CLUE WG A. Romanow
Internet-Draft Cisco Systems
Intended status: Informational M. Duckworth, Ed.
Expires: July 9, 2012 Polycom
A. Pepperell
B. Baldino
Cisco Systems
January 6, 2012
Framework for Telepresence Multi-Streams
draft-ietf-clue-framework-02.txt
Abstract
This memo offers a framework for a protocol that enables devices in a
telepresence conference to interoperate by specifying the
relationships between multiple RTP streams.
Status of this Memo
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This Internet-Draft will expire on July 9, 2012.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Framework Features . . . . . . . . . . . . . . . . . . . . . . 7
5. Stream Information . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Overview of the Model . . . . . . . . . . . . . . . . . . 9
5.2. Media capture -- Audio and Video . . . . . . . . . . . . . 9
5.3. Describing Spatial Relations . . . . . . . . . . . . . . . 10
5.4. Attributes for Media Captures . . . . . . . . . . . . . . 11
5.4.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 11
5.4.2. Composed . . . . . . . . . . . . . . . . . . . . . . . 12
5.4.3. Audio Channel Format . . . . . . . . . . . . . . . . . 12
5.4.4. Area of capture . . . . . . . . . . . . . . . . . . . 13
5.4.5. Point of capture . . . . . . . . . . . . . . . . . . . 14
5.4.6. Auto-switched . . . . . . . . . . . . . . . . . . . . 14
5.5. Capture Set . . . . . . . . . . . . . . . . . . . . . . . 14
5.6. Attributes for Capture Sets . . . . . . . . . . . . . . . 16
5.6.1. Area of Scene . . . . . . . . . . . . . . . . . . . . 16
5.6.2. Area Scale Millimeters . . . . . . . . . . . . . . . . 16
6. Choosing Streams . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Message Flow . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.1. Consumer Capability Message . . . . . . . . . . . . . 18
6.1.2. Provider Capabilities Announcement . . . . . . . . . . 18
6.1.3. Consumer Configure Request . . . . . . . . . . . . . . 19
6.2. Physical Simultaneity . . . . . . . . . . . . . . . . . . 19
6.3. Encoding Groups . . . . . . . . . . . . . . . . . . . . . 21
6.3.1. Encoding Group Structure . . . . . . . . . . . . . . . 21
6.3.2. Individual Encodes . . . . . . . . . . . . . . . . . . 22
6.3.3. More on Encoding Groups . . . . . . . . . . . . . . . 23
6.3.4. Examples of Encoding Groups . . . . . . . . . . . . . 24
7. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 26
8. Other aspects of the framework . . . . . . . . . . . . . . . . 27
9. Using the Framework . . . . . . . . . . . . . . . . . . . . . 27
9.1. The MCU Case . . . . . . . . . . . . . . . . . . . . . . . 32
9.2. Media Consumer Behavior . . . . . . . . . . . . . . . . . 32
9.2.1. One screen consumer . . . . . . . . . . . . . . . . . 33
9.2.2. Two screen consumer configuring the example . . . . . 33
9.2.3. Three screen consumer configuring the example . . . . 34
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
12. Security Considerations . . . . . . . . . . . . . . . . . . . 34
13. Informative References . . . . . . . . . . . . . . . . . . . . 34
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Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 35
A.1. Video layout arrangements and centralized composition . . 35
A.2. Source is selectable . . . . . . . . . . . . . . . . . . . 35
A.3. Media Source Selection . . . . . . . . . . . . . . . . . . 35
A.4. Endpoint requesting many streams from MCU . . . . . . . . 36
A.5. VAD (voice activity detection) tagging of audio streams . 36
A.6. Private Information . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
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1. Introduction
Current telepresence systems, though based on open standards such as
RTP [RFC3550] and SIP [RFC3261], cannot easily interoperate with each
other. A major factor limiting the interoperability of telepresence
systems is the lack of a standardized way to describe and negotiate
the use of the multiple streams of audio and video comprising the
media flows. This draft provides a framework for a protocol to
enable interoperability by handling multiple streams in a
standardized way. It is intended to support the use cases described
in draft-ietf-clue-telepresence-use-cases-00 and to meet the
requirements in draft-romanow-clue-requirements-xx.
The solution described here is strongly focused on what is being done
today, rather than on a vision of future conferencing. At the same
time, the highest priority has been given to creating an extensible
framework to make it easy to accommodate future conferencing
functionality as it evolves.
The purpose of this effort is to make it possible to handle multiple
streams of media in such a way that a satisfactory user experience is
possible even when participants are on different vendor equipment and
when they are using devices with different types of communication
capabilities. Information about the relationship of media streams
must be communicated so that audio/video rendering can be done in the
best possible manner. In addition, it is necessary to choose which
media streams are sent.
There is no attempt here to dictate to the renderer what it should
do. What the renderer does is up to the renderer.
After the following Definitions, a short section introduces key
concepts. The body of the text comprises three sections that deal
with in turn stream content, choosing streams and an implementation
example. The media provider and media consumer behavior are
described in separate sections as well. Several appendices describe
topics that are under discussion for adding to the document.
2. Terminology
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 [RFC2119].
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3. Definitions
The definitions marked with an "*" are new; all the others are from
draft-wenger-clue-definitions-00-01.txt.
*Audio Capture: Media Capture for audio. Denoted as ACn.
Camera-Left and Right: For media captures, camera-left and camera-
right are from the point of view of a person observing the rendered
media. They are the opposite of stage-left and stage-right.
Capture Device: A device that converts audio and video input into an
electrical signal, in most cases to be fed into a media encoder.
Cameras and microphones are examples for capture devices.
Capture Scene: the scene that is captured by a collection of Capture
Devices. A Capture Scene may be represented by more than one type of
Media. A Capture Scene may include more than one Media Capture of
the same type. An example of a Capture Scene is the video image of a
group of people seated next to each other, along with the sound of
their voices, which could be represented by some number of VCs and
ACs. A middle box may also express Capture Scenes that it constructs
from Media streams it receives.
A Capture Set includes Media Captures that all represent some aspect
of the same Capture Scene. The items (rows) in a Capture Set
represent different alternatives for representing the same Capture
Scene.
Conference: used as defined in [RFC4353], A Framework for
Conferencing within the Session Initiation Protocol (SIP).
*Individual Encode: A variable with a set of attributes that
describes the maximum values of a single audio or video capture
encoding. The attributes include: maximum bandwidth- and for video
maximum macroblocks, maximum width, maximum height, maximum frame
rate. [Edt. These are based on H.264.]
*Encoding Group: Encoding group: A set of encoding parameters
representing a device's complete encoding capabilities or a
subdivision of them. Media stream providers formed of multiple
physical units, in each of which resides some encoding capability,
would typically advertise themselves to the remote media stream
consumer as being formed multiple encoding groups. Within each
encoding group, multiple potential actual encodings are possible,
with the sum of those encodings' characteristics constrained to being
less than or equal to the group-wide constraints.
