I2=imread('tire.tif');%讀入圖像
imshow(I2)%顯示圖像
I=double(I2);%將數據轉換成double型，unit8型不能進行運算
[m,n]=size(I2);%原圖的大小
p=ceil(m*0.5);
q=ceil(n*0.5);%新圖的大小
for x=1:p-2
   for y=1:q-2
      I1(x,y)=0.25*(I(2*x,2*y)+I(2*x+1,2*y)+I(2*x+1,2*y+1)+I(2*x,2*y+1))+0.25*(I(2*x-1,2*y)+I(2*x-1,2*y+1)+I(2*x,2*y-1)+I(2*x+1,2*y-1)+I(2*x+2,2*y)+I(2*x+2,2*y+1)+I(2*x,2*y+2)+I(2*x+1,2*y+2))-0.5*(I(2*x-1,2*y-1)+I(2*x+2,2*y-1)+I(2*x+2,2*y+2)+I(2*x-1,2*y+2));
   end
end%實現算法
I3=uint8(I1);%將數據轉換回unit8型
I4=rgb2ind(I3);
figure,imshow(I4)%顯示新圖