Internet DRAFT - draft-greevenbosch-mmusic-sdp-parallax
draft-greevenbosch-mmusic-sdp-parallax
mmusic B. Greevenbosch
Internet-Draft Y. Hui
Intended status: Standards Track Huawei Technologies
Expires: April 25, 2013 October 22, 2012
SDP attribute to signal parallax
draft-greevenbosch-mmusic-sdp-parallax-01
Abstract
This document introduces a "ParallaxInfo" attribute in SDP. This
attribute can be used in stereoscopic applications, to signal the
depth positioning of 2D media data within the 3D space.
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Note
Discussion and suggestions for improvement are requested, and should
be sent to mmusic@ietf.org.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. The ParallaxInfo attribute . . . . . . . . . . . . . . . . . . 7
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Normative References . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
To see a 3D scene, the human brain interprets two different views as
perceived by the left and right eyes, and fuses these views into a
single 3D perception. The depth of the object is perceived by
interpreting the horizontal shift of that object between the views.
This shift is called "parallax".
In stereoscopic 3D multimedia applications, there are various ways to
transmit media streams in 3D. One way is to transmit two different
streams, one for the left eye and one for the right eye. These
streams are then projected to the relevant eyes using the appropriate
technology.
When sending text streams in 3D, the solution mentioned above would
imply sending the same text stream twice. Since the two streams
would only differ in horizontal positioning, this introduces a lot of
unnecessary overhead.
This document specifies a "ParallaxInfo" attribute in SDP [RFC4566],
which is used to transfer the parallax information. It eliminates
the need to send two different streams separately, as they can be
calculated from a single stream and the "ParallaxInfo" attribute
value.
The attribute transfers this information as two parameters: one
indicating which view (left/right/centre) is carried by the stream,
and another to signal the parallax between the objects.
The attribute is not restricted for signalling the parallax of text
streams, but it can also be used to place other 2D objects in the 3D
space. Examples include a channel logo, an electronic programme
guide and on-screen display of an audio volume indicator.
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2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Definitions
L-view
A visual entity that is to be projected to the left eye. In the
case of video, the L-view is a video frame designated for the left
eye. In the case of text, the L-view is the text positioned for
viewing by the left eye.
L-stream
A sequence of L-views, which is streamed to the device.
R-view
A visual entity that is to be projected to the right eye. In the
case of video, the R-view is a video frame designated for the
right eye. In the case of text, the R-view is the text positioned
for viewing by the right eye.
R-stream
A sequence of R-views, which is streamed to the device.
C-view
The centre view: a visual entity as seen from a viewpoint between
the left and right eyes. The C-view can be used to calculate the
L- and R-views.
C-stream
A sequence of C-views, which is streamed to the device.
stereoscopic device
A device that is able to produce and display different images for
the left and right eyes, such that the viewer can experience a 3D
view.
2D device
A device that is not able to produce and display different images
for the left and right eyes.
2D media stream
A sequence of two dimensional visual entities (such as text or 2D
graphics), which is streamed to the device.
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4. The ParallaxInfo attribute
The SDP attribute "ParallaxInfo" is used to transmit the depth
positioning of 2D media data, such as a text stream, in the 3D space.
The attribute has the following syntax:
a=ParallaxInfo:<transmitted position> <parallax>
The <transmitted position> indicates whether the L-, C- or R-stream
is transmitted, whereas <parallax> indicates the parallax (i.e.
shift) between corresponding L- and R-views in pixels, normalised to
a screen width of 11520 pixels. To convert the value of <parallax>
to match the display video screen width W, it has to be divided by a
factor F=11520/W.
The <transmitted position> can have the following values:
"L" indicates that the transmitted stream is the L-stream. A
stereoscopic device MUST calculate the corresponding R-views by
shifting the L-views <parallax>/F pixels towards the right.
"C" indicates that the transmitted stream is the C-stream. A
stereoscopic device MUST calculate the corresponding L-views by
shifting the C-views <parallax>/(2*F) pixels towards the left, and
the R-views by shifting the C-views <parallax>/(2*F) pixels
towards the right. <parallax> SHOULD be even. If it is odd, the
divided value MUST be rounded off towards zero.
"R" indicates that the transmitted stream is the R-stream. A
stereoscopic device MUST calculate the corresponding L-views by
shifting the R-views <parallax>/F pixels towards the left.
<parallax> MAY be negative. In this case, the shift direction is
reversed.
The "ParallaxInfo" attribute can be a session-level attribute or a
media-level attribute. As a session-level attribute, it specifies
the default parallax value which can be applied to all the 2D media
streams in the session being described. As a media-level attribute,
it specifies the parallax value which can be applied to the
associated 2D media stream, overriding any session-level parallax
value specified.
The stereoscopic device MAY use the session-level attribute value for
on-screen display, for example an audio volume indicaton, channel
indication or electronic programme guide.
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Notice that a 2D device that does not support the "ParallaxInfo"
attribute will ignore it, and therefore display the 2D media data on
the position as transmitted.
