Game Development Reference
In-Depth Information
unnoticeable distortions to which the HVS is insensitive. If the distortions are below
the threshold of detection, the distorted image is deemed to be perfect visual quality.
For the distortions beyond the threshold, the second stage is performed for evaluating
the distorted level based on the low-level visual property of perceived contrast and
the mid-level visual property of global precedence.
Let the I and I denote the original and the distorted images, the distortion image
is computed by E
I . Then M-level DWTs of I and E are performed to obtain
sub-bands S I and S E . The vector of spatial frequencies f is calculated via,
=
I
ʳʽ tan 180
2 m
f m =
(11.8)
where ʳ denotes the resolution of the display, and ʽ is the viewing distance. For each
f m , the contrast detection threshold CT
(
E fm |
I
)
is determined by,
I f )
a 0 f a 2 ln ( f ) + a 1
C
(
CT
(
E fm |
I
) =
(11.9)
If CT
, then the distortion is regarded as negligible. Otherwise
the perceived contrast of the distortions d pc =
(
E fm )<
CT
(
E fm |
I
)
C
(
E
)
is computed by,
1
N
2
1
2
N
1
u L ( I )
C
(
E
) =
1 [
L
(
E i +
u I )
u L ( E + u i ) ]
(11.10)
i
=
N i = 1 I i and u L ( I ) =
N i = 1 L
1
1
where u I
denote the average pixel and
average luminance of I , respectively. The disruption of global precedence d g p is
obtained by,
=
(
I i )
d g p =
M
[
C
· (
E fm )
C
]
(11.11)
m
=
1
Finally, the ultimate VSNR score can be calculated as follows,
C
(
I
)
.
VSNR
=
20log 10
(11.12)
d g p
2
ad pc + (
1
a
)
11.1.2.3 FSIM
The Feature SIMilarity (FSIM) index for FR-IQA is proposed based on the fact that
the HVS perceive the image mainly according to its low-level features. Specifically,
the phase congruency (PC) and gradient magnitude (GM) are employed as the two
features in FSIM. Based on the physiological and psychophysical evidences, the PC
theory provides a simple but biologically plausible model of how mammalian visual
 
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