求图像平均梯度运行是出现22行维数不一致问题

Question

function AVEGRAD=avegrad(img)
%%%%%%%%%%%%%%%%%%%%%%%%%
%%%% this function is used to calculate
%%%% the average gradient of an image.
%%%% editor: denghaibo Email:[e-mail]
%%%% date : 2009-4-15
%%%% 平均梯度可敏感地反映图像对微小细节反差表达的能力，可用来评价图像的模糊程度
%%%% 在图像中，某一方向的灰度级变化率大，它的梯度也就大。因此，可以用平均梯度值
%%%% 来衡量图像的清晰度，还同时反映出图像中微小细节反差和纹理变换特征。

img=double(img);
[M,N]=size(img);
gradval=zeros(M,N); %%% save the gradient value of single pixel
diffX=zeros(M,N); %%% save the differential value of X orient
diffY=zeros(M,N); %%% save the differential value of Y orient

tempX=zeros(M,N);
tempY=zeros(M,N);
tempX(1:M,1:(N-1))=img(1:M,2:N);
tempY(1:(M-1),1:N)=img(2:M,1:N);

diffX=img-tempX;
diffY=img-tempY;
diffX(1:M,N)=0; %%% the boundery set to 0
diffY(M,1:N)=0;
diffX=diffX.*diffX;
diffY=diffY.*diffY;
AVEGRAD=sum(sum(diffX+diffY));
AVEGRAD=sqrt(AVEGRAD/2);
AVEGRAD=AVEGRAD/((M-1)*(N-1));

end

请问每次运行时出现维数不一致22行diffX=img-tempX，新手看了一下感觉一样啊，请问是什么原因？求高手帮忙