator  --  Giving an ARS value of 37:30
20     720     14400                    30
If your NTSC movie has an aspect ratio of 2.35:1, then this can be represented as a fraction of 47/20. Meaning: -
Code:
47     480     22560                    47
--  x  ---  =  -----  lowest dominator  --  Giving an ARS value of 47:30
20     720     14400                    30
If your NTSC movie has an aspect ratio of 2.40:1, then this can be represented as a fraction of 12/5. Meaning: -
Code:
12     480     5760                    8
--  x  ---  =  ----  lowest dominator  -  Giving an ARS value of 8:5
 5     720     3600                    5
For working out the "Aspect Ratio Signalling" values of PAL sources, simply substitute 480/720 to 576/720 and recalculate.

The same formula can be used to work out the "Aspect Ratio Signalling" values of cropped and re-sized encodes too by entering the relevant resolutions.


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Last edited by SeeMoreDigital; 3rd February 2006 at 16:01.
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Old 3rd February 2006, 14:07   #10  |  Link
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Quote:
Originally Posted by SeeMoreDigital
The same formula can be used to work out the "Aspect Ratio Signalling" values of cropped and re-sized encodes too by entering the relevant resolutions.
See, that is what I have been doing, but what yaz says seems to contradict this.

So that is the question that I haven't seen everyone agree upon: If you have a movie/show that you "know" the aspect ratio of (16:9, 4:3, etc) but it has black bars which need to be cropped, do you a.) Use the cropped size in your SAR calculation, or b.) Use the original size before cropping?
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Old 3rd February 2006, 14:43   #11  |  Link
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Quote:
Originally Posted by bkman
So that is the question that I haven't seen everyone agree upon: If you have a movie/show that you "know" the aspect ratio of (16:9, 4:3, etc) but it has black bars which need to be cropped, do you a.) Use the cropped size in your SAR calculation, or b.) Use the original size before cropping?
What I do is establish the aspect ratio of the source movie. The popular "Movie Aspect Ratio's" (MAR's) are, 1.85:1, 2.35:1, 2.40:1

All MAR's can be converted to a fraction. For example: -
1.85:1 works out at 37/20
2.35:1 works out at 47/20
2.40:1 works out at 12/5

The MAR value can then be placed into the equation like this: -
Code:
MAR    FAR                              ARS 
47     368     17296                    1081
--  x  ---  =  -----  lowest dominator  ----  Giving an ARS value of 1081:900
20     720     14400                     900
Which can be further simplyfied by converting: -
Code:
1081      1080                    6
----  to  ----  lowest dominator  -  Giving an ARS value of 6:5
 900       900                    5
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Last edited by SeeMoreDigital; 3rd February 2006 at 16:00.
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Old 3rd February 2006, 15:06   #12  |  Link
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Well, yes I understand that, but it doesn't really answer my question about how to deal with DVD's that require cropping off the black bars.

Another related question is: Why are the padding bars even there on the DVD? Are they part of the original image as it was meant to be displayed or just artifacts of DVD conversion? Answering this will answer the first question also.
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Old 3rd February 2006, 15:27   #13  |  Link
yaz
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Quote:
Originally Posted by bkman
But why is it that the "true" DAR includes black bars? Surely they are not part of the actual image, and it should be resized to 4:3 without them.
definitely not. really, black bars are not part of the movie but they are part of the image/stream. they are used to maintain the stream dimensions which must(!) be always 720x576 for pal dvd and 720x480 for ntsc dvd. whithin this fixed-size box u can place any stream having quite arbitrary dimensions. (say, 654x321 is ok, if its 'bordered' to 720x576/480)
this box is resized during the playback according to the 'par' value (which simply defines how much it should be streched horizontally) calculated as showed above.

don't ask me why is it figured out like this but that's what we have

Quote:
Originally Posted by bkman
Like if you cropped to 704x576 to give the full image, shouldn't that then be resized to 768x576 (instead of 751x as with counting the black bars) to give proper AR since the original T.V. show was 4:3 (and had no black bars originally)?
hmm ... can't get u here, but ... the stream must be shown 4/3 on your screen/display/tv but how it's stored in a file/dvd/whatever depends on the intention of the creator. there are possibilities (see above).
about the borders ... maybe they just cut away some dirty/bleeding/shifted/ringing/... pixels round the pictures for getting better compression. dunno

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Old 3rd February 2006, 15:40   #14  |  Link
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Quote:
Originally Posted by bkman
Well, yes I understand that, but it doesn't really answer my question about how to deal with DVD's that require cropping off the black bars.
When you crop away the black mattes from a DVD source, the important thing is too make sure you end up with horizontal and vertical resolutions that are in multiples of 16 pixels (aka: "mod16").