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Endpoint: The logical point of final termination through receiving,
decoding and rendering, and/or initiation through capturing,
encoding, and sending of media streams. An endpoint consists of one
or more physical devices which source and sink media streams, and
exactly one [RFC4353] Participant (which, in turn, includes exactly
one SIP User Agent). In contrast to an endpoint, an MCU may also
send and receive media streams, but it is not the initiator nor the
final terminator in the sense that Media is Captured or Rendered.
Endpoints can be anything from multiscreen/multicamera rooms to
handheld devices.
Endpoint Characteristics: include placement of Capture and Rendering
Devices, capture/render angle, resolution of cameras and screens,
spatial location and mixing parameters of microphones. Endpoint
characteristics are not specific to individual media streams sent by
the endpoint.
Front: the portion of the room closest to the cameras. In going
towards back you move away from the cameras.
MCU: Multipoint Control Unit (MCU) - a device that connects two or
more endpoints together into one single multimedia conference
[RFC5117]. An MCU includes an [RFC4353] Mixer. [Edt. RFC4353 is
tardy in requiring that media from the mixer be sent to EACH
participant. I think we have practical use cases where this is not
the case. But the bug (if it is one) is in 4353 and not herein.
Media: Any data that, after suitable encoding, can be conveyed over
RTP, including audio, video or timed text.
*Media Capture: a source of Media, such as from one or more Capture
Devices. A Media Capture (MC) may be the source of one or more Media
streams. A Media Capture may also be constructed from other Media
streams. A middle box can express Media Captures that it constructs
from Media streams it receives.
*Media Consumer: an Endpoint or middle box that receives Media
streams
*Media Provider: an Endpoint or middle box that sends Media streams
Model: a set of assumptions a telepresence system of a given vendor
adheres to and expects the remote telepresence system(s) also to
adhere to.
Render: the process of generating a representation from a media, such
as displayed motion video or sound emitted from loudspeakers.
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*Simultaneous Transmission Set: a set of media captures that can be
transmitted simultaneously from a Media Provider.
Spatial Relation: The arrangement in space of two objects, in
contrast to relation in time or other relationships. See also
Camera-Left and Right.
Stage-Left and Right: For media captures, stage-left and stage-right
are the opposite of camera-left and camera-right. For the case of a
person facing (and captured by) a camera, stage-left and stage-right
are from the point of view of that person.
*Stream: RTP stream as in [RFC3550].
Stream Characteristics: include media stream attributes commonly used
in non-CLUE SIP/SDP environments (such as: media codec, bit rate,
resolution, profile/level etc.) as well as CLUE specific attributes
(which could include for example and depending on the solution found:
the I-D or spatial location of a capture device a stream originates
from).
Telepresence: an environment that gives non co-located users or user
groups a feeling of (co-located) presence - the feeling that a Local
user is in the same room with other Local users and the Remote
parties. The inclusion of Remote parties is achieved through
multimedia communication including at least audio and video signals
of high fidelity.
*Video Capture: Media Capture for video. Denoted as VCn.
Video composite: A single image that is formed from combining visual
elements from separate sources.
4. Framework Features
Two key functions must be accomplished so that multiple media streams
can be handled in a telepresence conference. These are:
o How to choose which streams the provider should send to the
consumer
o What information needs to be added to the streams to allow a
rendering of the capture scene
The framework/model we present here can be understood as specifying
these two functions.
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Media stream providers and consumers are central to the framework.
The provider's job is to advertise its capabilities (as described
here) to the consumer, whose job it is to configure the provider's
encoding capabilities as described below. Both providers and
consumers can each send and receive information, that is, we do not
have one party as the provider and one as the consumer exclusively,
but all parties have both sending and receiving parts to them. Most
devices function as both a media provider and as a media consumer.
For two devices to communicate bidirectionally, with media flowing in
both directions, both devices act as both a media provider and a
media consumer. The protocol exchange shown later in the "Choosing
Streams" section happens twice independently between the 2
bidirectional devices.
Both endpoints and MCUs, or more generally "middleboxes", can be
media providers and consumers.
5. Stream Information
This section describes the structure for communicating information
between providers and consumers. Figure illustrates how information
to be communicated is organized. Each construct illustrated in the
diagram is discussed in the sections below.
Diagram for Stream Content
+---------------+
| |
| Capture Set |
| |
+-------+-------+
_..-' | ``-._
_.-' | ``-._
_.-' | ``-._
+----------------+ +----------------+ +----------------+
| Media Capture | | Media Capture | | Media Capture |
| Audio or Video | | Audio or Video | | Audio or Video |
+----------------+ +----------------+ +----------------+
.' `. `-..__
.' `. ``-..__
,-----. ,---------. ``,----------.
,' Encode`. ,' `. ,'Simultaneous`.
( Group ) ( Attributes ) ( Transmission )
`. ,' `. ,' `. Sets ,'
`-----' `---------' `----------'
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5.1. Overview of the Model
The basic method of operation is that a provider describes to a
consumer what streams it has to offer. It describes them in terms
both of attributes of the media (e.g. audio and video) captures and
in terms of the encoding characteristics of the streams for these
captures. The consumer then tells the provider which streams it
wants to receive. Prior to this exchange, the consumer sends
information about itself to the provider which the provider may use
in determining what to advertise to the consumer.
A media provider provides media for one or more capture scenes. As
defined, a capture scene is the source scene that is captured by
media devices. An endpoint is likely to have more than one capture
scene, for example one for people and one for presentation. Each
capture scene is represented by a capture set, which describes all
the collections of media captures for that scene. A capture set
consists of one or more rows of media captures, where each row
represents a way of capturing the scene.
A media capture, typically audio or video, is the basic data
structure, as defined in definitions and described below in
Section 5.2. Media captures have attributes that describe them, such
as their spatial properties and relationships. These attributes are
described in Section 5.4 and Section 5.6.
Media Captures are also associated with data constructs that capture
encoding aspects of the streams - that is, simultaneous transmission
sets and encoding groups, described in Section 6.2 and Section 6.3.
Generally, the provider is capable of sending alternate captures of a
capture scene - different number of captures for the scene, or
captures with differing characteristics like bandwidth or resolution.
These are described by the provider as capabilities, using the
capture set and media capture model mentioned above, and chosen by
the consumer. The message exchange to accomplish this is described
in Section 6.1.
There are some additional separate aspects of the framework mentioned
in Section 8.
5.2. Media capture -- Audio and Video
A media capture, as defined in definitions, is a fundamental concept
of the model. Media can be captured in different ways, for example
by various arrangements of cameras and microphones. The model uses
the terms "video capture" (VC) and "audio capture" (AC) to refer to
sources of media streams. To distinguish between multiple instances,
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they are numbered for example VC1, VC2, and VC3 could refer to three
different video captures which can be used simultaneously.
