| CLUE WG | A. Romanow |
| Internet-Draft | Cisco Systems |
| Intended status: Informational | M. Duckworth, Ed. |
| Expires: September 11, 2012 | Polycom |
| A. Pepperell | |
| B. Baldino | |
| Cisco Systems | |
| March 12, 2012 |
Framework for Telepresence Multi-Streams
draft-ietf-clue-framework-04.txt
This memo offers a framework for a protocol that enables devices in a telepresence conference to interoperate by specifying the relationships between multiple media streams.
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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-02 and to meet the requirements in draft-ietf-clue-telepresence-requirements-01.
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 using different vendor equipment, and also when they are using devices with different types of communication capabilities. Information about the relationship of media streams at the provider's end must be communicated so that streams can be chosen and audio/video rendering can be done in the best possible manner.
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 several sections about the key elements of the framework, how a consumer chooses streams to receive, and some examples. The appendix describe topics that are under discussion for adding to the document.
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].
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: a structure representing the scene that is captured by a collection of capture devices. A capture scene includes attributes and one or more capture scene entries, with each entry including one or more media captures.
*Capture Scene Entry: a list of media captures of the same media type that together form one way to represent the capture scene.
Conference: used as defined in [RFC4353], A Framework for Conferencing within the Session Initiation Protocol (SIP).
*Individual Encoding: 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 (for H.264), maximum width, maximum height, maximum frame rate.
*Encoding Group: A set of encoding parameters representing a media provider's encoding capabilities. 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 using multiple encoding groups. Within each encoding group, multiple potential encodings are possible, with the sum of the chosen encodings' characteristics constrained to being less than or equal to the group-wide constraints.
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.
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.
*Plane of Interest: The spatial plane containing the most relevant subject matter.
Render: the process of generating a representation from a media, such as displayed motion video or sound emitted from loudspeakers.
*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: the 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, such as the ID of a capture or a spatial location.
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.
The CLUE framework specifies how multiple media streams are to be handled in a telepresence conference.
The main goals include:
Interoperability is achieved by the media provider describing the relationships between media streams in constructs that are understood by the consumer, who can then render the media. Extensibility is achieved through abstractions and the generality of the model, making it easy to add new parameters. Flexibility is achieved largely by having the consumer choose what content and format it wants to receive from what the provider is capable of sending.
A transmitting endpoint or MCU describes specific aspects of the content of the media and the formatting of the media streams it can send (advertisement); and the receiving end responds to the provider by specifying which content and media streams it wants to receive (configuration). The provider then transmits the asked for content in the specified streams.
This advertisement and configuration occurs at call initiation but may also happen at any time throughout the conference, whenever there is a change in what the consumer wants or the provider can send.
An endpoint or MCU typically acts as both provider and consumer at the same time, sending advertisements and sending configurations in response to receiving advertisements. (It is possible to be just one or the other.)
The data model is based around two main concepts: a capture and an encoding. A media capture (MC), such as audio or video, describes the content a provider can send. Media captures are described in terms of CLUE-defined attributes, such as spatial relationships and purpose of the capture. Providers tell consumers which media captures they can provide, described in terms of the media capture attributes.
A provider organizes its media captures that represent the same scene into capture scenes. A consumer chooses which media captures it wants to receive according to the capture scenes sent by the provider.
In addition, the provider sends the consumer a description of the streams it can send in terms of the media attributes of the stream, in particular, well-known audio and video parameters such as bandwidth, frame rate, macroblocks per second.
The provider also specifies constraints on its ability to provide media, and the consumer must take these into account in choosing the content and streams it wants. Some constraints are due to the physical limitations of devices - for example, a camera may not be able to provide zoom and non-zoom views simultaneously. Other constraints are system based constraints, such as maximum bandwidth and maximum macroblocks/second.
The following sections discuss these constructs and processes in detail, followed by use cases showing how the framework specification can be used.
In order for a consumer to perform a proper rendering, it is often necessary to provide spatial information about the streams it is receiving. CLUE defines a coordinate system that allows media providers to describe the spatial relationships of their media captures to enable proper scaling and spatial rendering of their streams. The coordinate system is based on a few principles:
The direction of increasing coordinate values is:
X increases from camera left to camera right
Y increases from front to back
Z increases from low to high
This section describes how media providers can describe the content of media to consumers.