However, because this is not always possible with cropping alone, you may end up having to re-size the image slightly until you obtain a "mod16" frame size.

In theory, it is possible to display an encode at any shape you want, once the required AR signalling value has been applied.

Quote:
Originally Posted by bkman
Another related question is: Why are the padding bars even there on the DVD? Are they part of the original image as it was meant to be displayed or just artifacts of DVD conversion?
The black mattes are there because that is what was agreed upon and later defined within the MPEG-2 DVD standard.

It was decided that the most effective way of displaying images with different aspect ratios was to overlay the "image portion" onto a "black background portion" (aka: matte) with a fixed Frame Aspect Ratio (FAR). So this is why all commercial PAL DVD's have an fixed FAR of 720x576 or 5/4. All commercial NTSC DVD's have a fixed FAR of 720x480 or 3/2.

Hope that helps.... a bit
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Last edited by SeeMoreDigital; 3rd February 2006 at 15:44.
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Old 3rd February 2006, 15:45   #15  |  Link
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Hmm, I'm still not quite satisfied with an answer, so let me phrase the question in another way with an example case:

You have T.V. show on an NTSC DVD. The video has 8 pixels of black padding on either size, so you crop them off and are left with a resolution of 704x480. What SAR would you set in this case? 10:11? 8:9? Something else?
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Old 3rd February 2006, 15:59   #16  |  Link
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Quote:
Originally Posted by bkman
Hmm, I'm still not quite satisfied with an answer, so let me phrase the question in another way with an example case:

You have T.V. show on an NTSC DVD. The video has 8 pixels of black padding on either size, so you crop them off and are left with a resolution of 704x480. What SAR would you set in this case? 10:11? 8:9? Something else?
If you followed the formula I provided you would be able to work it out....

Code:
MAR   FAR                             ARS 
4     480     1920                    10
-  x  ---  =  ----  lowest dominator  --  Giving an ARS value of 10:11
3     704     2112                    11
I'm presuming the source is intended to be displayed at 4:3?
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Old 3rd February 2006, 15:59   #17  |  Link
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Quote:
Well, yes I understand that, but it doesn't really answer my question about how to deal with DVD's that require cropping off the black bars.
I would say use the original size for calculations, because SAR is the heightXwidth of every little pixel, cropping off the black bars won't affect

lets say you have a pice of paper, 100 cm long, 10 cm width, you divide eqaully into 10 cells (similar to pixels), of course 100/10 = 10cm, therefore each box is 10cm x10cm, the sample ratio is 1x1, now cut off one cell, the piece of paper is now 90cm long, 10cm width, but the ratio of each cell is still 1x1, it never change, you can still use the previous equation 100/10 = 10cm , not 90/10.....

but for DAR, it is changed, before you cut, DAR is 100x10, after you cut it is 90x10

This is what I understand about cropping without resizing, hope you can imagine what i'm trying to tell

Last edited by kotrtim; 3rd February 2006 at 16:11.
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Old 3rd February 2006, 16:08   #18  |  Link
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kotrtim,

I think you are thinking about "square" pixels and not "anamorphic" (aka: non square) pixels.

Do you understand the differences?


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Old 3rd February 2006, 16:23   #19  |  Link
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I agree with kotrtim here. Your formula doesn't apply directly because 4:3 is _not_ the movie aspect ratio of the cropped image. However, the SAR doesn't change when cropping.
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Old 3rd February 2006, 16:24   #20  |  Link
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Quote:
Originally Posted by SeeMoreDigital
I'm presuming the source is intended to be displayed at 4:3?
Good question. How would you know definitively? Are all NTSC Television sources 4:3? If so, why the padding bars?
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