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5. Example
The following is an example of an SDP description of a session with
an audio stream, a video stream and a 3GPP timed text stream (see
[3gpp-tt]), streamed using RTP as per [RFC4396]. If the display
resolution is 1280x720, the parallax is scaled down with a factor
F=11952/1280=9. The transmitted text stream is the L-stream, and
with the example display resolution each R-view is 144/9=16 pixels on
the left of the L-view. This corresponds to the virtual positioning
of the text in front of the screen.
v=0
o=Alice 2890844526 2890842807 IN IP4 131.163.72.4
s=The technology of 3D-TV
c=IN IP4 131.164.74.2
t=0 0
a=ParallaxInfo:L -180
m=video 49170 RTP/AVP 99
a=rtpmap:99 H264/90000
m=video 52888 RTP/AVP 97
a=rtpmap:97 3gpp-tt/1000
a=ParallaxInfo:L -144
m=audio 52890 RTP/AVP 10
a=rtpmap:10 L16/16000/2
Notice that the default value "-180" is overridden by the value
"-144" for the text stream. However the "-180" value is still
signalled for on-screen display of e.g. volume control and other 2D
graphics.
In case each R-view is 24 pixels (216 pixels in the reference
resolution) on the right of the associated L-view, i.e. the virtual
positioning of the text is behind the screen, then the three lines
defined for 3gpp-tt can be replaced as follows:
m=video 52888 RTP/AVP 97
a=rtpmap:97 3gpp-tt/1000
a=ParallaxInfo:L 216
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6. Remarks
A positive parallax value indicates virtual positioning of the 2D
media data behind the screen. This is the case when the objects in
the L-view are on the left of the same objects in the R-view.
Similarly, a negative parallax value indicates that the objects in
the R-view are on the left of the same objects in the L-view, and
corresponds to virtual positioning in front of the screen.
Since the "ParallaxInfo" attribute indicates a shift of the
transmitted stream, it might happen that the L- or R-view trespasses
the bounderies of the display. In this case, clipping is necessary,
as illustrated by Figure 1.
Similarly, there are areas which are covered by the L-view but not by
the R-view and vice versa. These areas need to be filled in a
sensible way. Since the "ParallaxInfo" attribute is designed for
objects that overlay other video data (e.g. subtitles), it is trivial
to fill in uncovered areas by using the underlying video data.
However, if there is no underlying video data, other mechanisms to
fill in the uncovered areas need to be defined. Definition of these
mechanisms are out of scope of this document.
+-----------------------+ +-----------------------+
|SCREEN . | |SCREEN . |
| . | | . |
| +-------------------+ | +-------------------+|
| |TEXT . | | |TEXT . ||
| +-------------------+ | +-------------------+|
| . | | . |
| . | | . |
+-----------------------+ +-----------------------+
|
| CLIP
V
+-----------------------+ +-----------------------+
|SCREEN . | |SCREEN . |
| . | | . |
| +------------------+ | +-------------------+|
| |TEXT . | | |TEXT . ||
| +------------------+ | +-------------------+|
| . | | . |
| . | | . |
+-----------------------+ +-----------------------+
Figure 1
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The normalisation of the parallax towards a reference screen width of
11520 pixels was chosen to ensure correct display of the same stream
on different video display resolutions. This is especially useful
when the underlying video is coded with a scalable video codec, which
does not have a fixed video resolution.
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7. Security Considerations
The authors foresee no security issues in addition to those already
listed in [RFC4566].
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8. IANA Considerations
Following the guidelines in [RFC4566], the SDP attribute has to be
registered at IANA:
o Contact name/email: authors of this RFC
o Attribute name: ParallaxInfo
o Long-form attribute name: Parallax info for the depth positioning
of 2D media data in the 3D space
o Type of attribute: session level and media level
o Subject to charset: no
This attribute is used to signal how 2D media data is to be displayed
in the 3D space. It indicates the shift of the respective left and
right views.
The attribute has the following ABNF (see [RFC4234]) description:
ParalaxInfo = "a=ParallaxInfo:" TransmittedPosition Parallax
TransmittedPosition = "C"/"L"/"R"
Parallax = num-val
The attribute transfers this information as two parameters:
"TransmittedPosition" indicates which view of the 2D media data (left
"L", right "R" or centre "C") is carried by the stream, and
"Parallax" signals the parallax (in pixels) of objects in the 2D
media stream.
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9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4396] Rey, J. and Y. Matsui, "RTP Payload Format for 3rd
Generation Partnership Project (3GPP) Timed Text",
RFC 4396, February 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[3gpp-tt] 3GPP, "Transparent end-to-end packet switched streaming
service (PSS); Protocols and codecs (Release 5)",
TS 26.234 v5.3.0, December 2002.
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Authors' Addresses
Bert Greevenbosch
Huawei Technologies Co., Ltd.
Huawei Industrial Base
Bantian, Longgang District
Shenzhen 518129
P.R. China
Phone: +86-755-28978088
Email: bert.greevenbosch@huawei.com
Hui Yu
Huawei Technologies Co., Ltd.
Huawei Nanjing R&D Center
101 Software Avenue
Yuhuatai District
Nanjing 210012
P.R. China
Phone: +86-25-56620323
Email: huiyu@huawei.com
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