Media captures are the fundamental representations of streams that a device can transmit. What a Media Capture actually represents is flexible:
To distinguish between multiple instances, video and audio captures are numbered such as: VC1, VC2 and AC1, AC2. VC1 and VC2 refer to two different video captures and AC1 and AC2 refer to two different audio captures.
Each Media Capture can be associated with attributes to describe what it represents.
Media Capture Attributes describe static information about the captures. A provider uses the media capture attributes to describe the media captures to the consumer. The consumer will select the captures it wants to receive. Attributes are defined by a variable and its value. The currently defined attributes and their values are:
Content: {slides, speaker, sl, main, alt}
A field with enumerated values which describes the role of the media capture and can be applied to any media type. The enumerated values are defined by [RFC4796]. The values for this attribute are the same as the mediacnt values for the content attribute in [RFC4796]. This attribute can have multiple values, for example content={main, speaker}.
Composed: {true, false}
A field with a Boolean value which indicates whether or not the Media Capture is a mix (audio) or composition (video) of streams.
This attribute is not intended to describe the layout used when compositing video streams.
Audio Channel Format: {mono, stereo} A field with enumerated values which describes the method of encoding used for audio.
A value of 'mono' means the Audio Capture has one channel.
A value of 'stereo' means the Audio Capture has two audio channels, left and right.
This attribute applies only to Audio Captures. A single stereo capture is different from two mono captures that have a left-right spatial relationship. A stereo capture maps to a single RTP stream, while each mono audio capture maps to a separate RTP stream.
Switched: {true, false}
A field with a Boolean value which indicates whether or not the Media Capture represents the (dynamic) most appropriate subset of a 'whole'. What is 'most appropriate' is up to the provider and could be the active speaker, a lecturer or a VIP.
Point of Capture: {(X, Y, Z)} A field with a single Cartesian (X, Y, Z) point value which describes the spatial location, virtual or physical, of the capturing device (such as camera).
When the Point of Capture attribute is specified, it must include X, Y and Z coordinates. If the point of capture is not specified, it means the consumer should not assume anything about the spatial location of the capturing device. Even if the provider specifies an area of capture attribute, it does not need to specify the point of capture.
Area of Capture:
{bottom left(X1, Y1, Z1), bottom right(X2, Y2, Z2), top left(X3, Y3, Z3), top right(X4, Y4, Z4)}
A field with a set of four (X, Y, Z) points as a value which describe the spatial location of what is being "captured". By comparing the Area of Capture for different Media Captures within the same capture scene a consumer can determine the spatial relationships between them and render them correctly.
The four points should be co-planar. The four points form a quadrilateral, not necessarily a rectangle.
The quadrilateral described by the four (X, Y, Z) points defines the plane of interest for the particular media capture.
If the area of capture attribute is specified, it must include X, Y and Z coordinates for all four points. If the area of capture is not specified, it means the media capture is not spatially related to any other media capture (but this can change in a subsequent provider advertisement).
For a switched capture that switches between different sections within a larger area, the area of capture should use coordinates for the larger potential area.
EncodingGroup: {<encodeGroupID value>}
A field with a value equal to the encodeGroupID of the encoding group associated with the media capture.
In order for a provider's individual media captures to be used effectively by a consumer, the provider organizes the media captures into capture scenes, with the structure and contents of these capture scenes being sent from the provider to the consumer.
A capture scene is a structure representing the scene that is captured by a collection of capture devices. A capture scene includes one or more capture scene entries, with each entry including one or more media captures. A capture scene represents, for example, 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 in the capture scene entries. A middle box may also express capture scenes that it constructs from media streams it receives.
A provider may advertise multiple capture scenes or just a single capture scene. A media provider might typically use one capture scene for main participant media and another capture scene for a computer generated presentation. A capture scene may include more than one type of media. For example, a capture scene can include several capture scene entries for video captures, and several capture scene entries for audio captures.
A provider can express spatial relationships between media captures that are included in the same capture scene. But there is no spatial relationship between media captures that are in different capture scenes.
