pkg load image

clear
clc

I = imread('lena.bmp');

I2 = uint8(zeros(256));

for i = 1 : 256
  for j = 1 : 256
    if I(i,j)<64
      I2(i,j)=224;
    elseif I(i,j)<128
      I2(i,j)=160;
    elseif I(i,j)<192
      I2(i,j)=96;
    else
      I2(i,j)=32;
    endif
  endfor
endfor

subplot(1,2,1), imshow(I,[0 255]), title('Lena')
subplot(1,2,2), imshow(I2,[0 255]), title('Transformed')