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same, however, it was observed that for about 65% missing slices, the corresponding
slice from the previous frame was available. This allows error-concealment using
pixels from the previous frame in most cases, thus maintaining high spatial reso-
lution, which is important for virtual pan/tilt/zoom. The picture quality was better
even though the average PSNR improved by only about 0.05 dB. In experiments
with other high-spatial-resolution video sequences, the average PSNR improved by
about 1-2 dB compared to single tree per slice. The protocol overhead due to control
messages was observed to be between 5-10% of the total traffic.
5.4
Server Bandwidth Allocation
The slices hosted by the server constitute a set of P2P multicast streams which
generally vary in popularity. A framework for server bandwidth allocation among
multiple P2P multicast streams has been proposed in a related thread of our re-
search [68, 75]. The framework accommodates multiple multicast trees per stream
and can take into account the popularity, the rate-distortion operating point as well
as the peer churn rate associated with each stream. The framework allows minimiz-
ing different metrics like mean distortion among the peers, number of frame-freezes
overall, etc. When the available server bandwidth is scarce, it is very important to
judiciously allocate rate to the most important slices. For the above example with
100 peers and 2 trees per slice, the server capacity was set to 10 Mbps and the limits
on the numbers of direct children associated with the multicast trees were computed
by minimizing expected mean distortion. Note that the 10 Mbps server capacity
is less than the 14.1 Mbps bit-rate of the multi-resolution representation. The opti-
mized rate allocation among the slices was compared against a heuristic scheme that
sequentially allocates rate to slices with ascending slice IDs, stopping when the ca-
pacity exhausts. The optimized rate allocation resulted in about 21% missing slices
whereas the heuristic scheme resulted in about 82% missing slices.
6
Conclusions
Interactive pan/tilt/zoom allows watching user-selected portions of high-resolution
video even on displays of lower spatial resolution. In this chapter, we have reviewed
the technical challenges that must be overcome for watching IRoI video and possi-
ble solutions. From the gamut of solutions, we have elaborated those that facilitate
scaling to large numbers of users.
In the remote streaming scenario, the transmission of the entire high-resolution
video is generally not possible due to bandwidth limitations. Broadly speaking,
there are two approaches to provide a video sequence as controlled by the user's
pan/tilt/zoom commands. The RoI video sequence can either be cropped from
the raw high-resolution video and encoded prior to transmission or the adopted
compression format can allow easy extraction of the relevant portions from the
compressed representation. The first approach possesses the drawback that RoI
video encoding has to be performed for each user separately. Additionally, if the
