qGBbBShift - Emerging Trends in Image Processing, Computer Vision, and Pattern Recognition - page 136

(CBPF) is used. CBPF atenuates the details that the human visual system (HVS) is not able to
perceive, enhances those that are perceptually relevant and produces an approximation of the
image that the brain visual cortex perceives. CBPF takes an input image I and decomposes it
into a set of wavelet planes ω s , o of different spatial scales s (i.e., spatial frequency ν) and spatial
orientations o . It is described as:
(2)
where n is the number of wavelet planes, c n is the residual plane, and o is the spatial orientation
either v ertical, h orizontal, or d ia g ona l . The perceptual image I ρ is recovered by weighting these
ω s , o wavelet coefficients using the extended Contrast Sensitivity Function (e-CSF), which con-
siders spatial surround information (denoted by r ), visual frequency ( ν related to spatial fre-
quency by observation distance), and observation distance ( d ). Perceptual image I ρ can be ob-
tained by
(3)
where α ( ν , r ) is the e-CSF weighting function that tries to reproduce some perceptual proper-
ties of the HVS. The term α ( ν , r ) ω s,o ≡ ω s,o ; ρ , d can be considered the perceptual wavelet coefficients
of image I when observed at distance d . For details on the CBPF and the α ( ν , r ) function, see
Ref. [ 8 ] .
We employ the perceptual quantizer (ρSQ) either forward (F-ρSQ) and inverse (I-ρSQ),
deined by Moreno et al. [ 1 ] . Each transform sample at the perceptual image I ρ (from Equation
3 ) is mapped independently to a corresponding step size either Δ s or Δ n , thus I ρ is associated
with a specific interval on the real line. Then, the perceptually quantized coefficients Q (F-ρSQ),
from a known viewing distance d , are calculated as follows:
(4)