  The paper presents a non-uniform quantization method for the Detail
components in the JPEG2000 standard. Incorporating the fact that the
coefficients lying towards the ends of the histogram plot of each Detail
component represent the structural information of an image, the quantization
step sizes become smaller at they approach the ends of the histogram plot. The
variable quantization step sizes are determined by the actual statistics of the
wavelet coefficients. Mean and standard deviation are the two statistical
parameters used iteratively to obtain the variable step sizes. Moreover, the
mean of the coefficients lying within the step size is chosen as the quantized
value, contrary to the deadzone uniform quantizer which selects the midpoint of
the quantization step size as the quantized value. The experimental results of
the deadzone uniform quantizer and the proposed non-uniform quantizer are
objectively compared by using Mean-Squared Error (MSE) and Mean Structural
Similarity Index Measure (MSSIM), to evaluate the quantization error and
reconstructed image quality, respectively. Subjective analysis of the
reconstructed images is also carried out. Through the objective and subjective
assessments, it is shown that the non-uniform quantizer performs better than
the deadzone uniform quantizer in the perceptual quality of the reconstructed
image, especially at low bitrates. More importantly, unlike the deadzone
uniform quantizer, the non-uniform quantizer accomplishes better visual quality
with a few quantized values.
