Internet DRAFT - draft-abhishek-mmusic-superimposition-grouping
draft-abhishek-mmusic-superimposition-grouping
mmusic R. Abhishek
Internet-Draft S. Wenger
Intended status: Standards Track Tencent
Expires: December 3, 2021 June 1, 2021
SDP Superimposition Grouping framework
draft-abhishek-mmusic-superimposition-grouping-02
Abstract
This document defines semantics that allow for signaling a new SDP
group "supim" for superimposed media in an SDP session. The "supim"
attribute can be used by the application to relate all the fully or
partly superimposed visual media streams enabling them to be added as
an overlay on top of any one or more background visual media streams.
The superimposition grouping semantics is helpful if the media stream
data is separate and transported via different sessions.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 3, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Media Superimposition in SDP . . . . . . . . . . . . . . . . 3
4. Superimposition Group Identification Attribute . . . . . . . 4
5. Use of group and mid . . . . . . . . . . . . . . . . . . . . 5
6. "superimposition" Attribute for Superimposition Group
Identification Attribute . . . . . . . . . . . . . . . . . . 5
7. Example of Supim . . . . . . . . . . . . . . . . . . . . . . 6
8. Relationship with Existing Specifications (informative) . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This document defines semantics that allow for signaling a new SDP
group "supim" for superimposed media in an SDP session. The "supim"
attribute can be used by the application to relate all the fully or
partly superimposed visual media streams enabling them to be added as
an overlay on top of any one or more background visual media streams.
The superimposition grouping semantics is helpful if the media stream
data is separate and transported via different sessions.
Media superimposition herein is defined to be a visual media stream
(video/image/text) that is fully or partly superimposed on top of an
already existing visual media stream such that the resulting
foreground and background media can be displayed simultaneously.
Superimposition can be recursive in that visual media that is
superimposed against its background can, in turn, be the background
of another superimposed visual media. The superimposed visual media
displayed over a background media content may be anywhere between
opaque and transparent. Examples of applications for video
superimposition include real-time multi-party gaming, where these
superimposed media may be used to provide additional details or stats
about each player, or multi-party teleconferencing where visual media
from users in the teleconference may be superimposed over a
background media or over each other.
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This document describes new SDP group semantics for grouping the
superimposition in an SDP session. An SDP session description
consists of one or multiple media lines known as "m" lines which can
be identified by a token carried in a "mid" attribute. The SDP
session describes a session-level group-level attribute that groups
different media lines using a defined group semantics. The semantics
defined in this memo are to be used in conjunction with "The Session
Description Protocol (SDP) Grouping Framework" [RFC5888].
We have studied the existing specifications, including the CLUE
framework [RFC8845] and work in MPEG, and found that such work is not
covering our intended application space; please refer to Section 8
for details. The superimposition grouping as described below enables
a compliant receiver/renderer implementation to know the relative
relevance of the visual media as coded by the sender(s) and, in a
compliant implementation, observed by the renderer through
superimposition when needed.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Media Superimposition in SDP
SDP is predominantly used for describing the format for multimedia
communication sessions. Many SDP-based systems use open standards
such as RTP [RFC3550] for media transport and SIP [RFC3261] for
session setup and control. An SDP session may contain more than one
media description, with each media description identified by
"m"=line. Each line denotes a single media stream. If multiple
visual media lines are present in a session, at present, rendering
aspects, including their possible superimposition (foreground/
background), relationship at the rendering device is undefined. This
memo introduces a mechanism in which certain rendering information
becomes available. The rendering information herein is limited to
the foreground/background relationship of each grouped media to other
media streams through a layer order value, and optionally a
transparency value. Where, spatially, the media is rendered is not
covered by this memo, and is in many application scenarios a function
of the user interface. An example is shown in Figure 1, where three
foreground media streams have been superimposed over a background
media stream, with Media B being partly superimposed over Media C.
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_____________________________________
| ================= |
| ==== Media A ==== |
| ================= |
| ================= |
| +++++++++++++++++ |
| ++++ Media B ++++ |
| ############+++++++++++++++++ |
| ############+++++++++++++++++ |
| #### Media C #### |
| ################# |
|_____________________________________|
Figure 1: A example of media superimposition
Of course, assuming sufficient screen real-estate, a renderer may not
have to rely on superimposition mechanisms at all-when there is
enough screen real-estate available, a valid display strategy may
well be to show all media without overlapping and hence without
superimposition. However, when the screen real-estate becomes
insufficient, then the information provided by the mechanisms defined
in this memo can be used to order (in the sense of foreground to
background) the visual media according to a hierarchy chosen by the
sender or a MANE (media-aware network element), and based on their
application knowledge.
