Network Working Group M. Civanlar
Request for Comments: 2862 G. Cash
Category: Standards Track AT&T
June 2000
RTP Payload Format for Real-Time Pointers
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes an RTP [1] payload format for transporting
the coordinates of a dynamic pointer that may be used during a
presentation. Although a mouse can be used as the pointer, this
payload format is not intended and may not have all functionalities
needed to implement a general mouse event transmission mechanism.
1. IntrodUCtion
In most presentations, significant information is conveyed through
the use of viewgraphs and a pointer. This makes accurate transmission
of them vital in remote conferencing applications. Using regular
video of a presenter's display for this purpose is problematic
because, while the viewgraphs require a high spatial resolution, the
pointer movements need to be sampled and transmitted at a high
temporal resolution so that the presenter's pointing actions can be
displayed synchronously with the corresponding audio and video
signals. In many instances, this synchronization carries vital
information. As an example, consider a speaker pointing at two
alternatives on a viewgraph in sequence and saying "this one is
better than this". To satisfy both high spatial and high temporal
resolution requirements, at least S-VHS quality video may need to be
used. Codecs that can compress S-VHS video effectively in real-time
are eXPensive for this purpose, and transmitting such video
uncompressed requires very high bandwidths.
A much simpler and economical system can be designed by capturing and
transmitting the pointer coordinates separately [2]. The pointer
coordinates with respect to a displayed viewgraph can easily be
oBTained in electronic presentation systems. For presentations
prepared for optical systems, such as transparencies for overhead
projectors, an arrangement where the viewgraph is captured in a frame
buffer on a computer can be used to associate the pointer coordinates
with the displayed viewgraph. For capturing transparencies, printed
material, or even three dimensional objects, a document camera and a
personal computer or workstation based video capture card can be
used. This arrangement can handle electronic viewgraphs by feeding
the video output of the computer that displays them to the video
capture card through an appropriate converter also. A side benefit of
this is that it allows using a presenter's own computer to transmit
electronic viewgraphs without connecting it to, for example, an
intranet. The captured image is then displayed along with the
capturing computer's mouse pointer on the presenter's display using a
projector. The presenter moves the pointer on the display using a
regular or maybe a wireless mouse whose location can easily be
captured by appropriate software running on the capturing computer.
This document describes an RTP payload format to transmit the pointer
coordinates captured in one of the ways described above using RTP.
Although, a mouse can be used as the pointer, this payload format is
not intended and may not have all functionalities needed to implement
a general mouse event transmission mechanism.
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [3].
2. Payload Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
V=2PX CC M PT sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
timestamp
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
synchronization source (SSRC) identifier
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: contributing source (CSRC) identifiers :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
LMR x-coordinate PIN y-coordinate
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MBZ MBZ
Figure 1 - An RTP packet for Real-Time Pointer
Fig. 1 shows an RTP packet carrying real-time pointer coordinates.
This payload format does not have a payload specific header.
2.1. RTP Header Usage:
Payload Type (PT): The assignment of an RTP payload type for this new
packet format is outside the scope of this document, and will not be
specified here. It is expected that the RTP profile for a particular
class of applications will assign a payload type for this encoding,
or if that is not done then a payload type in the dynamic range shall
be chosen.
Marker (M) bit: Set to one if the pointer icon is changed in this
packet.
Extension (X) bit: Defined by the RTP profile used.
Sequence Number: Set as described in RFC1889 [1].
Timestamp: The sampling time for the pointer location measured by a
90kHz clock.
SSRC: Set as described in RFC1889 [1].
CC and CSRC fields are used as described in RFC1889 [1].
RTCP SHOULD be used as defined in RFC1889 [1].
2.2. Payload:
The pointer's x and y coordinates are measured from the upper left
corner of the associated display window. They are represented as a
fraction of the corresponding edge length of the display window using
12 bits, positive, fixed point numbers between 0 and (1 - 2^-12).
L (left), R (right) and/or M (middle) bits are pointer special
effects flags. Their use is application dependent and MUST be
established out-of-band. Applications MAY ignore these bits.
PIN: Pointer Icon Number (3 bits) selects a pointer icon. The
association between the PIN numbers and the icon pictures MUST be
established out-of-band. PIN = 0 represents a default pointer icon.
Applications which only support a single pointer icon SHOULD set the
PIN field to zero. Applications MAY ignore non-zero PIN values on
reception, and display a default icon.
3. MIME Media Type Registrations
This document defines a new RTP payload name, "pointer," and
associated MIME subtype, "video/pointer."
3.1. Registration of MIME media type video/pointer
MIME media type name: video
MIME subtype name: pointer
Required parameters: None
Optional parameters: None
Encoding considerations: Pointer video can be transmitted with RTP
as specified in this document.
Security considerations: As described in this document.
Interoperability considerations: None
Published specification: this document.
Applications which use this media type: Videoconferencing systems
that transmit VUgraphs with a real-time pointer.
Additional information: None
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
M. Reha Civanlar
e-mail: civanlar@research.att.com
Intended usage: COMMON Author/Change controller:
M. Reha Civanlar
e-mail: civanlar@research.att.com
4. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [1].
This payload type does not exhibit any significant non-uniformity in
the receiver side computational complexity for packet processing to
cause a potential denial-of-service threat.
5. References
[1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real Time Applications", RFC1889,
January 1996.
[2] M. R. Civanlar, G. L. Cash, "Networked Viewgraphs - NetVG"
Proceedings of The 9th Int. Workshop on Packet Video,
http://www.research.att.com/~mrc/PacketVideo99.Html.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
6. Authors' Addresses
M. Reha Civanlar
AT&T Labs - Research
100 Schultz Drive, Room 3-205
Red Bank, NJ 07701, USA
EMail: civanlar@research.att.com
Glenn L. Cash
AT&T Labs - Research
100 Schultz Drive, Room 3-213
Red Bank, NJ 07701, USA
EMail: glenn@research.att.com
7. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Acknowledgement
Funding for the RFCEditor function is currently provided by the
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