>> I=imread('lena.bmp');
>> I1=rgb2gray(I);
>> imshow(I1),title('a');
>> I2=I1(1:2:end,1:2:end);
>> figure,imshow(I2),title('b');
>> I3=I1(1:4:end,1:4:end);
>> figure,imshow(I3),title('c');
>> figure,subplot(2,2,1),imshow(I),title('color image');
>> subplot(2,2,2),imshow(I1),title('512X512');
>> subplot(2,2,3),imshow(I2),title('256X256');
>> subplot(2,2,4),imshow(I3),title('128X128');