A media provider arranges media captures in a capture scene to help the media consumer choose which captures it wants. The capture scene entries in a capture scene are different alternatives the provider is suggesting for representing the capture scene. The media consumer can choose to receive all media captures from one capture scene entry for each media type (e.g. audio and video), or it can pick and choose media captures regardless of how the provider arranges them in capture scene entries.
Media captures within the same capture scene entry must be of the same media type - it is not possible to mix audio and video captures in the same capture scene entry, for instance. The provider must be capable of encoding and sending all media captures in a single entry simultaneously. A consumer may decide to receive all the media captures in a single capture scene entry, but a consumer could also decide to receive just a subset of those captures. A consumer can also decide to receive media captures from different capture scene entries.
When a provider advertises a capture scene with multiple entries, it is essentially signaling that there are multiple representations of the same scene available. In some cases, these multiple representations would typically be used simultaneously (for instance a "video entry" and an "audio entry"). In some cases the entries would conceptually be alternatives (for instance an entry consisting of 3 video captures versus an entry consisting of just a single video capture). In this latter example, the provider would in the simple case end up providing to the consumer the entry containing the number of video captures that most closely matched the media consumer's number of display devices.
The following is an example of 4 potential capture scene entries for an endpoint-style media provider:
The first entry in this capture scene example is a list of video captures with a spatial relationship to each other. Determination of the order of these captures (VC0, VC1 and VC2) for rendering purposes is accomplished through use of their Area of Capture attributes. The second entry (VC3) and the third entry (VC4) are additional alternatives of how to capture the same room in different ways. The inclusion of the audio capture in the same capture scene indicates that AC0 is associated with those video captures, meaning it comes from the same scene. The audio should be rendered in conjunction with any rendered video captures from the same capture scene.
Attributes can be applied to capture scenes as well as to individual media captures. Attributes specified at this level apply to all constituent media captures.
Area of Scene attribute
The area of scene attribute for a capture scene 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 scene entries. If the provider does not specify the area of scene, but does specify areas of capture, then the consumer may assume the area of scene is greater than or equal to the outer extents of the individual areas of capture.
Scale attribute
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, as described in Section 5. If any media captures have an area of capture attribute or point of capture attribute, then this scale attribute must also be defined. The possible values for this attribute are:
The provider may have constraints or limitations on its ability to send media captures. One type is caused by the physical limitations of capture mechanisms; these constraints are represented by a simultaneous transmission set. The second type of limitation reflects the encoding resources available - bandwidth and macroblocks/second. This type of constraint is captured by encoding groups, discussed below.
An endpoint or MCU can send multiple captures simultaneously, however sometimes there are constraints that limit which captures can be sent simultaneously with other captures. A device may not be able to be used in different ways at the same time. Provider advertisements are made so that the consumer will choose one of several possible mutually exclusive usages of the device. This type of constraint is expressed in a Simultaneous Transmission Set, which lists all the media captures that can be sent at the same time. 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, but not both at the same time.
Simultaneous transmission sets are expressed as sets of the MCs 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 shown in Table 1. The consumer must make sure that it chooses one and not more of the mutually exclusive sets. A consumer may choose any subset of the media captures in a simultaneous set, it does not have to choose all the captures in a simultaneous set if it does not want to receive all of them.
| Simultaneous Sets |
|---|
| {VC0, VC1, VC2} |
| {VC0, VC3, VC2} |
A media provider includes the simultaneous sets in its provider advertisement. These simultaneous set constraints apply across all the captures scenes in the advertisement. The simultaneous transmission sets MUST allow all the media captures in a particular capture scene entry to be used simultaneously.
We have considered how providers can describe the content of media to consumers. We will now consider how the providers communicate information about their abilities to send streams. We introduce two constructs - individual encodings and encoding groups. Consumers will then map the media captures they want onto the encodings with encoding parameters they want. This process is then described.
An individual encoding represents a way to encode a media capture to become an encoded media stream sent from the media provider to the media consumer. An individual encoding has a set of parameters characterizing how the media is encoded. Different media types have different parameters, and different encoding algorithms may have different parameters. An individual encoding can be used for only one actual encoded media stream at a time.
