Subject: Sending "cliprect" over SDP

From: Alagu Periyannan <[email-masked]>
Date: Tue, 2 Sep 1997 12:17:05 -0700

Hi all:

Popular video compressions schemes used with RTP
(H.262, H.263) offer a restricted set of picture sizes.

These are CIF (376x288), QCIF (188x144) and in the
case of H.263, 4CIF, 16CIF. Since these sizes are fixed
many of today's conferencing and broadcast tools
place a gray border around an image captured at a
different size.

This looks fine when the aspect ratio is close to
the CIF ratio as is the case for NTSC. However, if one
needs to send say cinema-style letterbox format the
gray borders will be fat and will not look good.
Letterbox format will be popular for sending movies
over RTP in conjuction with RTSP.

I would like to propose a scheme that can be used to
send the real video size over SDP. This will not solve
the gray border problem for conferencing since each
participant can potential have a different size border.
However, it will solve the problem in broadcast and
unicast (in conjunction with RTSP) situtations.

The "cliprect", i.e. the rectangle containing the useful
part of the picture, will be sent in an "a=" attribute
line within the "m=video" context.

a=cliprect: <top> <left> <bottom> <right>

Examples:

if we want to send 160x120 centered
inside a QCIF RTP session we send,

a=cliprect: 12 14 132 174

if we want to send 160x120 top left justified
inside a QCIF RTP session we send,

a=cliprect: 0 0 120 160

if we want to send letterbox (160x69) top left justified
inside a QCIF RTP session we send,

a=cliprect: 0 0 69 160

It would be useful if we all agree to this and generate/recognize it.

Any suggestions?

Also, how does one formalize an "a=" attribute? (IANA?)

---------------------------------------------------
Alagu Periyannan                   [email-masked]

Interactive Multimedia Group
Apple Computer, Inc.

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From: Mark Handley <[email-masked]>
Date: Tue, 02 Sep 1997 16:01:06 -0400

>It would be useful if we all agree to this and generate/recognize it.

This seems like a reasonable suggestion to me.  It would be even
better if there was a simple way to indicate the same thing for
multi-party conferencing, but short of adding a new RTCP message type
(which isn't really a good idea) I can't think of one.

>Also, how does one formalize an "a=" attribute? (IANA?)

The process isn't formalized yet, but we expect this to happen through
IANA.  Whether we expect people to write a one-page RFC for each
attribute, or whether we'll collect new attributes together
periodically, or whatever is not decided yet, but will I expect be
sorted out when the IESG approve SDP for Proposed Standard.

Mark

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From: "Dr. Stephan Wenger" <[email-masked]>
Date: Tue, 02 Sep 1997 23:34:56 +0200

>place a gray border around an image captured at a

I think that everyone agrees to the necessity of having a
more flexible choice of picture sizes than the old CIF
model. ITU-T expert groups on video compression realized this
as well. The new version of H.263, currently known as it's
working name H.263+, offers pictures sizes between 4 and 2048
pixels in X and Y dimension, which can be negotiated in an
4 pixel granularity. The signalling of the size of the coded
picture is done within the Picture Header of H.263+. Similar
mechanisms are available in MPEG 2 alias H.262.

H.263+ will be a ITU-T recommendation around January 1998. It
might be worth to wait for this new standard, especially
because the implementation effort for the enhancement of
the current (1995) version of H.263 to the new standard is
not very high, if none of the optional modes are implemented.

However, it might be very useful to think today about the
necessity of having any announcement of this information,
if it is already contained in the bitstream. One reason for
that, which comes in mind, is recource allocation in the receivers.

Opinions?

Regards

Stephan
--
--
Dr. Stephan Wenger   [email-masked]

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From: Alagu Periyannan <[email-masked]>
Date: Thu, 4 Sep 1997 11:09:01 -0700

Hi all,

There was an error in the CIF/QCIF numbers I sent out
in my previous email. Thanks to Kipp (H.A. Kippenhan Jr.) for
pointing this out.

Actually the CIF size is 352x288 and the QCIF size is 176x144.
(I had incorrectly used 376x288 and 188x144)

Also please note that the origin for the purposes of sending the
cliprect is located at the top left corner of the image.

So the examples will read,

Examples:

a=cliprect: 12 8 132 168

a=cliprect: 0 0 120 160

a=cliprect: 0 0 69 160

---

From: Alagu Periyannan <[email-masked]>
Date: Thu, 4 Sep 1997 11:09:01 -0700

Hi all,

Examples:

a=cliprect: 12 8 132 168

a=cliprect: 0 0 120 160

a=cliprect: 0 0 69 160