A media capture can be a media source such as video from a specific
camera, or it can be more conceptual such as a composite image from
several cameras, or an automatic dynamically switched capture
choosing from several cameras depending on who is talking or other
factors.
A media capture can also come from synthetically generated sources,
such as a computer generated audiovisual presentation. Or from the
playback of a recording. Any media type that can be carried over RTP
can be represented by a media capture.
A media capture is described by Attributes and associated with an
Encode Group, and Simultaneous Transmission Set.
Media captures are aggregated into Capture Sets as described below.
5.3. Describing Spatial Relations
Within a capture set, an optional coordinate system can be used to
model relative or absolute positions of its constituent media
captures. This can allow precise physical sizes to be determined if
required, as well as the order in which media captures that form a
group should be displayed. This coordinate model is based on a few
basic principles:
o Systems which have no multi-capture elements (e.g. those that
supply only a single participant video stream perhaps with an
additional presentation stream) do not have to use the coordinate
model at all.
o Systems may use the co-ordinate model for some capture sets and
not use it for others - for example it might be appropriate to
signal real-world measurements for 3 side-by-side participant
streams captured from real cameras but to not use co-ordinates for
a presentation with no physical camera (sourced from a
presentation running on a laptop, for instance).
o Coordinates can either be real physical units (millimeters), have
an unknown scale, or not have a physical scale. All coordinates
used within a particular capture set are of the same type.
Systems that know their actual physical dimensions should always
provide those real world measurements in order that the far end
can make use of those measurements if it is capable of doing so.
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o The co-ordinate system is Cartesian X, Y, Z with the origin at a
spot of the implementer's choosing.
The direction of increasing coordinate values is:
X increases from camera left to camera right
Y increases from audience front to audience back
Z increases from low to high
5.4. Attributes for Media Captures
Media capture attributes describe information about streams and their
relationships. [Edt: We do not mean to duplicate SDP, if an SDP
description can be used, great.] The attributes of media captures
refer to static aspects of those captures that can be used by the
consumer for selecting the captures offered by the provider.
The mechanism of Attributes make the framework extensible. Although
we are defining some attributes now based on the most common use
cases, new attributes can be added for new use cases as they arise.
In general, the way to extend the solution to handle new features is
by adding attributes and/or values.
We describe attributes by variables and their values. The current
attributes are listed below and then described. The variable is
shown in parentheses, and the values follow after the colon:
o (Purpose): main, presentation
o (Composed): true, false
o (Audio Channel Format): mono, stereo, tbd
o (Area of Capture): A set of four points describing the relevant
area being captured by a capture device
o (Point of Capture): A 'Point' describing the location of the
capture device or pseudo-device
o (Auto-switched): true, false
5.4.1. Purpose
A variable with enumerated values describing the purpose or role of
the Media Capture. It could be applied to any media type. Possible
values: main, presentation, others TBD.
Main:
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The audio or video capture is of one or more people participating in
a conference (or where they would be if they were there). It is of
part or all of the Capture Scene.
Presentation:
The capture provides a presentation, e. g., from a connected laptop
or other input device.
5.4.2. Composed
A Boolean variable to indicate whether the MC is a mix or composition
of other MCs or Streams. (This could indicate for example a
continuous presence view of multiple images in a grid, or a large
image with smaller picture-in-picture images in it. When applied to
an audio capture, it indicates a composition of ACs by some mixing
algorithm)
This attribute is not intended to differentiate between different
ways of composing or mixing images. For possible extension of the
framework, additional attributes could be defined to distinguish
between different ways of composing or mixing captures. For example,
with different video layout arrangements of composing multiple images
into one, or different audio mixing algorithms.
5.4.3. Audio Channel Format
The "channel format" attribute of an Audio Capture indicates how the
meaning of the channels is determined. It is an enumerated variable
describing the type of audio channel or channels in the Audio
Capture. The possible values of the "channel format" attribute are:
o mono
o stereo
o TBD - other possible future values (to potentially include other
things like 3.0, 3.1, 5.1 surround sound and binaural)
All ACs in the same row of a Capture Set MUST have the same value of
the "channel format" attribute.
There can be multiple ACs of a particular type, or even different
types. These multiple ACs could each have an area of capture
attribute to indicate they represent different areas of the capture
scene.
If there are multiple audio streams, they might be correlated (that
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is, someone talking might be heard in multiple captures from the same
room). Echo cancellation and stream synchronization in consumers
should take this into account.
Mono:
An AC with channel format="mono" has one audio channel.
Stereo:
An AC with channel format = "stereo" has exactly two audio channels,
left and right, as part of the same AC. [Edt: should we mention RFC
3551 here? The channel format may be related to how Audio Captures
are mapped to RTP streams. This stereo is not the same as the effect
produced from two mono ACs one from the left and one from the right.]
5.4.4. Area of capture
The area_of_capture attribute is used to describe the relevant area
of which a media capture is "capturing". By comparing the area of
capture for different media captures, a consumer can determine the
spatial relationships of the captures on the provider so that they
can be rendered correctly.
The area of capture is a quadrilateral in the plane of interest for
the particular media capture. The plane of interest is the plane
containing the most interesting or relevant subject matter, such as
the first row of seats in a telepresence room. The provider chooses
a plane of interest for the purpose of specifying the points that
define the area of capture.
For example, consider the case of a telepresence room with several
primary seats. The provider should choose a plane of interest that
is vertical (or nearly vertical) that intersects the seating
positions. Each media capture can have its own plane of interest,
but the planes should intersect at the boundary between spatially
related captures. The plane of interest does not have to be
perpendicular to the line of sight of a camera that produces a video
capture.
The four points of an area of capture are defined using X, Y, Z
coordinates in a Cartesian coordinate system.
The four points should be coplanar. The four points form a
quadrilateral, not necessarily a rectangle.
The area of capture attribute is optional for a media capture. But
if it is given, it must include X, Y, Z coordinates for all four
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points.
5.4.5. Point of capture
The point_of_capture attribute can be used to describe the location
of a capture device or pseudo-device. If there are multiple captures
which share the same 'area_of_capture' value, then it is useful to
know the location from which they are capturing that area (e.g. a
device which has multiview). Point of capture is expressed as a
single {x, y, z} coordinate.
5.4.6. Auto-switched
A Boolean variable that may be used for audio and/or video streams.
In this case the offered AC or VC varies depending on some rule; it
is auto-switched between possible VCs, or between possible ACs. The
most common example of this is sending the video capture associated
with the "loudest" speaker according to an audio detection algorithm.
5.5. Capture Set
A capture set describes the alternative media streams that the
provider offers to send to the consumer. As shown in the content
diagram above, the capture set is an aggregation of all audio and
video captures for a particular scene that a provider is willing to
send.
A provider can have more than one capture set, each representing a
different scene. For example one capture set can be for main people
audio and video, and another capture set can be for a computer
generated presentation.