When multiple superimposed streams are transmitted within a session,
the receiver needs to be able to relate the media streams to each
other. This is achieved by the SDP grouping framework [RFC5888] by
using the "group" attribute that groups different "m" lines in a
session. By using a new superimpose group semantic defined in this
memo, a group's media streams can be uniquely identified across
multiple SDP descriptions exchanged with different receivers, thereby
identifying the streams in terms of their role in the session
irrespective of their media type and transport protocol. These
superimposed streams within the group may be multiplexed based on the
guidelines defined in [draft-ietf-avtcore-multiplex-guidelines-12].
4. Superimposition Group Identification Attribute
The "superimposition media stream identification" attribute, "supim",
is used to identify the relationship of superimposed media streams
within a session description. In a superimposition group, the media
lines MAY have different media formats. There is no defined behavior
for the rendering of non- visual media being grouped in a
superimposition group. It is assumed that all the media streams are
that need to be time- synchronized are time-synchronized. Its
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formatting follows [RFC5888] in the use of the 'mid' attribute to
identify the media line to be included in the superimposition.
It is used for grouping the foreground and the background media
streams intended for the purpose of composition with foreground media
to be superimposed over the background media stream. A media player
that chooses to implement the extension and receives a session
description that contains "m" lines grouped together using "supim"
semantics is able to superimpose the foreground media streams on top
of the background media stream in cases where there is overlap. For
non-supporting devices, these media streams are treated as
independent media streams.
5. Use of group and mid
All group and mid attributes MUST follow the rules defined in
[RFC5888]. The "mid" attribute MUST be used for all "m" lines
covering visual media within a session description for which a
foreground/background relationship is to be defined. The foreground/
background relationship of visual media within a session description
that is not covered in a group is undefined. Multiple groups MUST
not be used within one session. If the identification-tags
associated with "a=group" lines do not map to any "m" lines, the
identification-tags MUST be ignored.
semantics = "supim" /; semantics extension
as defined in RFC5888
6. "superimposition" Attribute for Superimposition Group Identification
Attribute
This memo defines a new media-level attribute, "superimposition",
with the following ABNF [RFC5234]. The identification-tag is defined
in [RFC5888].
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superimposition-attribute =
"superimposition:" super-opt *(SP super-opt)
super-opt = super-trans / super-layer
super-trans = "transparency:" super-trans-val
super-layer = "layer:" super-layer-val
super-trans-val = signed-integer ; range [-128, 127]
super-layer-val = signed-integer ; range [0, 255]
signed-integer =
<zero-based-integer defined in RFC8866>
/ "-" <integer defined in RFC8866>
attribute = <attribute defined in RFC4566>
attribute =/ superimposition-attribute
The transparency for the media stream is identified by its super-
trans-val values in the super-trans attribute. The value MUST be an
ASCII representation of an 8 bit signed integer with values between
"-128" and "127", and linear weighting between the two extremes. A
value of -128 means the media stream is opaque, and the highest value
of 127 means it is transparent. Further details of interpretion is
to be left open to the implementer. The layering order value for the
media stream is identified by super-layer-val. It MUST be an integer
value between 0 and n, where the value 0 represents the deepest
background layer. For each k within 0..n, a reconstructed sample of
the k-th media is superimposed (while perhaps applying an super-
trans-val value) on the 0 to k-th reconstructed samples in the same
spatial position. Each "m" line in a session MUST NOT contain more
than one instance of super-opt attribute.
7. Example of Supim
The following example shows a session description for superimposed
media streams in an SDP session. The "group" line indicates that the
"m" lines with tokens 1, 2 and 3 are grouped for the purpose of
superimposition.
In the example shown below, three media streams are being transmitted
for superimposition. The background media stream along with the
foreground media streams are grouped together using "supim". All
media streams are videos with "superimposition" attribute. The media
stream with layer order value 0 is intended for background.
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v=0
o=Alice 292742730 29277831 IN IP4 233.252.0.74
c=IN IP4 233.252.0.79
t=0 0
a=group:supim 1 2 3
m=video 30000 RTP/AVP 31
a=mid:1
a= superimposition:transparency= -128, layer=0
m=video 30002 RTP/AVP 31
a=mid:2
a= superimposition:transparency=35, layer=1
m=video 30003 RTP/AVP 31
a=mid:3
a= superimposition:transparency=75, layer=2
The transparency value is used for composing the foreground with the
background media [Wiki.Alpha-compositing]. This value itself does
not define the transparency of each pixel but is applied to each
pixel within a frame and defines the factor by which the transparency
of each pixel within a frame is to be increased or decreased. The
"layer" value is relevant when two or more media streams are to be
composed. When the transparency value of the foreground is -128, the
composed image will be the foreground image, as it is being displayed
as opaque. Similarly, if the transparency value for the foreground
media is 127, the resulting image will be the background media, as
the foreground media stream is being presented fully transparent,
hence invisible. The details of the weighting of foreground and
background sample values based on a given super-trans value is left
to the implementation, beyond the abstract definition that value
equal to -128 means opaque, and value equal to 127 means transparent,
and the weighting is to be implemented such that it is visually
linear for the values in between. We do not define a weighting
formula in this specification as these formulae would depend on many
factors such as the colorspace and the sampling structure of the
media.