The parameters of an individual encoding represent the maximimum values for certain aspects of the encoding. A particular instantiation into an encoded stream might use lower values than these maximums.
The following tables show the variables for audio and video encoding.
| Name | Description |
|---|---|
| encodeID | A unique identifier for the individual encoding |
| maxBandwidth | Maximum number of bits per second |
| maxH264Mbps | Maximum number of macroblocks per second: ((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 |
| Name | Description |
|---|---|
| maxBandwidth | Maximum number of bits per second |
An encoding group includes a set of one or more individual encodings, plus some parameters that apply to the group as a whole. By grouping multiple individual encodings together, an encoding group describes additional constraints on bandwidth and other parameters for the group. Table 4 shows the parameters and individual encoding sets that are part of an encoding group.
| Name | Description |
|---|---|
| encodeGroupID | A unique identifier for the encoding group |
| maxGroupBandwidth | Maximum number of bits per second relating to all encodings combined |
| maxGroupH264Mbps | Maximum number of macroblocks per second relating to all video encodings combined |
| videoEncodings[] | Set of potential encodings (list of encodeIDs) |
| audioEncodings[] | Set of potential encodings (list of encodeIDs) |
When the individual encodings in a group are instantiated into actual encoded media streams, each stream has a bandwidth that must be less than or equal to the maxBandwidth for the particular individual encoding. The maxGroupBandwidth parameter gives the additional restriction that the sum of all the individual instantiated bandwidths must be less than or equal to the maxGroupBandwidth value.
Likewise, the sum of the macroblocks per second of each instantiated encoding in the group must not exceed the maxGroupH264Mbps value.
The following diagram illustrates the structure of a media provider's Encoding Groups and their contents.
,-------------------------------------------------.
| Media Provider |
| |
| ,--------------------------------------. |
| | ,--------------------------------------. |
| | | ,--------------------------------------. |
| | | | Encoding Group | |
| | | | ,-----------. | |
| | | | | | ,---------. | |
| | | | | | | | ,---------.| |
| | | | | Encoding1 | |Encoding2| |Encoding3|| |
| `.| | | | | | `---------'| |
| `.| `-----------' `---------' | |
| `--------------------------------------' |
`-------------------------------------------------'
A media provider advertises one or more encoding groups. Each encoding group includes one or more individual encodings. Each individual encoding can represent a different way of encoding media. For example one individual encoding may be 1080p60 video, another could be 720p30, with a third being CIF.
While a typical 3 codec/display system might have one encoding group per "codec box", there are many possibilities for the number of encoding groups a provider may be able to offer and for the encoding values in each encoding group.
There is no requirement for all encodings within an encoding group to be instantiated at once.
Every media capture is associated with an encoding group, which is used to instantiate that media capture into one or more encoded streams. Each media capture has an encoding group attribute. The value of this attribute is the encodeGroupID for the encoding group with which it is associated. More than one media capture may use the same encoding group.
The maximum number of streams that can result from a particular encoding group constraint is equal to the number of individual encodings in the group. The actual number of streams used at any time may be less than this maximum. Any of the media captures that use a particular encoding group can be encoded according to any of the individual encodings in the group. If there are multiple individual encodings in the group, then a single media capture can be encoded into multiple different streams at the same time, with each stream following the constraints of a different individual encoding.
The Encoding Groups MUST allow all the media captures in a particular capture scene entry to be used simultaneously.
After receiving the provider's advertised media captures and associated constraints, the consumer must choose which media captures it wishes to receive, and which individual encodings from the provider it wants to use to encode the capture. Each media capture has an encoding group ID attribute which specifies which individual encodings are available to be used for that media capture.
For each media capture the consumer wants to receive, it configures one or more of the encodings in that capture's encoding group. The consumer does this by telling the provider the resolution, frame rate, bandwidth, etc. when asking for streams for its chosen captures. Upon receipt of this configuration command from the consumer, the provider generates streams for each such configured encoding and sends those streams to the consumer.
The consumer must have received at least one capture advertisement from the provider to be able to configure the provider's generation of media streams.
