Page 250 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 9.8. Interactive Techniques 227

concealment is successful and the error of a given block is su$ciently small,
then the encoder may decide against INTRA encoding. Note that this method
requires the encoder to perform the same error concealment process that was
used at the decoder.

9.8.2.2 Restricted Prediction Based on Feedback
Based on the error trace, prediction of the current frame is restricted to use
only error-free areas in the reference frame. For example, in Figure 9.9(c) the
a ected pels in the reference frame, n + d − 1, are not used for predicting the
current frame, n + d. This stops error propagation to the next frame, n + d +1.
This restricted prediction based on feedback and error tracking was proposed
by Wada in the selective recovery technique [203].
Again, this technique can also bene t from the reduced-complexity error-
tracking algorithm of Steinbach et al. [202], and the coding e$ciency can
also be improved by performing error concealment in the encoder so that both
encoder and decoder use the same reference frames for prediction.

9.8.3 Reference Picture Selection

In reference picture selection (RPS), both the encoder and decoder store mul-
tiple previous frames to be used as reference frames. When the encoder learns,
through feedback messages from the decoder, that the most recent reference
frame contains errors, the encoder switches to use another older reference
frame that is known to be error free. Provided the alternative reference frame
is not too far away from the current frame, the loss in coding e$ciency is not
signi cant. In particular, this technique is more e$cient than the INTRA re-
fresh technique. The RPS technique has been adopted by H.263+ in annex N,
and an enhanced version of the technique has been included in annex U of
H.263++.
Figure 9.10 shows the RPS technique with two types of feedback messages.
In the negative acknowledgment mode, illustrated in Figure 9.10(a), the de-
coder sends a negative acknowledgment (NACK) message whenever errors
are detected in a frame. In the example shown, the decoder detects an error
in frame 3 and sends a NACK(3) message to the encoder. At the encoder,
the encoding operation proceeds in the normal way (i.e., using the most re-
cent reference frame for prediction) until the NACK(3) message arrives before
encoding frame 6. Based on this message, the encoder knows that errors oc-
curred in frame 3 and propagated up to the most recent reference frame 5.
To stop this error propagation, the encoder uses the older error-free reference
frame 2 instead of the most recent reference frame 5 to encode the current
frame 6.