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.
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.
Civanlar & Cash Standards Track [Page 1]
RFC 2862 RTP Payload Format for Real-Time Pointers June 2000
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 RFC 2119 [3].
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RFC 2862 RTP Payload Format for Real-Time Pointers June 2000
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.
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.
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.
Copyright (C) The Internet Society (2000). All Rights Reserved.
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