EN161 Image Understanding
                     Matlab lab section
                  
                        Sep 16, 2004
                     
                     Ming-Ching Chang
                  mcchang@lems.brown.edu


=======================================================
Matlab 6 introduction

- Matrix Laboratory
   . environment for scientific computation
   . workspace: commandline, editor, figures
   . plotform independent

- a tool for mathematical computation
   . process matrices, variables, strings
      - basic array operations
      - multi-dementional arrays
   . solve algebra, polynomial, equations
   . do optimization, differentiation/intergration,
     solve differential equations
   
- a tool for visualization/GUI (Graphic User Interface)
   . 2D/3D plotting
   . showing images, grids
   . figures, subplots, axis labels
   . contour plots, surface plots, volumn visualization
   . videos, multimedia/web support
   . GUI: mouse interactivity/controls, dialog boxes

- a programming language   
   . variables, strings
   . logical operations
   . flow control
      - if, for loop, while loop
   . function *.M files
   
- a programming environment
   . command line interpreter
   . *.m program editor
   . debugger, variable watcher
   . profiler
   . C, FORTRAN, JAVA
   
=======================================================
Useful Matlab commands/functions

help
help imread      (help ANY_FUNCTION)
help complex

demo            (good starting demonstration)

who            (see current variables)
whos            (with detailed info)
A=1            (variable name '3A' is not allowed.)
%               comment

pi               3.1416
format long
pi               3.14159265358979

ver
path
cd..            (or select the 'Current Directory...' control)
pwd

sin, cos, tan, exp, log, log2, sqrt, floor, ceil, round, erf, gamma, dot, cross

imread         (image I/O, try 'help imread')
imwrite
imshow

BASIC OPERATIONS:
54.4 + 23*6

input A
pause

ARRAY OPERATIONS:

A = [1 2 3;4 5 6;7 8 9]
=======================================================
Matlab matrix manupulation:

Learn to use ':' syntex to speedup computation (avoid loops)

K>> A=[1 2 3; 4 5 6]
A =
     1     2     3
     4     5     6
K>> size(A)
ans =
     2     3
K>> A(2:end)                     (column than row)
ans =
     4     2     5     3     6     
K>> A(1:2,2:3)                     (part of an array)
ans =
     2     3
     5     6     
K>> A(:,2)=7
A =
     1     7     3
     4     7     6
K>> A(:,:)=8
A =
     8     8     8
     8     8     8
K>> A=7                           (re-assign A)
A =
     7
     
K>> B=(0:0.1:1)*pi
B =
  Columns 1 through 8 
         0    0.3142    0.6283    0.9425    1.2566    1.5708    1.8850    2.1991
  Columns 9 through 11 
    2.5133    2.8274    3.1416  
       
ARRAY TRANSPOSE

C=1:5
C'       

INITIALIZE AN ARRAY
K>> ones(3,4) 
ans =
     1     1     1     1
     1     1     1     1
     1     1     1     1
K>> zeros(2,3)
ans =
     0     0     0
     0     0     0
K>> d=pi
d =
    3.1416
K>> D=repmat(d,3,4)                  (Replicate matrix, 3x4)
ans =
    3.1416    3.1416    3.1416    3.1416
    3.1416    3.1416    3.1416    3.1416
    3.1416    3.1416    3.1416    3.1416  
K>> E=reshape(D, 4, 3)               (Reshape to 4x3)
E =
    3.1416    3.1416    3.1416
    3.1416    3.1416    3.1416
    3.1416    3.1416    3.1416
    3.1416    3.1416    3.1416  
K>> E(2:3,1:3)=[1 2 3; 4 5 6]         (process sub-image of an image)
E =
    3.1416    3.1416    3.1416
    1.0000    2.0000    3.0000
    4.0000    5.0000    6.0000
    3.1416    3.1416    3.1416    
    
LOGICAL ARRAY
K>> B=[5 -3; 2 -4]
B =
     5    -3
     2    -4
K>> x=abs(B)>2
x =
     1     1
     0     1
K>>  
K>> y=-3:3
y =
    -3    -2    -1     0     1     2     3
K>> k=find(abs(y)>1)
k =
     1     2     6     7   
K>> max(y)
ans =
     3
K>> [my, index]=max(y)               (return 2 values of a function call)
my =
     3
index =
     7
K>> 
=======================================================
Image Manipulation:
   - read image from file (JPG, GIF, BMP, TIFF, PNG, PCX...)
   - write image to file
   - show image on screen, zoom in/out
      . get pixel value
   - process image as an array of pixels
      . UINT8: unsigned 8-bit (0 to 255)         % what happen if we subtract the image?
      . DOUBLE: double-precision (0 to 1)
      . convert between the 2 format: im2double, im2uint8
        'whos' to check the image format

EXAMPLE CODE OF PROBLEM1:

THE SLOW VERSION:

IMAGE = imread ('quart.jpg');
INFO = imfinfo ('quart.jpg');
height = INFO.Height;
width  = INFO.Width;
threshold = 150;
for i=1:height
   for j=1:width
      if IMAGE(i,j) > threshold
             IMAGE(i,j) = 0;
        else IMAGE(i,j) = 255;
      end     
   end
end

imshow (IMAGE);
imwrite (IMAGE, 'problem1.jpg');


THE FAST VERSION (WITHOUT LOOP):

IMAGE = imread ('quart.jpg');
IMAGE = im2double(IMAGE);
IMAGE(:,:) = (IMAGE(:,:)>150/255);
imshow (IMAGE);


THE FAST AND NEAT VERSION:

threshold = 182
QUARTERS = imread('quart.jpg');

PROBLEM1 = QUARTERS<threshold;

subplot(2,1,1)
imshow(QUARTERS); 
subplot(2,1,2)
imshow(PROBLEM1); 

imwrite(PROBLEM1, 'lab1p1.jpg', 'jpg');

=======================================
PROBLEM3:

BRUTE_FORCE APPROACH:
double for loop, im2double, img=1-img

invert_image = bitxor(QUARTERS,255);

help format
format hex
>> invert_image(1,2)
ans =
   1e                                 1*16+14=30
>> invert_image(PROBLEM3(1,2),255)
ans =
   e1
>>