A provider describes its ability to send alternative media streams in
the capture set, which lists the media captures in rows, as shown
below. Each row of the capture set consists of either a single
capture or a group of captures. A group means the individual
captures in the group are spatially related with the specific
ordering of the captures described through the use of attributes.
Here is an example of a simple capture set with three video captures
and three audio captures:
(VC0, VC1, VC2)
(AC0, AC1, AC2)
The three VCs together in a row indicate those captures are spatially
related to each other. Similarly for the 3 ACs in the second row.
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The ACs and VCs in the same capture set are spatially related to each
other.
Multiple Media Captures of the same media type are often spatially
related to each other. Typically multiple Video Captures should be
rendered next to each other in a particular order, or multiple audio
channels should be rendered to match different speakers in a
particular way. Also, media of different types are often associated
with each other, for example a group of Video Captures can be
associated with a group of Audio Captures meaning they should be
rendered together.
Media Captures of the same media type are associated with each other
by grouping them together in a single row of a Capture Set. Media
Captures of different media types are associated with each other by
putting them in different rows of the same Capture Set.
Since all captures can have an area_of_capture associated with them,
a consumer can determine the spatial relationships of captures by
comparing the locations of their areas of capture with one another.
Association between audio and video can be made by finding audio and
video captures which share overlapping areas of capture.
The items (rows) in a capture set represent different alternatives
for representing the same Capture Scene. For example the following
are alternative ways of capturing the same Capture Scene - two
cameras each viewing half of a room, or one camera viewing the whole
room, or one stream that automatically captures the person in the
room who is currently speaking. Each row of the Capture Set contains
either a single media capture or one group of media captures.
The following example shows a capture set for an endpoint media
provider where:
o (VC0, VC1, VC2) - camera-left video capture, center video capture,
camera-right video capture
o (VC3) - capture associated with loudest
o (VC4) - zoomed out view of all people in the room
o (AC0) - main audio
The first item in this capture set example is a group of video
captures with a spatial relationship to each other. These are VC0,
VC1, and VC2. VC3 and VC4 are additional alternatives of how to
capture the same room in different ways. The audio capture is
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included in the same capture set to indicate AC0 is associated with
those video captures, meaning the audio should be rendered along with
the video in the same set.
The idea is to have sets of captures that represent the same
information ("information" in this context might be a set of people
and their associated audio / video streams, or might be a
presentation supplied by a laptop, perhaps with an accompanying audio
commentary). Spatial ordering of media captures is described through
the use of attributes.
A media consumer could choose one row of each media type (e.g., audio
and video) from a capture set. For example a three stream consumer
could choose the first video row plus the audio row, while a single
stream consumer could choose the second or third video row plus the
audio row. An MCU consumer might choose to receive multiple rows.
The Simultaneous Transmission Sets and Encoding Groups as discussed
in the next section apply to media captures listed in capture sets.
The Simultaneous Transmission Sets and Encoding Groups MUST allow all
the Media Captures in a particular row of the capture set to be used
simultaneously. But media captures in different rows of the capture
set might not be able to be used simultaneously.
5.6. Attributes for Capture Sets
These are attributes that can be applied to a capture set.
o (Area of Scene): four points describing the area of the entire
capture scene
o (Area scale): indicating if area numbers are in millimeters,
unknown scale, or no scale
5.6.1. Area of Scene
The area of scene attribute for a capture set has the same format as
the area of capture attribute for a media capture. The area of scene
is for the entire scene, which is captured by the one or more media
captures in the capture set rows.
5.6.2. Area Scale Millimeters
An optional attribute indicating if the numbers used for area of
scene, area of capture and point of capture are in terms of
millimeters, unknown scale factor, or not any scale.
This attribute applies to all the MCs that are part of the capture
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set.
Values for this attribute:
o "millimeters" means the scale is in millimeters
o "Unknown" means the scale is not necessarily mm, but the scale is
the same for every capture in the capture set. This might be
useful for an endpoint provider that does not know physical scale
but does know relative distances.
o "No Scale" means the scale could be different for each capture,
such as for an MCU provider that advertises two adjacent captures,
but it picks sources (which can change quickly) from different
endpoints to send for each capture, so the scale could be
different and changing for each capture.
6. Choosing Streams
This section describes the process of choosing which streams the
provider sends to the consumer. In order for appropriate streams to
be sent from providers to consumers, certain characteristics of the
multiple streams must be understood by both providers and consumers.
Two separate aspects of streams suffice to describe the necessary
information to be shared by providers and consumers. The first
aspect we call "physical simultaneity" and the other aspect we refer
to as "encoding group". These are described in the following
sections, after the message flow is discussed.
6.1. Message Flow
The following diagram shows the flow of messages between a media
provider and a media consumer. The provider sends information about
its capabilities (as specified in this section), then the consumer
chooses which streams it wants, which we refer to as "configure".
The consumer sends its own capability message to the provider which
may contain information about its own capabilities or restrictions,
in which case the provider might tailor its announcements to the
consumer.
Diagram for Message Flow
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Media Consumer Media Provider
-------------- ------------
| |
|----- Consumer Capability ---------->|
| |
| |
|<---- Capabilities (announce) -------|
| |
| |
|------ Configure (request) --------->|
| |
Media captures are dynamic. They can come and go in a conference -
and their parameters can change. A provider can advertise a new list
of captures at any time. Both the media provider and media consumer
can send "their messages" (i.e., capture set announcements, stream
configurations) any number of times during a call, and the other end
is always required to act on any new information received (e.g.,
stopping streams it had previously configured that are no longer
valid).
These messages do not always have to occur with all three messages
together as part of an exchange. A provider can send a new
capabilities announce message any time, without first receiving a new
consumer capability message. Similarly, a consumer can send a new
configure request at any time, to change what it wants to receive.
The new configure request must be compatible with the most recently
received capabilities announce message.
6.1.1. Consumer Capability Message
In order for a maximally-capable provider to be able to advertise a
manageable number of video captures to a consumer, there is a
potential use for the consumer being able, at the start of CLUE to be
able to inform the provider of its capabilities. One example here
would be the video capture attribute set - a consumer could tell the
provider the complete set of video capture attributes it is able to
understand and so the provider would be able to reduce the capture
set it advertises to be tailored to the consumer.
TBD - the content of this message needs to be better defined. The
authors believe there is a need for this message, but have not worked
out the details yet.
6.1.2. Provider Capabilities Announcement
The provider capabilities announce message includes:
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o the list of captures and their attributes
o the list of capture sets
o the list of Simultaneous Transmission Sets
o the list of the encoding groups
6.1.3. Consumer Configure Request
After receiving a set of video capture information from a provider
and making its choice of what media streams to receive based on the
consumer's own capabilities and any provider-side simultaneity
restrictions, the consumer needs to essentially configure the
provider to transmit the chosen set.