The consumer is able to change its configuration of the provider's encodings any number of times during the call, either in response to a new capture advertisement from the provider or autonomously. The consumer need not send a new configure message to the provider when it receives a new capture advertisement from the provider unless the contents of the new capture advertisement cause the consumer's current configure message to become invalid.
When choosing which streams to receive from the provider, and the encoding characteristics of those streams, the consumer needs to take several things into account its local preference, simultaneity restrictions, and encoding limits.
A variety of local factors will influence the consumer's choice of streams to be received from the provider:
There may be physical simultaneity constraints imposed by the provider that affect the provider's ability to simultaneously send all of the captures the consumer would wish to receive. For instance, a middle box such as an MCU, when connected to a multi-camera room system, might prefer to receive both individual camera streams of the people present in the room and an overall view of the room from a single camera. Some endpoint systems might be able to provide both of these sets of streams simultaneously, whereas others may not (if the overall room view were produced by changing the zoom level on the center camera, for instance).
Each of the provider's encoding groups has limits on bandwidth and macroblocks per second, and the constituent potential encodings have limits on the bandwidth, macroblocks per second, video frame rate, and resolution that can be provided. When choosing the media captures to be received from a provider, a consumer device must ensure that the encoding characteristics requested for each individual media capture fits within the capability of the encoding it is being configured to use, as well as ensuring that the combined encoding characteristics for media captures fit within the capabilities of their associated encoding groups. In some cases, this could cause an otherwise "preferred" choice of streams to be passed over in favour of different streams - for instance, if a set of 3 media captures could only be provided at a low resolution then a 3 screen device could switch to favoring a single, higher quality, stream.
The following diagram shows the basic flow of messages between a media provider and a media consumer. The usage of the "capture advertisement" and "configure encodings" message is described above. The consumer also sends its own capability message to the provider which may contain information about its own capabilities or restrictions.
Diagram for Message Flow
Media Consumer Media Provider
-------------- ------------
| |
|----- Consumer Capability ---------->|
| |
| |
|<---- Capture advertisement ---------|
| |
| |
|------ Configure encodings --------->|
| |
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, 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 scene it advertises to be tailored to the consumer.
TBD - the content of the consumer capability message needs to be better defined. The authors believe there is a need for this message, but have not worked out the details yet.
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:
The infrastructure is designed to be extended rather than requiring 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.
This section shows some examples 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 description:
The audio and video captures for this endpoint can be described as follows.
Video Captures:
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.
,-. 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 an area scale of millimeters.
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 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 planar 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.
Audio Captures:
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:
This constraint indicates it is not possible to use all the VCs at the same time. VC5 can not be used at the same time as VC1 or VC3 or VC4. Also, using every member in the set simultaneously may not make sense - for example VC3(loudest) and VC4 (loudest with PIP). (In addition, there 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 encoding groups, not in the simultaneous transmission sets.)
In this example there are no restrictions on which audio captures can be sent simultaneously.
Encoding Groups:
This example has three encoding groups associated with the video captures. Each group can have 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 in the group(as maxMbps for the entire encoding group is also 489600). Significantly, as up to 3 encodings are available per group, it is possible to transmit some video captures simultaneously that are not in the same entry in the capture scene. For example VC1 and VC3 at the same time.
It is also possible to transmit multiple encodings of a single video capture. For example VC0 can be encoded using ENC0 and ENC1 at the same time, as long as the encoding parameters satisfy the constraints of ENC0, ENC1, and EG0, such as one at 1080p30 and one at 720p30.
encodeGroupID=EG0, maxGroupH264Mbps=489600, maxGroupBandwidth=6000000
encodeID=ENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC1, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC2, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
encodeGroupID=EG1 maxGroupH264Mbps=489600 maxGroupBandwidth=6000000
encodeID=ENC3, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC4, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC5, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
encodeGroupID=EG2 maxGroupH264Mbps=489600 maxGroupBandwidth=6000000
encodeID=ENC6, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC7, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC8, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
For audio, there are five potential encodings available, so all five audio captures can be encoded at the same time.
encodeGroupID=EG3, maxGroupH264Mbps=0, maxGroupBandwidth=320000
encodeID=ENC9, maxBandwidth=64000
encodeID=ENC10, maxBandwidth=64000
encodeID=ENC11, maxBandwidth=64000
encodeID=ENC12, maxBandwidth=64000
encodeID=ENC13, maxBandwidth=64000
Capture Scenes:
The following table represents the capture scenes for this provider. Recall that a capture scene is composed of alternative capture scene entries covering the same scene. Capture Scene #1 is for the main people captures, and Capture Scene #2 is for presentation.
