求助：帮忙改改这程序

[紫馨86]

求图像的峰值信噪比
clear all;
close all;
X=imread('toys.bmp');
[height width]=size(X);
X=im2double(X);
A=imnoise(X,'gaussian',0,0.02);
X=double(X);
A=double(A);
sigma=0;msey=0;
for i=1:256
for j=1:256
sigma=sigma+(X(i,j)-A(i,j))^2;
end
end
sigma
msey=(1/(256*256))*sigma
psny=10*(log10(255^2/msey))


数值结果始终不对，大家帮帮忙忙啊