The expectation is that this message will enumerate each of the
encoding groups and potential encoders within those groups that the
consumer wishes to be active (this may well be a subset of the
complete set available). For each such encoder within an encoding
group, the consumer would specify the video capture (i.e., VC<n> as
described above) along with the specifics of the video encoding
required, i.e. width, height, frame rate and bit rate. At this
stage, the consumer would also provide RTP demultiplexing information
as required to distinguish each stream from the others being
configured by the same mechanism.
6.2. Physical Simultaneity
An endpoint or MCU can send multiple captures simultaneously.
However, there may be constraints that limit which captures can be
sent simultaneously with other captures.
Physical or device simultaneity refers to fact that a device may not
be able to be used in different ways at the same time. This shapes
the way that offers are made from the provider. The offers are made
so that the consumer will choose one of several possible usages of
the device. This type of constraint is expressed in Simultaneous
Transmission Sets. This is easier to show in an example.
Consider the example of a room system where there are 3 cameras each
of which can send a separate capture covering 2 persons each- VC0,
VC1, VC2. The middle camera can also zoom out and show all 6
persons, VC3. But the middle camera cannot be used in both modes at
the same time - it has to either show the space where 2 participants
sit or the whole 6 seats. We refer to this as a physical device
simultaneity constraint.
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The following illustration shows 3 cameras with 4 video streams. The
middle camera can be used as main video zoomed in on 2 people or it
could be used in zoomed out mode and capture the whole endpoint. The
idea here is that the middle camera cannot be used for both zoomed in
and zoomed out captures simultaneously. This is a constraint imposed
by the physical limitations of the devices.
Diagram for Simultaneity
`-. +--------+ VC2
.-'_Camera 3|---------->
.-' +--------+
VC3
-------->
`-. +--------+ /
.-'|Camera 2|<
.-' +--------+ \ VC1
-------->
`-. +--------+ VC0
.-'|Camera 1|---------->
.-' +--------+
VC0- video zoomed in on 2 people VC2- video zoomed in on 2 people
VC1- video zoomed in on 2 people VC3- video zoomed out on 6 people
Simultaneous transmission sets can be expressed as sets of the VCs
that could physically be transmitted at the same time, though it may
not make sense to do so.
In this example the two simultaneous sets are:
{VC0, VC1, VC2}
{VC0, VC3, VC2}
In this example VC0, VC1 and VC2 can be sent OR VC0, VC3 and VC2.
Only one set can be transmitted at a time. These are physical
capabilities describing what can physically be sent at the same time,
not what might make sense to send. For example, in the second set
both VC0 and VC2 are redundant if VC3 is included.
In describing its capabilities, the provider must take physical
simultaneity into account and send a list of its Simultaneous
Transmission Sets to the consumer, along with the Capture Sets and
Encoding Groups.
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6.3. Encoding Groups
The second aspect of multiple streams that must be understood by
providers and consumers in order to create the best experience
possible, i. e., for the "right" or "best" streams to be sent, is the
encoding characteristics of the possible audio and video streams
which can be sent. Just as in the way that a constraint is imposed
on the multiple streams due to the physical limitations, there are
also constraints due to encoding limitations. These are described by
four variables that make up an Encoding Group, as shown in the
following table:
Table: Encoding Group
+----------------+--------------------------------------------------+
| Name | Description |
+----------------+--------------------------------------------------+
| maxBandwidth | Maximum number of bits per second relating to |
| | all encodes combined |
| maxVideoMbps | Maximum number of macroblocks per second |
| | relating to a all video encodes combined ((width |
| | + 15) / 16) * ((height + 15) / 16) * |
| | framesPerSecond |
| videoEncodes[] | Set of potential video encodes can be generated |
| audioEncodes[] | Set of potential encodes that can be generated |
+----------------+--------------------------------------------------+
An encoding group is the basic concept for describing encoding
capability. As shown in the Table, it has an overall maxMbps and
bandwidth limits, as well as being comprised of sets of individual
encodes, which will be described in more detail below.
Each media stream provider includes one or more encoding groups.
There may be multiple encoding groups per endpoint. For example,
each video capture device might have an associated encoding group
that describes the video streams that can result from that capture.
A remote receiver (i. e., stream consumer)configures some or all of
the specific encodings within one or more groups in order to provide
it with media streams to decode.
6.3.1. Encoding Group Structure
This section shows more detail on the media stream provider's
encoding group structure. The encoding group includes several
individual encodes, each has different encoding values. For example
one may be high definition video 1080p60, and another 720p30, with a
third being CIF. While a typical 3 codec/display system would have
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one encoding group per "box", there are many possibilities for the
number of encoding groups a provider may be able to offer and for
what encoding values there are in each encoding group.
Diagram for Encoding Group Structure
,-------------------------------------------------.
| Media Provider |
| |
| ,--------------------------------------. |
| | ,--------------------------------------. |
| | | ,--------------------------------------. |
| | | | Encoding Group | |
| | | | ,-----------. | |
| | | | | | ,---------. | |
| | | | | | | | ,---------.| |
| | | | | Encode1 | | Encode2 | | Encode3 || |
| `.| | | | | | `---------'| |
| `.| `-----------' `---------' | |
| `--------------------------------------' |
`-------------------------------------------------'
As shown in the diagram, each encoding group has multiple potential
individual encodes within it. Not all encodes are equally capable,
the stream consumer chooses the encodes it wants by configuring the
provider to send it what it wants to receive.
Some encoding endpoints are fixed, others are flexible, e. g., a
single box with multiple DSPs where the resources are shared.
6.3.2. Individual Encodes
An encoding group is associated with a media capture through the
individual encodes, that is, an audio or video capture is encoded in
one or more individual encodes, as described by the videoEncodes[]
and audioEncodes[]variables.
The following table shows the variables for a Video Encode. (There
is a similar table for audio.)
Table: Individual Video Encode
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+--------------+----------------------------------------------------+
| Name | Description |
+--------------+----------------------------------------------------+
| maxBandwidth | Maximum number of bits per second relating to a |
| | single video encoding |
| maxMbps | Maximum number of macroblocks per second relating |
| | to a single video encoding: ((width + 15) / 16) * |
| | ((height + 15) / 16) * framesPerSecond |
| maxWidth | Video resolution's maximum supported width, |
| | expressed in pixels |
| maxHeight | Video resolution's maximum supported height, |
| | expressed in pixels |
| maxFrameRate | Maximum supported frame rate |
+--------------+----------------------------------------------------+
A remote receiver configures (i. e., instantiates) some or all of the
specific encodes such that:
o The configuration of each active ENC<n> does not exceed that
individual encode's maxWidth, maxHeight, maxFrameRate.
o The total bandwidth of the configured ENC<n> does not exceed the
maxBandwidth of the encoding group.
o The sum of the macroblocks per second of each configured encode
does not exceed the maxMbps attribute of the encoding group.
