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 Doom9's Forum Sample Aspect Ratio calcutions
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 1st February 2006, 04:52 #1  |  Link kotrtim Registered User   Join Date: Dec 2005 Posts: 145 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
 1st February 2006, 10:28 #2  |  Link yaz n00b ever   Join Date: May 2002 Posts: 627 @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``` the bests y Last edited by yaz; 1st February 2006 at 10:31.
 2nd February 2006, 20:57 #3  |  Link Raithmir Registered User   Join Date: Dec 2003 Posts: 51 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.
 2nd February 2006, 21:03 #4  |  Link Raithmir Registered User   Join Date: Dec 2003 Posts: 51 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.
 3rd February 2006, 05:06 #5  |  Link kotrtim Registered User   Join Date: Dec 2005 Posts: 145 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
3rd February 2006, 06:41   #6  |  Link
bkman
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Join Date: Jan 2006
Posts: 294
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??

 3rd February 2006, 12:02 #7  |  Link yaz n00b ever   Join Date: May 2002 Posts: 627 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 the bests y
 3rd February 2006, 12:29 #8  |  Link bkman Registered User   Join Date: Jan 2006 Posts: 294 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)?
 4th February 2006, 22:34 #9  |  Link DigitalDivide Registered User   Join Date: Jul 2005 Posts: 240 Not being the best at math, is there any easy way to work out the lowest common denominator? Is it possible to enter in 1920:2112 as the SAR in MeGUI or must I absolutely break it down further? __________________ Toronto,ON
 4th February 2006, 22:50 #10  |  Link DigitalDivide Registered User   Join Date: Jul 2005 Posts: 240 So for my 2.35 NTSC DVD after cropping 47 368 17296 -- x --- = ----- which breaks down to ? This us entered as SAR in MeGUI 20 704 14080 Is this correct? __________________ Toronto,ON
5th February 2006, 00:50   #11  |  Link
SeeMoreDigital
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Location: Notts, UK
Posts: 11,328
Quote:
 Originally Posted by DigitalDivide So for my 2.35 NTSC DVD after cropping 47 368 17296 -- x --- = ----- which breaks down to? This us entered as SAR in MeGUI 20 704 14080 Is this correct?
Yes... Initially this would provide an ARS of 1081:880.

However, this could be further converted to say: -
Code:
```1080                             27
---- which, in turn decimates to --
880                             22```
That said, 199:162 would be more exact

Cheers
__________________
| I've been testing hardware media playback devices and software A/V encoders and decoders since 2001 | My Network Layout & A/V Gear |

Last edited by SeeMoreDigital; 5th February 2006 at 01:05.

5th February 2006, 18:00   #12  |  Link
DigitalDivide
Registered User

Join Date: Jul 2005
Posts: 240
Quote:
 Originally Posted by SeeMoreDigital Yes... Initially this would provide an ARS of 1081:880. However, this could be further converted to say: - Code: ```1080 27 ---- which, in turn decimates to -- 880 22``` That said, 199:162 would be more exact Cheers
SMD,

Is there a formula as to how to work out the lowest common den? Is it just guess work with a calculator? Give me something to go on..lol
__________________
Toronto,ON

5th February 2006, 18:17   #13  |  Link
SeeMoreDigital
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Join Date: Jun 2003
Location: Notts, UK
Posts: 11,328
Quote:
 Originally Posted by DigitalDivide SMD, Is there a formula as to how to work out the lowest common den? Is it just guess work with a calculator? Give me something to go on..lol
I use a Sharp EL-530V calculator to work out and simplify all these fractions.... But I'm thinking about developing a little "ARS Calculator" tool... Something like this: -

EDIT: Here's a 2.35:1 video sample, with a pixel frame size of 704x368 and aspect ratio sigalling at 199:162.

Cheers
__________________
| I've been testing hardware media playback devices and software A/V encoders and decoders since 2001 | My Network Layout & A/V Gear |

Last edited by SeeMoreDigital; 5th February 2006 at 22:33.

 5th February 2006, 18:53 #14  |  Link DigitalDivide Registered User   Join Date: Jul 2005 Posts: 240 You should definitely create that ARS calculator. It would definitely help a lot and I'm sure everyone would appreciate it. __________________ Toronto,ON
 6th February 2006, 02:37 #15  |  Link iceborne Registered User     Join Date: Aug 2003 Posts: 43 i second that
 6th February 2006, 10:43 #16  |  Link Wilbert Moderator   Join Date: Nov 2001 Location: Netherlands Posts: 6,310 SMD likes to stick to non-standard terminology. I think his MAR is the playback AR (after cropping away the black borders). If that's the case, than in many cases you don't know the MAR in advance, unless it is specified on the box of the dvd.
6th February 2006, 19:09   #17  |  Link
SeeMoreDigital
Life looks better in UHD

Join Date: Jun 2003
Location: Notts, UK
Posts: 11,328
Quote:
 Originally Posted by Wilbert SMD likes to stick to non-standard terminology. I think his MAR is the playback AR (after cropping away the black borders). If that's the case, than in many cases you don't know the MAR in advance, unless it is specified on the box of the dvd.
Yes.... MAR (Movie Aspect Ratio) is the term I'm proposing we adopt to refer to the aspect ratio of the original source.

After looking at the back of a few DVD cases I've found several other expressions, such as: Original Theatrical Presentation, Original Theatrical Exhibition, Presented in x.xx:x, TV Aspect Ratio.

Cheers
__________________
| I've been testing hardware media playback devices and software A/V encoders and decoders since 2001 | My Network Layout & A/V Gear |

 8th February 2006, 11:40 #18  |  Link Seb.26 20-35 a win for Revolver.     Join Date: Mar 2004 Location: FR.26 Posts: 221 Hum ... long & interesting post ! so I've a question : is my way is right or fully ...bad... ? My way : For both PAL and NTSC DVD, I just crop up and bottom ( if side borders are not too large ) ... never resize ! And I know : > PAL : 720*576 -> 1024*576 > NTSC : 720*480 -> 854*480 So if my cropped frame is 720*416 (PAL) for exemple , my AR in MkvMergeGui is 1024*416 ... is it right or not ?! Thanks everybody
 8th February 2006, 11:47 #19  |  Link SeeMoreDigital Life looks better in UHD     Join Date: Jun 2003 Location: Notts, UK Posts: 11,328 If we are not following the ITU specification to the letter, which most (if not all) software players, don't appear to follow.... then the answer is yes __________________ | I've been testing hardware media playback devices and software A/V encoders and decoders since 2001 | My Network Layout & A/V Gear |
8th February 2006, 11:55   #20  |  Link
Seb.26
20-35 a win for Revolver.

Join Date: Mar 2004
Location: FR.26
Posts: 221
Quote:
 Originally Posted by SeeMoreDigital If we are not following the ITU specification to the letter[...]
Please : Where could I read some papers about that ? ... I don't know what is it
 -> I will read all on your www ... it looks great !

Quote:
 Originally Posted by SeeMoreDigital [...] then the answer is yes
Ok, so I'm not fully idiot ...

My way looks really simple beside all your heavy-sci-math-computing

So if I'm wrong for 1 ou 2 pixels in height ... what the matter ?!

( from where is your avatar ? a movie, but which ? )

Last edited by Seb.26; 8th February 2006 at 12:06.