qGBbBShift - Emerging Trends in Image Processing, Computer Vision, and Pattern Recognition - page 134

FIGURE 2 ROI mask generation, wavelet domain.
(3) Wavelet coefficients are quantized and stored in a sign magnitude representation, using the
most significant part of the precision. It will allow to downscale BG coefficients.
(4) A specified scaling value, , downscales the coefficients inside the BG.
(5) The most significant bitplanes are progressively entropy encoded.
The input of ROI scaling-based method is the scaling value φ , while MaxShift method cal-
culates it. Hence, the encoder defines from quantized coefficients this scaling value such that,
(1)
where max{MBG} is the maximum coefficient in the BG. Thus, when ROI is scaled up φ bit-
planes, the minimum coefficient belonging to ROI will be place one bitplane up of BG ( Figure
1(c) ). Namely, 2 φ is the smallest integer that is greater than any coefficient in the BG. MaxShift
method is shown in Figure 1(c) . Bitplane mask (BPmask) will be explained in Section 2.1 .
At the decoder side, the ROI and BG coefficients are simply identified by checking the coef-
icient magnitudes. All coefficients that are higher or equal than the φ th bitplane belong to the
ROI, otherwise, they are a part of BG. Hence, it is not important to transmit the shape informa-
tion of the ROI or ROIs to the decoder. The ROI coefficients are scaled down φ bitplanes before
inverse wavelet transformation is applied.
2 Related work
2.1 BbB Shift
Wang and Bovik proposed the bitplane-by-bitplane shift (BbBShift) method in Ref. [ 6 ].
BbBShift shifts bitplanes on a bitplane-by-bitplane strategy. Figure 3(a) shows an illustration
of the BbBShift method. BbBShift uses two parameters, φ 1 and φ 2, whose sum is equal to the
number of bitplanes for representing any coefficient inside the image, indexing the top bit-
plane as bitplane 1. Summarizing, the BbBShift method encodes the first φ 1 bitplanes with ROI
coeicients, then, BG and ROI bitplanes are alternately shifted, refining gradually both ROI
and BG of the image ( Figure 3(a) ). The encoding process of the BbBShift method is defined as