An equivalent set of attributes holds for audio encodes within an
audio encoding group.
6.3.3. More on Encoding Groups
An encoding group EG<n> comprises one or more potential encodings
ENC<n>. For example,
EG0: maxMbps=489600, maxBandwidth=6000000
VIDEO_ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
VIDEO_ENC1: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
AUDIO_ENC0: maxBandwidth=96000
AUDIO_ENC1: maxBandwidth=96000
AUDIO_ENC2: maxBandwidth=96000
Here, the encoding group is EG0. It can transmit up to two 1080p30
encodings (Mbps for 1080p = 244800), but it is capable of
transmitting a maxFrameRate of 60 frames per second (fps). To
achieve the maximum resolution (1920 x 1088) the frame rate is
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limited to 30 fps. However 60 fps can be achieved at a lower
resolution if required by the consumer. Although the encoding group
is capable of transmitting up to 6Mbit/s, no individual video
encoding can exceed 4Mbit/s.
This encoding group also allows up to 3 audio encodings,
AUDIO_ENC<0-2>. It is not required that audio and video encodings
reside within the same encoding group, but if so then the group's
overall maxBandwidth value is a limit on the sum of all audio and
video encodings configured by the consumer. A system that does not
wish or need to combine bandwidth limitations in this way should
instead use separate encoding groups for audio and video in order for
the bandwidth limitations on audio and video to not interact.
Audio and video can be expressed in separate encode groups, as in
this illustration.
VIDEO_EG0: maxMbps=489600, maxBandwidth=6000000
VIDEO_ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
VIDEO_ENC1: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
AUDIO_EG0: maxBandwidth=500000
AUDIO_ENC0: maxBandwidth=96000
AUDIO_ENC1: maxBandwidth=96000
AUDIO_ENC2: maxBandwidth=96000
6.3.4. Examples of Encoding Groups
This section illustrates further examples of encoding groups. In the
first example, the capability parameters are the same across ENCs.
In the second example, they vary.
An endpoint that has 3 similar video capture devices would advertise
3 encoding groups that can each transmit up to 2 1080p30 encodings,
as follows:
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EG0: maxMbps = 489600, maxBandwidth=6000000
ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
ENC1: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
EG1: maxMbps = 489600, maxBandwidth=6000000
ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
ENC1: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
EG2: maxMbps = 489600, maxBandwidth=6000000
ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
ENC1: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=244800, maxBandwidth=4000000
A remote consumer configures some or all of the specific encodings
such that:
o The configuration of each active ENC<n> parameter values does not
cause that encoding's maxWidth, maxHeight, maxFrameRate to be
exceeded
o The total bandwidth of the configured ENC <n> encodings does not
exceed the maxBandwidth of the encoding group
o The sum of the "macroblocks per second" values of each configured
encoding does not exceed the maxMbps of the encoding group
There is no requirement for all encodings within an encoding group to
be activated when configured by the consumer.
Depending on the provider's encoding methods, the consumer may be
able to request fixed encode values or choose encode values in the
range less than the maximum offered. We will discuss consumer
behavior in more detail in a section below.
6.3.4.1. Sample video encoding group specification #2
This example specification expresses a system whose encoding groups
can each transmit up to 3 encodings, but with each potential encoding
having a progressively lower specification. In this example, 1080p60
transmission is possible (as ENC0 has a maxMbps value compatible with
that) as long as it is the only active encoding (as maxMbps for the
entire encoding group is also 489600). Significantly, as up to 3
encodings are available per group, some sets of captures which
weren't able to be transmitted simultaneously in example #1 above now
become possible, for instance VC1, VC3 and VC6 together. In common
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with example #1, all encoding groups have an identical specification.
EG0: maxMbps = 489600, maxBandwidth=6000000
ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=489600, maxBandwidth=4000000
ENC1: maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxMbps=108000, maxBandwidth=4000000
ENC2: maxWidth=960, maxHeight=544, maxFrameRate=30,
maxMbps=61200, maxBandwidth=4000000
EG1: maxMbps = 489600, maxBandwidth=6000000
ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=489600, maxBandwidth=4000000
ENC1: maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxMbps=108000, maxBandwidth=4000000
ENC2: maxWidth=960, maxHeight=544, maxFrameRate=30,
maxMbps=61200, maxBandwidth=4000000
EG2: maxMbps = 489600, maxBandwidth=6000000
ENC0: maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxMbps=489600, maxBandwidth=4000000
ENC1: maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxMbps=108000, maxBandwidth=4000000
ENC2: maxWidth=960, maxHeight=544, maxFrameRate=30,
maxMbps=61200, maxBandwidth=4000000
7. Extensibility
One of the most important characteristics of the Framework is its
extensibility. Telepresence is a relatively new industry and while
we can foresee certain directions, we also do not know everything
about how it will develop. The standard for interoperability and
handling multiple streams must be future-proof.
The framework itself is inherently extensible through expanding the
data model types. For example:
o Adding more types of media, such as telemetry, can done by
defining additional types of captures in addition to audio and
video.
o Adding new functionalities , such as 3-D, say, will require
additional attributes describing the captures, such as x,y, z
coordinates.
o Adding a new codecs, such as H.265, can be accomplished by
defining new encoding variables.
The infrastructure is designed to be extended rather than requiring
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new infrastructure elements. Extension comes through adding to
defined types.
Assuming the implementation is in something like XML, adding data
elements and attributes makes extensibility easy.
8. Other aspects of the framework
A few other aspects of the framework are separate from the provider
capture set model. These include:
o Voice activity detection
o Indications about stream switching/composing, information about
the source media captures
o associating captures/streams with a conference roster
o mapping the model to specific protocol messages
[Edt. much of this is work in progress and will need to be updated]
9. Using the Framework
This section shows in more detail how to use the framework to
represent a typical case for telepresence rooms. First an endpoint
is illustrated, then an MCU case is shown.
Consider an endpoint with the following characteristics:
o 3 cameras, 3 displays, a 6 person table
o Each video device can provide one capture for each 1/3 section of
the table
o A single capture representing the active speaker can be provided
o A single capture representing the active speaker with the other 2
captures shown picture in picture within the stream can be
provided
o A capture showing a zoomed out view of all 6 seats in the room can
be provided
The audio and video captures for this endpoint can be described as
follows. The Encode Group specifications can be found above in
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Section 6.3.4.1, Sample video encoding group specification #2.
Video Captures:
o VC0- (the camera-left camera stream), encoding group:EG0,
attributes:purpose=main;auto-switched:no
o VC1- (the center camera stream), encoding group:EG1, attributes:
purpose=main; auto-switched:no
o VC2- (the camera-right camera stream), encoding group:EG2,
attributes: purpose=main;auto-switched:no
o VC3- (the loudest panel stream), encoding group:EG1, attributes:
purpose=main;auto-switched:yes
o VC4- (the loudest panel stream with PiPs), encoding group:EG1,
attributes: purpose=main; composed=true; auto-switched:yes
o VC5- (the zoomed out view of all people in the room), encoding
group:EG1, attributes: purpose=main;auto-switched:no
o VC6- (presentation stream), encoding group:EG1, attributes:
purpose=presentation;auto-switched:no
Summary of video captures - 3 codecs, center one is used for center
camera stream, presentation stream, auto-switched, and zoomed views.
