Cryptography Reference
In-Depth Information
Prediction
Output Bit-stream (010101)
Quantization
Input Video
+
Reconstruction
In-Loop
Frame Buffer
Reconstruction
(a) Closed-Loop Encoder
Invers e
Quantization
+
Prediction
Output Video
Output Bit-stream (000101)
In-Loop
Frame Buffer
Quantization
(c) Closed-Loop Decoder
Input Video
Mismatch!
Out-Loop
Frame Buffer
(b) Open-Loop Encoder
Fig. 3.7.
Comparison of open-loop and closed-loop encoder control.
Closed-Loop and Open-Loop
In the current algorithm for SVC [9], the MCTF or hierarchical prediction can
be done using the closed-loop or open-loop configuration. In setting the open-
loop, the encoder performs prediction by referring to a reference frame that
is not available at decoder. This is contrary to the closed-loop configuration.
Here both the encoder and the decoder employ the same reference frame for
prediction.
Fig. 3.7 illustrates the encoding structures for both of these configurations,
where the DCT and Entropy Coder are omitted for simplicity. In Fig. 3.7(a),
the in-loop frame buffer keeps identical content as the one at the decoder
in Fig. 3.7(c). Particularly, the reference frame at encoder is produced by a
Reconstruction Loop, which has the same structure as the decoder. For the
open-loop configuration, the out-loop frame buffer in Fig. 3.7(b) may have
different contents from the in-loop frame buffer at the decoder. In practice,
the reference frame in the out-loop frame buffer can be arbitrarily generated
by the encoder.
The configuration of prediction loop affects the prediction e
ciency and
the quality of decoded video. With the closed-loop configuration, the encoder
uses the reconstructed frame for prediction. Since the quality of reconstructed
frame depends on the target bit rate, the prediction becomes less e
cient
when the video is encoded at a very low bit rate. Poor prediction e
ciency
causes reduced coding e
ciency and rate-distortion performance. On the other
hand, one can freely refer to the original frame for better prediction in the
open-loop configuration. However, mismatch errors are created because the
original frame is only available at the encoder. At the decoder, the reference
frame is a quantized version of the original one. Quantization errors cause the
