Graphics Reference
In-Depth Information
Fig. 7.16
Example of BO,
where the
dotted curve
represents original samples
and the
solid curve
denotes
reconstructed samples.
Reproduced with permission
from [
13
], © 2012 IEEE
band index = k+3
band index = k+2
band index = k+1
band index = k
values). The sample values range is divided into 32 equal bands. For 8-bit samples
in the range from 0 to 255, the width of a band is 8. Thus, sample values from 8
k
to
8
k
C
7 belong to band
k
,where
k
ranges from 0 to 31. The difference between the
original samples and reconstructed samples in a band (i.e. the offset of a band) can
be signaled to the decoder. There is no constraint on the offset sign for the BO.
Figure
7.16
demonstrates how the BO compensates sample intensity offset of
a region. The horizontal axis denotes the sample position and the vertical axis
denotes the sample value. The dotted curve represents the original samples, while
the solid curve denotes the reconstructed samples, affected by quantization errors of
prediction residues and phase shifts because of the coded motion vectors that deviate
from the true motion. As shown in Fig.
7.16
, if there is a phase shift (difference)
between the reconstructed motion vector and the “true” motion vector, a smooth
region with a gradient may be offset with a certain value compared to the original
signal. In this example, the reconstructed samples are shifted to the left compared
to the original samples, which results in a systematic negative error that can be
corrected by BO for bands
k
,
k
C
1,
k
C
2, and
k
C
3, where the samples ranging
from
k
*8 to ((
k
C
1) * 8)
1 are classified as belonging to band
k
, and can be
modified by using the corresponding offset value.
In HEVC, only offsets of four consecutive bands and the starting (or minimum)
band position of the current region are signaled to the decoder [
25
,
29
]. Four offsets
are signaled in the BO, which is equal to the number of signaled offsets in EO (the
number of offsets is limited to reduce the line buffer requirement). The reason for
signaling only four bands is that the range of sample values in a region formed
by CTBs can be quite limited. Therefore, by signaling the starting band position
of current region, BO can identify the minimum sample value to be compensated
in the current region so that the decoder can recover it, as shown in the example
in Fig.
7.17
. This is especially true for chroma CTBs. In natural images, chroma
components are often represented by a narrow-band signal, which means that by
several band offsets, the encoder can recover most samples in the region.