8. Relationship with Existing Specifications (informative)
Edt. Note: maybe we remove this section later once there is a general
understanding why the existing specifications in its current form is
unsuitable. The CLUE framework [RFC8845] is the IETF's chosen
technology for the applications requiring defining multiple
"captures" (camera views), and their geo-spatial relationship to
each. However, information pertaining to display/rendering is
outside of CLUE's scope. While many CLUE-capable receivers infer
appropriate rendering strategies from the information offered by
CLUE, the CLUE framework has generally assumed non-overlapped
rendering of transmitted and reconstructed video streams from the
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multiple captures, often on different physical rendering devices.
Insofar, we concluded that the CLUE framework neither supports the
application we contemplate in this memo, nor would it be sensible to
enhance the CLUE specifications with rendering-related mechanisms.
There are certain technologies from standards bodies such as MPEG
[MPEG-4], often described as "scene descriptions", that to a certain
extent can address the applications we contemplate. We evaluated the
technologies we are aware of and concluded that something different
is required. We base our assumption on a) the complexity of these
mechanisms, and b) their design as a metadata media stream, which in
the IETF context would be conveyed in RTP sessions or similar, rather
than a static or semi-static stream description that is best conveyed
at session setup or renegotiation using SDP.
9. Security Considerations
All security considerations as defined in [RFC5888] apply:
Using the "group" parameter with FID semantics, an entity that
managed to modify the session descriptions exchanged between the
participants to establish a multimedia session could force the
participants to send a copy of the media to any destination of its
choosing.
Integrity mechanisms provided by protocols used to exchange session
descriptions and media encryption can be used to prevent this attack.
In SIP, Secure/Multipurpose Internet Mail Extensions (S/MIME)
[RFC8550] and Transport Layer Security (TLS) [RFC8446] can be used to
protect session description exchanges in an end-to-end and a hop-
byhop fashion, respectively.
10. IANA Considerations
The following contact information shall be used for all registrations
included here:
Rohit Abhishek <rabhishek@rabhishek.com>
Stephan Wenger <stewe@stewe.org>
The IETF MMUSIC working group <mmusic@ietf.org> or its successor
as designated by the IESG.
This document defines a new SDP group semantics value for media
superimposition for a SDP session. This attribute can be used by the
application to group the foreground and the background media streams
to be superimposed together in a session. Semantics values to be
used with this framework should be registered by the IANA following
the Standards Action policy [RFC8126]. This document adds a new
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group semantics value to the sdp-paramters registry group defined in
[RFC5888] [RFC8859].
IANA is requested to register the following semantics value in the
"sdp-parameters" in the registry.
Semantics Token Reference
----------------------------------------------
Superimposition supim RFCXXXX
The "supim" attribute is used to group different media streams to be
superimposed together with one background media stream and the rest
foreground streams. Its format is defined in Section 4.
IANA is requested to register the semantics value for SDP media-level
attribute "superimposition" for "sdp-attributes(media-level only)".
The registration procedure in [RFC8866] applies.
SDP Attribute ("sdp-attributes(media level only)"):
Attribute name: superimposition: transparency, layer
Long form: superimposition transparency, superimposition layer
Type of name: att-field
Type of attribute: media level only
Subject to charset: no
Purpose: RFC 5583
Reference: RFC 5583
Values: super-trans-val, super-layer-val
11. Acknowledgements
The authors would like to thank Christer Holmberg and Paul Kyzivat
for reviewing the draft and providing key ideas.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<https://www.rfc-editor.org/info/rfc5888>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8550] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
April 2019, <https://www.rfc-editor.org/info/rfc8550>.
[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/info/rfc8859>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.
12.2. Informative References
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[draft-ietf-avtcore-multiplex-guidelines-12]
Westerlund, M., Burman, B., Perkins, C., Alvestrand, H.,
and R. Even, "Guidelines for using the Multiplexing
Features of RTP to Support Multiple Media Streams", draft-
ietf-avtcore-multiplex-guidelines-12 (work in progress),
June 2020.
[MPEG-4] "MPEG-4 Scene Description and Application Engine",
<https://mpeg.chiariglione.org/standards/mpeg-4/scene-
description-and-application-engine>.
[RFC8845] Duckworth, M., Ed., Pepperell, A., and S. Wenger,
"Framework for Telepresence Multi-Streams", RFC 8845,
DOI 10.17487/RFC8845, January 2021,
<https://www.rfc-editor.org/info/rfc8845>.
[Wiki.Alpha-compositing]
"Alpha compositing",
<https://en.wikipedia.org/wiki/Alpha_compositing>.
Authors' Addresses
Rohit Abhishek
Tencent
2747 Park Blvd
Palo Alto 94588
USA
Email: rabhishek@rabhishek.com
Stephan Wenger
Tencent
2747 Park Blvd
Palo Alto 94588
USA
Email: stewe@stewe.org
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