下面的MATLAB程序计算两幅图像的加权峰值信噪比。相比常用的PSNR而言，考虑到HVS(human visual sytem)的影响。
function f = WPSNR(A,B,varargin)
% This function computes WPSNR (weighted peak signal-to-noise ratio) between
% two images. The answer is in decibels (dB).
%
% Using contrast sensitivity function (CSF) to weight spatial frequency
% of error image.
%
% Using: WPSNR(A,B)
%
% Written by Ruizhen Liu, http://www.assuredigit.com
if A == B
error('Images are identical: PSNR has infinite value')
end
max2_A = max(max(A));
max2_B = max(max(B));
min2_A = min(min(A));
min2_B = min(min(B));
if max2_A > 1 | max2_B > 1 | min2_A < 0 | min2_B < 0
error('input matrices must have values in the interval [0,1]')
end
e = A - B;
if nargin<3
fc = csf; % filter coefficients of CSF
else
fc = varargin{1};
end
ew = filter2(fc, e); % filtering error with CSF
decibels = 20*log10(1/(sqrt(mean(mean(ew.^2)))));
% disp(sprintf('WPSNR = +%5.2f dB',decibels))
f=decibels;
%=============
function fc = csf()
%=============
% Program to compute CSF
% Compute contrast sensitivity function of HVS
%
% Output: fc --- filter coefficients of CSF
%
% Reference:
% Makoto Miyahara
% "Objective Picture Quality Scale (PQS) for Image Coding"
% IEEE Trans. on Comm., Vol 46, No.9, 1998.
%
% Written by Ruizhen Liu, http://www.assuredigit.com
% compute frequency response matrix
Fmat = csfmat;
% Plot frequency response
%mesh(Fmat); pause
% compute 2-D filter coefficient using FSAMP2
fc = fsamp2(Fmat);
%mesh(fc)
%========================
function Sa = csffun(u,v)
%========================
% Contrast Sensitivity Function in spatial frequency
% This file compute the spatial frequency weighting of errors
%
% Reference:
% Makoto Miyahara
% "Objective Picture Quality Scale (PQS) for Image Coding"
% IEEE Trans. on Comm., Vol 46, No.9, 1998.
%
% Input : u --- horizontal spatial frequencies
% v --- vertical spatial frequencies
%
% Output: frequency response
%
% Written by Ruizhen Liu, http://www.assuredigit.com
% Compute Sa -- spatial frequency response
%syms S w sigma f u v
sigma = 2;
f = sqrt(u.*u+v.*v);
w = 2*pi*f/60;
Sw = 1.5*exp(-sigma^2*w^2/2)-exp(-2*sigma^2*w^2/2);
% Modification in High frequency
sita = atan(v./(u+eps));
bita = 8;
f0 = 11.13;
w0 = 2*pi*f0/60;
Ow = ( 1 + exp(bita*(w-w0)) * (cos(2*sita))^4) / (1+exp(bita*(w-w0)));
% Compute final response
Sa = Sw * Ow;
%===================
function Fmat = csfmat()
%===================
% Compute CSF frequency response matrix
% Calling function csf.m
% frequency range
% the rang of frequency seems to be:
% w = pi = (2*pi*f)/60
% f = 60*w / (2*pi), about 21.2
%
min_f = -20;
max_f = 20;
step_f = 1;
u = min_f:step_f:max_f;
v = min_f:step_f:max_f;
n = length(u);
Z = zeros(n);
for i=1:n
for j=1:n
Z(i,j)=csffun(u(i),v(j)); % calling function csffun
end
end
Fmat = Z;
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