[opendtv] Re: Definition of Anamorphic
- From: Kilroy Hughes <Kilroy.Hughes@xxxxxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 7 Oct 2009 15:49:47 +0000
Just to complicate things ...
Everyone is using "720" as the width, but the display width and 4:3/16:9
display aperture is actually supposed to be 704 (implying a Sample Aspect Ratio
in an MPEG codec too dumb to specify it as 11/10 H/W, 1.10), and it can be
encoded at 704 or 720. There are various conflicting derivations of what NTSC
SAR should be from analog to D1 digital sampling to the world of macroblocks;
but the DVD spec expresses the intent of 704 and 720 in microseconds of line
length, which translates to 704 x 480 being the "exact scan" visible picture.
We codified that in H.264 with enumerated SARs that make it clear the display
process is assumed to throw away a macroblock on each side with 720. There are
also cropping parameters that should make it clear you are supposed to throw
away half a macroblock on the bottom of a 1088 line encoded picture (you can't
encode 1080) and shift the center up 4 lines. If the cropping parameters were
used on all black padded pictures, the world would be a better place because
decode/display devices wouldn't have to guess where the real picture is.
It pays to be picky about these things in the digital world because there is a
big penalty in quality and performance to unnecessarily scale by a few pixels
(and Nyquist filter) when you should leave it along and crop.
Kilroy
-----Original Message-----
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] On
Behalf Of Craig Birkmaier
Sent: Wednesday, October 07, 2009 8:07 AM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: Definition of Anamorphic
At 3:58 PM -0500 10/6/09, Manfredi, Albert E wrote:
>Poor choice of words. I meant to say, that would translate to an
>aspect ratio of 1.5:1 were the pixels meant to be square.
>
>The point of my thesis remains the same. Which is, pixels in the DVD
>format are never square, whether for 4:3 or for 16:9 content. So
>that the term "anamorphic," when related to DVD content, cannot mean
>distorted with respect to a square pixel frame.
>
>It must indicate, instead, "distorted with respect to a 4:3 frame,
>requiring the compensating distortion in playback."
NO. Both ATSC and DVD use non-square samples for BOTH 4:3 and 16:9
aspect ratio sources.
In the case of 4:3 the pixels are narrower than they are tall (i.e.
720 x 480 versus 640 x 480 for square sample 4:3). Note that ATSC
does allow the square sample 640 x 480 format for the 4:3 aspect
ratio. To present in pillarbox on a square sample 16:9 display the
samples must be filtered to square at the resolution of the display
(e.g. 960 x 720 on a 720 line display).
In the case of 16:9 the samples are also narrower than they are tall
( i.e. 720 x 480 versus 854 x 480 for square sample 16:9). ATSC does
not permit the square sample version of 16:9 at 480 lines (854 x
480). SD-DVD does not support 854 x 480 either, as this exceeds the
sample rate for MPEG-2 MP@ML (10.4 Msamples per second). To present
widescreen on a square sample 16:9 display the samples must be
filtered to square at the resolution of the display (e.g. 1280 x 720
on a 720 line display).
>Also true for "anamorphic" DVDs. The horizontal dimension in
>playback is created from fewer pixels per unit length than the
>vertical. I'm talking about source data pixels here, not diplay
>system pixels.
True and false. For 4:3 source there are more samples than needed for
the square sample format at 480 lines, thus for a 480P display
samples would be reduced to 640. For a 16:9 source there are less
samples than needed for the square sample format at 480 lines, thus
for a 480P widescreen display the number of samples would increase to
854. For HD displays an SD-DVD must be upsampled to square at 720 or
1080 lines.
Regards
Craig
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