[opendtv] Re: News: See the Big Picture? Don't Forget to Examine the Fine Print
- From: "John Willkie" <jmwillkie@xxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 4 May 2004 12:50:41 -0700
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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