Graphics Reference
In-Depth Information
where
bitdepth: Bit depth of each pixel
W: Number of horizontal pixels
H: Number of vertical pixels
O
i
: Pixel value of the reference picture
D
i
: Pixel value of the decoded picture
i: Pixel address
PSNR is calculated for each YCbCr component. In YCbCr domain, human
visual system is more sensitive to luminance (Y) than to chrominance (Cb or Cr);
accordingly, and in practice, PSNR for luminance (PSNR Y) is a more important
metric for objective quality measurements.
In order to compare the coding efficiency of a reference codec vs. the one
being evaluated, the average difference of the two R-D curves is calculated. The
average bit rate difference (difference in horizontal direction) is referred to as BD
(Bjøntegaard's Delta) Rate and the average PSNR difference (difference in vertical
direction) is referred to as BD PSNR [
1
].
In order to calculate BD Rate and BD PSNR, the two R-D curves (corresponding
to reference and tested codecs) are approximated by the following cubic polynomial.
PSNR
D
a
C
b
.
bit rate
/
C
c
.
bit rat
e/
2
C
d
.
bit rat
e/
3
Parameters a-d in the above equation can be derived by using four data points
(PSNR and bit rate points). This polynomial approximation will then allow us to
derive the BD Rate by integrating the difference of two curves in horizontal direction
and BD PSNR by integrating the difference of two curves in vertical direction (see
Fig.
9.4
).
BD Rate and BD PSNR have been widely used to evaluate coding tools, in the
HEVC standardization work. It is however known that such approximation could
sometimes lead to large errors, especially for large pictures (e.g. class A sequences).
To further improve the approximation accuracy, a piece-wise cubic interpolation is
proposed as an alternative [
2
].
9.2.7
R-D Optimization
HEVC encoder flexibility stems from the fact that it contains an increased number
of coding tools, beyond those provided by earlier video coding standards e.g., AVC.
This added flexibility allows an encoder to adaptively determine block dependent
coding parameters in terms of:
1. Coding unit (CU) quadtree structure, prediction unit (PU) partition modes and
transform unit (TU) quadtree structure;
2. Intra PU prediction mode;
