Page 43 - ITU Journal, ICT Discoveries, Volume 3, No. 1, June 2020 Special issue: The future of video and immersive media

ITU Journal: ICT Discoveries, Vol. 3(1), June 2020

3.1.2 Temporal property overlays, as the system could take smart
decisions based on its available network
OMAFv2 provides tools that enable altering the
temporal properties of the overlay based on user bandwidth and resources, such as memory
interaction and the background timeline. A user and system buffers.
may choose to switch to a new overlay from the 4. Opacity and Alpha Channel: the standard
currently viewing overlay, potentially altering the allows us to signal the opacity and the alpha
temporal property of the overlay relative to the values of an overlay. This helps in controlling
background video (see also section 3.1.3). The the transparency of the overlay.
standard also allows a content provider to control
the playback of the overlay based on the user’s 4. MULTI-VIEWPOINT IN THE MPEG
viewing direction. This feature enables the control OMAF STANDARD
of the temporal property of the overlay based on the
spatial direction. For example, the content provider Omnidirectional cameras typically capture subjects
may choose to pause the playback of an overlay with sufficient details if they are close to it. Subjects
when the overlay is not visible in the user’s which are further away from the camera appear
viewport, and once the user returns to the viewport with lower details. In addition, OMAF (v1) content
where the overlay is visible, the overlay is played enables a single viewpoint to have three degrees of
back from the pause state. Such a scenario can be freedom. 3DoF allows perusing content in all
realized in case of both 2D and spherical overlays of directions around a single location. However,
Fig. 2 and Fig. 3, respectively. a single viewpoint does not allow watching a person,
event or object of interest from a different location.
3.1.3 Interactivity property Hence, OMAFv2 has incorporated support for
The OMAFv2 standard allows the flexibility to multiple viewpoints to address the need to enable
enable user interaction with an overlay. Some of the high quality content capture as well as provide the
possible interactions that could be performed on an possibility of experiencing any subject or event of
overlay are the following: interest from a different perspective. Furthermore,
multiple viewpoints facilitate leveraging the well-
1) rotate an overlay; established cinematic rules for multi-camera

2) resize an overlay; directors that make use of different shot types, such
as wide-angles, mid-shots, close-ups, etc. [21].
3) switch on-off an overlay;
The standard supports enablers which provide
4) change the spatial position within the more freedom for content creators to design
omnidirectional system; content for diverse scenarios and effective
5) switch to a new overlay. storytelling.

3.1.4 Inherent property
OMAFv2 provides for the following inherent
properties to be either explicitly signaled or implied
by the usage of an overlay:

1. Source: an overlay could be a separate bit
stream from the background bit stream or
could be part of the same stream as the
background stream. The latter indicates that
the overlay video spatially coexists with the
background video.
2. Representation: an overlay could be a 2D Fig. 5 – Example of a basketball game with multiple
video/image or a spherically projected viewpoints.
video/image. Fig. 5 illustrates an example of OMAFv2 content
with multiple viewpoints (VPk, VPl and VPm). This
3. Priority: the standard allows us to signal a allows the user to experience the action close to
priority bound to an overlay. Signaling the where it happens, as well as from different
priority is helpful for the omnidirectional perspectives. In the following, the key concepts and
system in the presence of a large number of

© International Telecommunication Union, 2020 21