Each row in the table is a separate entry in the capture scene
| Capture Scene #1 |
|---|
| VC0, VC1, VC2 |
| VC3 |
| VC4 |
| VC5 |
| AC0, AC1, AC2 |
| AC3 |
| Capture Scene #2 |
|---|
| VC6 |
| AC4 |
Different capture scenes are unique to each other, non-overlapping. A consumer can choose an entry from each capture scene. 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 entries of audio in Capture Scene #1 represent the "same thing", in that one way to receive the audio is with the 3 audio captures (AC0, AC1, AC2), and another way is with the mixed AC3. The Media Consumer can choose an audio capture entry it is capable of receiving.
The spatial ordering is understood by the media capture attributes area and point of capture.
A Media Consumer would likely want to choose a capture scene entry 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 entry of Capture Scene #1 (VC0, VC1, VC2) and not receive the other entries. A consumer that can receive only one people stream would probably choose one of the other entries.
If the consumer can receive a presentation stream too, it would also choose to receive the only entry from Capture Scene #2 (VC6).
This is an example of an encoding group to illustrate how it can express dependencies between encodings.
encodeGroupID=EG0, maxGroupH264Mbps=489600, maxGroupBandwidth=6000000
encodeID=VIDENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=VIDENC1, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=AUDENC0, maxBandwidth=96000
encodeID=AUDENC1, maxBandwidth=96000
encodeID=AUDENC2, 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 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, AUDENC<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 encoding groups, as in this illustration.
encodeGroupID=EG0, maxGroupH264Mbps=489600, maxGroupBandwidth=6000000
encodeID=VIDENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=VIDENC1, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeGroupID=EG1, maxGroupH264Mbps=0, maxGroupBandwidth=500000
encodeID=AUDENC0, maxBandwidth=96000
encodeID=AUDENC1, maxBandwidth=96000
encodeID=AUDENC2, maxBandwidth=96000
This section shows how an MCU might express its Capture Scenes, 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 Scene #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 Scene #2 | note |
|---|---|
| VC10 | video capture for presentation |
| AC1 | presentation audio to accompany VC10 |
This section gives an example of how a media consumer might behave when deciding how to request streams from the three screen endpoint described in the previous section.
The receive side of a call needs to balance its requirements, based 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 scene advertised by the provider.
A sane, basic, algorithm might be for the consumer to go through each capture scene 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 entries in the video capture scenes 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 encoding groups in order to make best use of the available network bandwidth and its own decoding capabilities.
VC3, VC4 and VC5 are all different entries by themselves, not grouped together in a single entry, 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.
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:
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.
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 encoding group's capability by 3 to determine what resolution and frame rate to configure the provider with or to configure the individual video captures' encoding 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).
Mark Gorzyinski contributed much to the approach. We want to thank Stephen Botzko for helpful discussions on audio.
TBD
TBD
NOTE TO THE RFC-Editor: Please remove this section prior to publication as an RFC.
Changes from 03 to 04:
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate 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. |
| [RFC4796] | Hautakorpi, J. and G. Camarillo, "The Session Description Protocol (SDP) Content Attribute", RFC 4796, February 2007. |
| [RFC5117] | Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008. |
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 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.
The use cases include a case where the person at a receiving endpoint can request to receive media from a particular other endpoint, for 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 scene. The mechanism to do this will probably need to be different than selecting media captures based on capture scenes and attributes.
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?
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 an entry within a capture scene 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 scene. Specifically, there should be a list of active coordinates and their VAD characteristics within the audio stream in addition to the overall VAD information for the capture scene. 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 scene. Additionally, coordinate information can be mapped to video captures by a media stream consumer in order that it can perform "panel switching" if required.
Do we want a way to include private information?