Graphics Reference
In-Depth Information
9.2.2
Test Conditions
During the development of the HEVC specification, establishment of Common
Test Conditions (CTC) provided a well-defined platform on which experiments
for coding tool evaluations are performed [
3
]. Since HEVC coding performance
evaluations are carried out according to the CTC, a detailed description of CTC key
elements will follow.
9.2.3
Prediction Structure
For performance evaluation, CTC defines the following prediction structures.
1. All Intra (AI)
2. Random Access (RA)
3. Low Delay P picture (LDP)
4. Low Delay B picture (LDB)
In these configurations, QP (Quantization Parameter) value can be modified by
adding to it a “QP offset” value. That is, CTC defines QP of the first picture (QP of
an I picture, QPI, with I picture defined below) and the QP of the following pictures
are derived as QP
D
(QPI
C
QP offset), with QP offset being determined according
to the picture type (e.g., P & B pictures, defined below) or a picture temporal ID.
An I (intra) picture refers to a picture that can be decoded independently without
requiring prediction data from other decoded pictures. A P (predicted) picture, in
general, requires picture sample data from one other I, P or B picture to generate
each predicted sample block. A B (bi-predicted), in general, requires picture sample
data from two other I, P or B pictures to generate each predicted sample block.
9.2.3.1
All Intra (AI)
In this configuration, each picture is encoded as an I picture. Because no inter
picture prediction is used, it is thus suitable for low delay and higher bit rate
applications. QP offset in this configuration is 0 since QP is kept constant over the
whole sequence. Figure
9.1
shows an example of this prediction structure.
9.2.3.2
Random Access (RA)
In this configuration, a hierarchical B structure is used [
21
]. Figure
9.2
shows an
example of this prediction structure. The coding efficiency achieved by the bi-
directional hierarchical prediction structure is higher than the other configurations.
It has however a larger delay due to the reordering of the pictures. To control
possible error propagation and ease of random access, I pictures are inserted
periodically. QP offset values for each picture are summarized in Fig.
9.2
.
