[opendtv] Re: News: See the Big Picture? Don't Forget to Examine the Fine Print
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 4 May 2004 22:31:53 -0400
Thanks John
I don't know how I missed that.
:-(
Regards
Craig
At 3:34 PM -0700 5/4/04, John Willkie wrote:
>really? You mean that SAR in the legend for the table you quoted means
>something other than "screen aspect ratio" and DAR in the same legend means
>something other than "display aspect ratio?" I use the definitions there.
>Which you already alluded to.
>
>You quoted the text. Didn't you peruse it? Did you fail to understand it?
>
>Samples can be square: in this context (I learn from my mistakes) that means
>that the "aspect ratio" of the sample matches the display aspect ratio.
>Anything else is a "non-square" picture element.
>
>And, to nip your tailtext in the end: I am not talking about private data
>elements. I am speaking of mandatory PUBLIC data; MPEG-2 metadata, which
>comprises a good chunk of broadcast metadata, a field that I am somewhat
>familiar with.
>
>Happy to be of assistance.
>
>John Willkie
>
>-----Original Message-----
>From: opendtv-bounce@xxxxxxxxxxxxx
>[mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Craig Birkmaier
>Sent: Tuesday, May 04, 2004 1:43 PM
>To: opendtv@xxxxxxxxxxxxx
>Subject: [opendtv] Re: News: See the Big Picture? Don't Forget to
>Examine the Fine Print
>
>
>Please define "screen aspect ratio." This is not a term that is used
>in the MPEG-2 context.
>
>Are you trying to say that the sample aspect ratio is NOT 1:1 (i.e. square)?
>
>Regards
>Craig
>
>I would also add, that it is possible to signal almost anything in
>the private data area of the MPEG-2 transport standard.
>
>
>
>At 12:50 PM -0700 5/4/04, John Willkie wrote:
>>Really? it doesn't limit? Then, pray tell (note the tone, if you call that
>>rude, well, that's a clue) please indicate how, using the existing
>>mechanisms, I would indicate a screen aspect ratio of 1:2.88 with a display
>>aspect ratio of 1:0.72. Note: I made it "somewhat" easy, as 0.72 can be
>>easily factored into 1:2.88.
>>
>>But, you say that MPEG-2 is flexible. So, tell me how I would signal such
>a
>>combination using the MPEG-2 infrastructure.
>>
>>And, if you can't, rather than just provide material that proves MY POINT,
>>would you for once concede?
>>
>>John Willkie
>>
>>-----Original Message-----
>>From: opendtv-bounce@xxxxxxxxxxxxx
>>[mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Craig Birkmaier
>>Sent: Tuesday, May 04, 2004 11:57 AM
>>To: opendtv@xxxxxxxxxxxxx
>>Subject: [opendtv] Re: News: See the Big Picture? Don't Forget to
>>Examine the Fine Print
>>
>>
>>At 10:00 AM -0700 5/4/04, John Willkie wrote:
>>>You are VERY incorrect.
>>
>>Really?
>>
>>I will disregard this statement and its rudeness,=20
>>as i do much of what you spout in this forum.
>>
>>=2EI've worked in the MPEG process for years and I=20
>>know exactly of what I speak. If you were to have=20
>>included the actual text from the MPEG-2 standard=20
>>everyone would see just how wrong you are.
>>
>>
>>
>>>MPEG-2 provides several mechanisms that signal video size ans aspect ratio
>>>(section 1: systems), including a flag that signals a video frame as being
>>>1:1.33; 1:1.78 and 1:2.21, and the size relative size of the picture
>>>elements. Look up the extended segment.
>>
>>Correct. From section 6.3.3 of the standard (I do=20
>>not have the latest version, but I do not think=20
>>this has been amended). I am only quoting the=20
>>relevant parts of this section:
>>
>>6.3.3 Sequence header
> >
>>horizontal_size_value -- This word forms the 12=20
>>least significant bits of horizontal_size.
>>
>>vertical_size_value -- This word forms the 12=20
>>least significant bits of vertical_size.
>>
>>horizontal_size -- The horizontal_size is a=20
>>14-bit unsigned integer, the 12 least significant=20
>>bits are defined in horizontal_size_value, the 2=20
>>most significant bits are defined in=20
>>horizontal_size_extension. The horizontal_size is=20
>>the width of the displayable part of the=20
>>luminance component of pictures in samples. The=20
>>width of the encoded luminance component of=20
>>pictures in macroblocks, mb_width, is=20
> >(horizontal_size + 15)/16. The displayable part=20
>>is left-aligned in the encoded pictures.
>>In order to avoid start code emulation=20
>>horizontal_size_value shall not be zero. This=20
>>precludes values of horizontal_size that are=20
>>multiples of 4096.