The following diagram is a top view of the room with 3 cameras, 3
displays, and 6 seats. Each camera is capturing 2 people. The six
seats are not all in a straight line.
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,-. d
( )`--.__ +---+
`-' / `--.__ | |
,-. | `-.._ |_-+Camera 2 (VC2)
( ).' ___..-+-''`+-+
`-' |_...---'' | |
,-.c+-..__ +---+
( )| ``--..__ | |
`-' | ``+-..|_-+Camera 1 (VC1)
,-. | __..--'|+-+
( )| __..--' | |
`-'b|..--' +---+
,-. |``---..___ | |
( )\ ```--..._|_-+Camera 0 (VC0)
`-' \ _..-''`-+
,-. \ __.--'' | |
( ) |..-'' +---+
`-' a
The two points labeled b and c are intended to be at the midpoint
between the seating positions, and where the fields of view of the
cameras intersect.
The plane of interest for VC0 is a vertical plane that intersects
points 'a' and 'b'.
The plane of interest for VC1 intersects points 'b' and 'c'.
The plane of interest for VC2 intersects points 'c' and 'd'.
This example uses a millimeter scale
Areas of capture:
bottom left bottom right top left top right
VC0 (-2011,2850,0) (-673,3000,0) (-2011,2850,757) (-673,3000,757)
VC1 ( -673,3000,0) ( 673,3000,0) ( -673,3000,757) ( 673,3000,757)
VC2 ( 673,3000,0) (2011,2850,0) ( 673,3000,757) (2011,3000,757)
VC3 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
VC4 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
VC5 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
VC6 none
Points of capture:
VC0 (-1678,0,800)
VC1 (0,0,800)
VC2 (1678,0,800)
VC3 none
VC4 none
VC5 (0,0,800)
VC6 none
In this example, the right edge of the VC0 area lines up with the
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left edge of the VC1 area. It doesn't have to be this way. There
could be a gap or an overlap. One additional thing to note for this
example is the distance from a to b is equal to the distance from b
to c and the distance from c to d. All these distances are 1346 mm.
This is the width of each area of capture for VC0, VC1, and VC2.
Note the text in parentheses (e.g. "the camera-left camera stream")
is not explicitly part of the model, it is just explanatory text for
this example, and is not included in the model with the media
captures and attributes.
[edt. It is arbitrary that for this example the alternative views
are on EG1 - they could have been spread out- it was not a necessary
choice.]
Audio Captures:
o AC0 (camera-left), attributes: purpose=main;channel format=mono
o AC1 (camera-right), attributes: purpose=main;channel format=mono
o AC2 (center) attributes: purpose=main;channel format=mono
o AC3 being a simple pre-mixed audio stream from the room (mono),
attributes: purpose=main;channel format=mono
o AC4 audio stream associated with the presentation video (mono)
attributes: purpose=presentation;channel format=mono
Areas of capture:
bottom left bottom right top left top right
AC0 (-2011,2850,0) (-673,3000,0) (-2011,2850,757) (-673,3000,757)
AC1 ( 673,3000,0) (2011,2850,0) ( 673,3000,757) (2011,3000,757)
AC2 ( -673,3000,0) ( 673,3000,0) ( -673,3000,757) ( 673,3000,757)
AC3 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
AC4 none
The physical simultaneity information is:
{VC0, VC1, VC2, VC3, VC4, VC6}
{VC0, VC2, VC5, VC6}
It is possible to select any or all of the rows in a capture set.
This is strictly what is possible from the devices. However, using
every member in the set simultaneously may not make sense- for
example VC3(loudest) and VC4 (loudest with PIP). (In addition, there
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are encoding constraints that make choosing all of the VCs in a set
impossible. VC1, VC3, VC4, VC5, VC6 all use EG1 and EG1 has only 3
ENCs. This constraint shows up in the Capture list and encoding
groups, not in the simultaneous transmission sets.)
In this example there are no restrictions on which audio captures can
be sent simultaneously.
The following table represents the capture sets for this provider.
Recall that a capture set is composed of alternative captures
covering the same scene. Capture Set #1 is for the main people
captures, and Capture Set #2 is for presentation.
+----------------+
| Capture Set #1 |
+----------------+
| VC0, VC1, VC2 |
| VC3 |
| VC4 |
| VC5 |
| AC0, AC1, AC2 |
| AC3 |
+----------------+
+----------------+
| Capture Set #2 |
+----------------+
| VC6 |
| AC4 |
+----------------+
Different capture sets are unique to each other, non-overlapping. A
consumer chooses a capture row from each capture set. In this case
the three captures VC0, VC1, and VC2 are one way of representing the
video from the endpoint. These three captures should appear adjacent
next to each other. Alternatively, another way of representing the
Capture Scene is with the capture VC3, which automatically shows the
person who is talking. Similarly for the VC4 and VC5 alternatives.
As in the video case, the different rows of audio in Capture Set #1
represent the "same thing", in that one way to receive the audio is
with the 3 linear position audio captures (AC0, AC1, AC2), and
another way is with the single channel monaural format AC3. The
Media Consumer would choose the one audio capture row it is capable
of receiving.
The spatial ordering is understood by the media capture attributes
area and point of capture.
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The consumer finds a "row" in each capture set #x section of the
table that it wants. It configures the streams according to the
encoding group for the row.
A Media Consumer would likely want to choose a row to receive based
in part on how many streams it can simultaneously receive. A
consumer that can receive three people streams would probably prefer
to receive the first row of Capture Set #1 (VC0, VC1, VC2) and not
receive the other rows. A consumer that can receive only one people
stream would probably choose one of the other rows.
If the consumer can receive a presentation stream too, it would also
choose to receive the only row from Capture Set #2 (VC6).
9.1. The MCU Case
This section shows how an MCU might express its Capture Sets,
intending to offer different choices for consumers that can handle
different numbers of streams. A single audio capture stream is
provided for all single and multi-screen configurations that can be
associated (e.g. lip-synced) with any combination of video captures
at the consumer.
+--------------------+---------------------------------------------+
| Capture Set #1 | note |
+--------------------+---------------------------------------------+
| VC0 | video capture for single screen consumer |
| VC1, VC2 | video capture for 2 screen consumer |
| VC3, VC4, VC5 | video capture for 3 screen consumer |
| VC6, VC7, VC8, VC9 | video capture for 4 screen consumer |
| AC0 | audio capture representing all participants |
+--------------------+---------------------------------------------+
If / when a presentation stream becomes active within the conference,
the MCU might re-advertise the available media as:
+----------------+--------------------------------------+
| Capture Set #2 | note |
+----------------+--------------------------------------+
| VC10 | video capture for presentation |
| AC1 | presentation audio to accompany VC10 |
+----------------+--------------------------------------+
9.2. Media Consumer Behavior
[Edt. Should this be moved to appendix?]
