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RE: [AVT] supporting variable picture sizesindraft-ietf-avt-rtp-h264-10.txt
Reha et al,
Picture size changes are not as uncommon as you might think. In fact I believe it is common practice in broadcast using MPEG-2 for picture sizes to change. The viewer typically never notices it because whatever size of pictures is received is automatically adjusted to fit appropriately onto whatever display the viewer happens to be watching.
In fact the same content must be viewable on a wide variety of displays, and encoders often have no idea what displays will be used (if any) after the video they produce is decoded.
After the decoding process, upsampling or downsampling can be used to adjust pictures to the type of display in use.
I would think that typically, in a well-designed receiving system, the viewer would not notice a change unless the aspect ratio of the overall picture changed. One common example of an aspect ratio change is the way people often will see a "letterbox" view during a program and a "full-screen" view during commercial breaks. The experience doesn't seem to bother people all that much.
Of course, if the encoder does something that causes a bad experience for the viewer, it is the encoder's fault. So if you think that some strange thing might be experienced if you encode your video in a particular way (such as frequently jumping between unusual picture shapes and sizes), it might be advisable for you not to design your encoder to encode video that way.
Things like methods of resampling for displays don't affect the ability to communicate -- to understand the video -- so they don't necessarily always need to be standardized.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On
+> Behalf Of REHA CIVANLAR
+> Sent: Friday, July 23, 2004 2:05 AM
+> To: lindbergh at 92f1.com
+> Cc: avt at ietf.org; MURAT TEKALP
+> Subject: Re: [AVT] supporting variable picture
+> sizesindraft-ietf-avt-rtp-h264-10.txt
+>
+> Let's say I am considering to send H.264 video with changing
+> picture resolutions to a mobile device or a set top box.
+> If my target display, which I know to be H.264 compatible,
+> is not capable of adjusting its final display size to a
+> constant resolution, I, probably, wouldn't want to use
+> variable resolutions, because the end user will obviously
+> not be happy to watch video with changing picture sizes...
+> (By the way, this argument applies to variable frame rates also.)
+> Moreover, even if the display device displays everything at
+> a set resolution, I may still need to know what kind of
+> interpolation is being used for this purpose before deciding
+> to use variable resolutions.
+>
+> My question is shouldn't these be specified e.g., in the set
+> of attributes for the end device? Should another standard
+> body be addressing this issue?
+>
+> -Reha
+>
+> >>> Dave Lindbergh <lindbergh at 92f1.com> 7/22/2004 3:49:42 PM >>>
+> Displays are outside the scope of H.264 - the standard covers only
+> bitstream syntax and decoding.
+>
+> So displays are free to show the decoded video any way they
+> prefer - they
+> can scale to the largest size, display in actual format,
+> drop frames,
+> whatever...
+>
+> (A H.264 conformant decoder does not have to have a display
+> at all. Things
+> like MCUs or recorders usually don't.)
+>
+> --Dave
+>
+>
+> At 10:41 AM 7/22/2004, REHA CIVANLAR wrote:
+>
+> >We noticed that in Section 1.2 of
+> draft-ietf-avt-rtp-h264-10.txt seems to
+> >imply that more than one picture size
+> >can be supported in the same video bitstream by toggling
+> between two or
+> >more parameter sets.
+> >
+> >The question is how is the decoder supposed to display
+> video containing
+> >varying picture sizes (e.g., does it interpolate
+> >all the pictures to do largest size?) Is this specified in
+> any part of the
+> >standard?
+> >
+> >
+> >
+> >------------------------------------------------------
+> >Prof. M. Reha Civanlar, Ph.D.
+> >Computer Engineering Department
+> >Koç University
+> >Istanbul, TURKEY
+> >+90 212 338 1719
+> >rcivanlar at ku.edu.tr
+> >
+> >
+> >_______________________________________________
+> >Audio/Video Transport Working Group
+> >avt at ietf.org
+> >https://www1.ietf.org/mailman/listinfo/avt
+>
+> ----------
+> Dave Lindbergh
+> Polycom, Inc.
+> 100 Minuteman Road
+> Andover MA 01810 USA
+> Voice: +1 978 292 5366
+> Email: lindbergh at 92F1.com
+> H.320, H.323 video by arrangement
+>
+>
+> _______________________________________________
+> Audio/Video Transport Working Group
+> avt at ietf.org
+> https://www1.ietf.org/mailman/listinfo/avt
+>
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