Image Processing Reference
In-Depth Information
FIGURE 3 ROI coding methods. (a) BbBShift, φ 1 = 3 and φ 2 = 4, (b) GBbBShift and (c)
ρGBbBShift. Background is denoted as BG (for ρGBbBShift method is perceptually quantized
by ρSQ at d 2 ), region of interest as ROI (for ρGBbBShift method is perceptually quantized at
d 1 by ρSQ) and bitplane mask as BPmask.
(1) For a given bitplane bpl with at least one ROI coefficient:•
• If bpl ≤ φ 1, bpl is not shifted.
• If φ 1 < bpl ≤ φ 1 + φ 2, bpl is shifted down to φ 1 + 2 (bpl − φ 1)
(2) For a given bitplane bpl with at least one BG coefficient:•
• If bpl ≤ φ 2, bpl is shifted down to φ 1 + 2bpl − 1
• If bpl > φ 2, bpl is shifted down to φ 1 + φ 2 + bpl
2.2 GBbBShift
In practice, the quality reinement patern of the ROI and BG used by BbBShift method is sim-
ilar to the general scaling-based method. Thus, when the image is encoded and this process is
truncated in a specific point, the quality of the ROI is high while there is no information of BG.
Hence, Wang and Bovik [ 7 ] modified BbBShift method and proposed the generalized
bitplane-by-bitplane shift (GBbBShift) method, which introduces the option to improve visual
quality either of ROI or BG or both. Figure 3(b) shows that with GBbBShift method it is pos-
sible to decode some bitplanes of BG after the decoding of same ROI bitplanes. It allows to
improve the overall quality of the recovered image. This is possible gathering BG bitplanes.
Thus, when the encoding process achieves the lowest bitplanes of ROI, the quality of BG could
be good enough in order to portray an approximation of BG.
Therefore, the main feature of GBbBShift is to give the opportunity to arbitrary chose the
order of bitplane decoding, grouping them in ROI bitplanes and BG bitplanes. This is pos-
sible using a binary bitplane mask or BPmask, which contains one bit per each bitplane,
that is, twice the amount of bitplanes of the original image. An ROI bitplane is represented
by 1, while a BG bitplane by 0. For example, the BPmask for MaxShift method in Figure
1(c) is 11111110000000, while for BbBShift in Figure 3(a) and GBbBShift in Figure 3(b) are
11101010101000 and 11100011110000, respectively.
At the encoder side, the BPmask has the order of shifting both the ROI and BG bitplanes.
Furthermore, BPmask is encoded in the bitstream, while the scaling values φ or φ 1 and φ 2
from the MaxShift and BbBShift methods, respectively, have to be transmited.
3 Perceptual GBbBShift
3.1 Quantization
In order to generate an approximation to how every pixel is perceived from a certain distance
taking into account the value of its neighboring pixels the Chromatic Induction Wavelet Model
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