The receive side of a call needs to balance its requirements, based
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on number of screens and speakers, its decoding capabilities and
available bandwidth, and the provider's capabilities in order to
optimally configure the provider's streams. Typically it would want
to receive and decode media from each capture set advertised by the
provider.
A sane, basic, algorithm might be for the consumer to go through each
capture set in turn and find the collection of video captures that
best matches the number of screens it has (this might include
consideration of screens dedicated to presentation video display
rather than "people" video) and then decide between alternative rows
in the video capture sets based either on hard-coded preferences or
user choice. Once this choice has been made, the consumer would then
decide how to configure the provider's encode groups in order to make
best use of the available network bandwidth and its own decoding
capabilities.
9.2.1. One screen consumer
VC3, VC4 and VC5 are all on different rows by themselves, not in a
group, so the receiving device should choose between one of those.
The choice would come down to whether to see the greatest number of
participants simultaneously at roughly equal precedence (VC5), a
switched view of just the loudest region (VC3) or a switched view
with PiPs (VC4). An endpoint device with a small amount of knowledge
of these differences could offer a dynamic choice of these options,
in-call, to the user.
9.2.2. Two screen consumer configuring the example
Mixing systems with an even number of screens, "2n", and those with
"2n+1" cameras (and vice versa) is always likely to be the
problematic case. In this instance, the behavior is likely to be
determined by whether a "2 screen" system is really a "2 decoder"
system, i.e., whether only one received stream can be displayed per
screen or whether more than 2 streams can be received and spread
across the available screen area. To enumerate 3 possible behaviors
here for the 2 screen system when it learns that the far end is
"ideally" expressed via 3 capture streams:
1. Fall back to receiving just a single stream (VC3, VC4 or VC5 as
per the 1 screen consumer case above) and either leave one screen
blank or use it for presentation if / when a presentation becomes
active
2. Receive 3 streams (VC0, VC1 and VC2) and display across 2 screens
(either with each capture being scaled to 2/3 of a screen and the
centre capture being split across 2 screens) or, as would be
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necessary if there were large bezels on the screens, with each
stream being scaled to 1/2 the screen width and height and there
being a 4th "blank" panel. This 4th panel could potentially be
used for any presentation that became active during the call.
3. Receive 3 streams, decode all 3, and use control information
indicating which was the most active to switch between showing
the left and centre streams (one per screen) and the centre and
right streams.
For an endpoint capable of all 3 methods of working described above,
again it might be appropriate to offer the user the choice of display
mode.
9.2.3. Three screen consumer configuring the example
This is the most straightforward case - the consumer would look to
identify a set of streams to receive that best matched its available
screens and so the VC0 plus VC1 plus VC2 should match optimally. The
spatial ordering would give sufficient information for the correct
video capture to be shown on the correct screen, and the consumer
would either need to divide a single encode group's capability by 3
to determine what resolution and frame rate to configure the provider
with or to configure the individual video captures' encode groups
with what makes most sense (taking into account the receive side
decode capabilities, overall call bandwidth, the resolution of the
screens plus any user preferences such as motion vs sharpness).
10. Acknowledgements
Mark Gorzyinski contributed much to the approach. We want to thank
Stephen Botzko for helpful discussions on audio.
11. IANA Considerations
TBD
12. Security Considerations
TBD
13. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
Appendix A. Open Issues
A.1. Video layout arrangements and centralized composition
In the context of a conference with a central MCU, there has been
discussion about a consumer requesting the provider to provide a
certain type of layout arrangement or perform a certain composition
algorithm, such as combining some number of most recent talkers, or
producing a video layout using a 2x2 grid or 1 large cell with 5
smaller cells around it. The current framework does not address
this. It isn't clear if this topic should be included in this
framework, or maybe a different part of CLUE, or maybe outside of
CLUE altogether.
A.2. Source is selectable
A Boolean variable. True indicates the media consumer can request a
particular media source be mapped to a media capture. Default is
false.
TBD - how does the consumer make the request for a particular source?
How does the consumer know what is available? Need to explain better
how multiple media captures are different from a single media capture
with choices for the source, and when each concept should be used.
A.3. Media Source Selection
The use cases include a case where the person at a receiving endpoint
can request to receive media from a particular other endpoint, for
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example in a multipoint call to request to receive the video from a
certain section of a certain room, whether or not people there are
talking.
TBD - this framework should address this case. Maybe need a roster
list of rooms or people in the conference, with a mechanism to select
from the roster and associate it with media captures. This is
different from selecting a particular media capture from a capture
set. The mechanism to do this will probably need to be different
than selecting media captures based on capture sets and attributes.
A.4. Endpoint requesting many streams from MCU
TBD - how to do VC selection for a system where the endpoint media
consumers want to receive lots of streams and do their own
composition, rather than MCU doing transcoding and composing.
Example is 3 screen consumer that wants 3 large loudest speaker
streams, and a bunch of small ones to render as PiP. How the small
ones are chosen, which could potentially be chosen by either the
endpoint or MCU. There are other more complicated examples also. Is
the current framework adequate to support this?
A.5. VAD (voice activity detection) tagging of audio streams
TBD - do we want to have VAD be mandatory? All audio streams
originating from a media provider must be tagged with VAD
information. This tagging would include an overall energy value for
the stream plus information on which sections of the capture scene
are "active".
Each audio stream which forms a constituent of a row within a capture
set should include this tagging, and the energy value within it
calculated using a fixed, consistent algorithm.
When a system determines the most active area of a capture scene
(either "loudest", or determined by other means such as a button
press) it should convey that information to the corresponding media
stream consumer via any audio streams being sent within that capture
set. Specifically, there should be a list of active linear positions
and their VAD characteristics within the audio stream in addition to
the overall VAD information for the capture set. This is to ensure
all media stream consumers receive the same, consistent, audio energy
information whichever audio capture or captures they choose to
receive for a capture set. Additionally, linear position information
can be mapped to video captures by a media stream consumer in order
that it can perform "panel switching" if required.
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A.6. Private Information
Do we want a way to include private information?
Authors' Addresses
Allyn Romanow
Cisco Systems
San Jose, CA 95134
USA
Email: allyn@cisco.com
Mark Duckworth (editor)
Polycom
Andover, MA 01810
US
Email: mark.duckworth@polycom.com
Andrew Pepperell
Cisco Systems
Langley, England
UK
Email: apeppere@cisco.com
Brian Baldino
Cisco Systems
San Jose, CA 95134
US
Email: bbaldino@cisco.com
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