>>
>>vertical_size -- The vertical_size is a 14-bit=20
>>unsigned integer, the 12 least significant bits=20
>>are defined in vertical_size_value, the 2 most=20
>>significant bits are defined in=20
>>vertical_size_extension. The vertical_size is the=20
>>height of the displayable part of the luminance=20
>>component of the frame in lines.
>>In the case that progressive_sequence is '1' the=20
>>height of the encoded luminance component of=20
>>frames in macroblocks, mb_height, is=20
>>(vertical_size + 15)/16.
>>In the case that progressive_sequence is '0' the=20
>>height of the encoded luminance component of=20
>>frame pictures in macroblocks, mb_height, is=20
>>2*((vertical_size + 31)/32). The height of the=20
>>encoded luminance component of field pictures in=20
>>macroblocks, mb_height, is ((vertical_size +=20
>>31)/32).
>>The displayable part is top-aligned in the encoded pictures.
>>In order to avoid start code emulation=20
>>vertical_size_value shall not be zero. This=20
>>precludes values of vertical_size that are=20
>>multiples of 4096.
>>
>>aspect_ratio_information -- This is a four-bit=20
>>integer defined in the Table 6-3.
>>
>>aspect_ratio_information either specifies that=20
>>the "sample aspect ratio" (SAR) of the=20
>>reconstructed frame is 1,0 (square samples) or=20
>>alternatively it gives the "display aspect ratio"=20
>>(DAR).
>>
>>* If sequence_display_extension() is not=20
>>present then it is intended that the entire=20
>>reconstructed frame is intended to be mapped to=20
>>the entire active region of the display. The=20
>>sample aspect ratio may be calculated as follows:
>>
>> SAR=3DDAR x horizontal_size/vertical_size
>>
>>NOTE - In this case horizontal_size and=20
>>vertical_size are constrained by the SAR of the=20
>>source and the DAR selected.
>>
>>* If sequence_display_extension() is=20
>>present then the sample aspect ratio may be=20
>>calculated as follows:
>> SAR=3DDAR x
>>display_horizontal_size/display_vertical_size
>>
>>
>>
>>Table 6-3 - aspect_ratio_information
>>aspect_ratio_information Sample Aspect Ratio DAR
>> 0000 forbidden forbidden
>> 0001 1,0 (Square Sample) -
>> 0010 - 3=F74
>> 0011 - 9=F716
>> 0100 - 1=F72,21
>> 0101 - reserved
>> =BA =BA
>> 1111 - reserved
>>
>>---------
>>
>>Now, what does all of this mean.
>>
>>=46irst, the standard accommodates two different=20
>>meanings of aspect ratio, which together can=20
>>influence the actual display aspect ratio. First,=20
>>it considers the sample aspect ratio (SAR);=20
>>second it considers the display aspect ratio DAR.
>>
>>John is correct that the standard does provide=20
>>values for the common 4:3 and 16:9 aspect ratios=20
>>as well as 2.21. But this does not limit the=20
>>ability to designate other SARs or DARs.
>>
>>=46or example, How does MPEG deal with ITU-R BT.601 samples?
>>
>>The size of the raster is stated in=20
>>horizontal_size and vertical_size. Let's say it=20
>>is the full 720 x 480/576. But are these samples=20
>>representing a 4:3 or 16:9 raster. You must look=20
>>at aspect_ratio_information to figure this out.=20
>>You can also use any fractional line length in=20
>>Horizontal size - the display will then stretch=20
>>the sample to fill the display aspect ratio.
>>
>>=46ortunately "601" is the only video standard in=20
> >use today that uses variable sample aspect ratio=20
>>9actually the compressed HD camcorders do this=20
>>internally, but they always produce square=20
>>samples at the output.
>>
>>Now how would I designate something other than 4:3 or 16:9 or 2.21:1?
>>
>>Simple. I would use horizontal and vertical size=20
>>to designate the raster size, and 0001 in aspect_=20
>>ratio_information to designate square samples.=20
>>Voila... I can represent ANY aspect ratio. It=20
>>gets a bit more tricky if I do not use square=20
>>samples; I would then need to use a reserved=20
>>extension to indicate the DAR and calculate the=20
>>SAR using the formulas.
> >
>>John must have been confused by the fact that=20
>>MPEG took the time to include values for the=20
>>standards that exist. But this does not mean that=20
>>you cannot use the standard as I have described.
>>
>>
>>>
>>>ATSC supports only specific video formats, and extended segment support is
>>>specified.
>>>
>>>DVB-SI provides mechanisms -- in addition to MPEG-2 section 1 -- to signal
>>>video frame aspect ratio.
>>>
>>>Wanna try again?
>>
>>Why don't you give up before you make an ass out of yourself again.
>>
>>Regards
>>Craig
>>
>>
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