Introduction to MatLab: Image Processing

2. Introduction to MatLab: Image Processing • Most of the programming operations have as input or output a matrix or a vector. • Images are often represented a matrices. • - MatLab is a powerful tool to manipulate graphics and images.

3. Robot Vision Development in Practice Build your robot (you are done) Install a camera on it. Learn software that comes with camera, understand formats. JPG Install Matlab on your laptop – student version, much software for free on WWW – Octave Be able to display image from camera in one window so that you will see it and Matlab code or processing in other windows. Guide your robot through stage or labyrinth and see with your own eyes what robot sees. Think what is a good processing method for the image that you see. Protytype this in Matlab using existing functions. If necessary, use parallel Matlab on CUDA If necessary, rewrite to C++ or Python or Java.

4. MATLAB • This introduction will give • a brief overview, it’s not a MATLAB tutorial ! • Some basic ideas • Main advantages and drawbacks compared to other languages

5. MATLAB • high-performance language for technical computing • computation, visualization, and programming in an easy-to-use environment • Typical uses include: • Math and computation • Algorithm development • Modelling, simulation, and prototyping • Data analysis, exploration, and visualization • Scientific and engineering graphics • Application development, including Graphical User Interface building

6. Why MATLAB • A good choice for vision program development because: • Easy to do very rapid prototyping • Quick to learn, and good documentation • A good library of image processing functions • Excellent display capabilities • Widely used for teaching and research in universities and industry • Another language to impress your boss with !

7. Why not MATLAB Has some drawbacks: • Slow for some kinds of processes • Not geared to the web • Not designed for large-scale system development

8. MATLAB Components • MATLAB consists of: • The MATLAB language • a high-level matrix/array language with control flow statements, functions, data structures, input/output, and object-oriented programming features. • The MATLAB working environment • the set of tools and facilities that you work with as the MATLAB user or programmer, including tools for developing, managing, debugging, and profiling • Handle Graphics • the MATLAB graphics system. It includes high-level commands for two-dimensional and three-dimensional data visualization, image processing, animation, and presentation graphics. • …(cont’d)

9. MATLAB Components • … • The MATLAB function library. • a vast collection of computational algorithms ranging from elementary functions like sum, sine, cosine, and complex arithmetic, to more sophisticated functions like matrix inverse, matrix eigenvalues, Bessel functions, and fast Fourier transforms as well as special image processing related functions • The MATLAB Application Program Interface (API) • a library that allows you to write C and Fortran programs that interact with MATLAB. It include facilities for calling routines from MATLAB (dynamic linking), calling MATLAB as a computational engine, and for reading and writing MAT-files.

10. MATLAB • Some facts for a first impression • Everything in MATLAB is a matrix ! • MATLAB is an interpreted language, no compilation needed (but possible) • MATLAB does not need any variable declarations, no dimension statements, has no packaging, no storage allocation, no pointers • Programs can be run step by step, with full access to all variables, functions etc.

11. What does Matlab code look like? A simple example: a = 1 while length(a) < 10 a = [0 a] + [a 0] end which prints out Pascal’s triangle: 1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 1 7 21 35 35 21 7 1 1 8 28 56 70 56 28 8 1 1 9 36 84 126 126 84 36 9 1 (with “a=” before each line).

12. What does Matlab code look like? Another simple example: t = 0:pi/100:2*pi; y = sin(t); plot(t,y)

13. What does Matlab code look like? Another simple example: t = 0:pi/100:2*pi; y = sin(t); plot(t,y) Remember: EVERYTHING IN MATLAB IS A MATRIX ! creates 1 x 200 Matrix Argument and result: 1 x 200 Matrix

14. jpg bmp

15. Formats of Images in MATLAB • MATLAB can import/export several image formats • BMP (Microsoft Windows Bitmap) • GIF (Graphics Interchange Files) • HDF (Hierarchical Data Format) • JPEG (Joint Photographic Experts Group) • PCX (Paintbrush) • PNG (Portable Network Graphics) • TIFF (Tagged Image File Format) • XWD (X Window Dump) • MATLAB can also load raw-data or other types of image data • Data types in MATLAB • Double (64-bit double-precision floating point) • Single (32-bit single-precision floating point) • Int32 (32-bit signed integer) • Int16 (16-bit signed integer) • Int8 (8-bit signed integer) • Uint32 (32-bit unsigned integer) • Uint16 (16-bit unsigned integer) • Uint8 (8-bit unsigned integer)

16. Images in MATLAB • Binary images : {0,1} • Intensity images : [0,1] or uint8, double etc. • RGB images : m-by-n-by-3 • Indexed images : m-by-3 color map • Multidimensional images m-by-n-by-p (p is the number of layers)

17. Image import and export • Read and write images in Matlab >> I=imread('cells.jpg'); >> imshow(I) >> size(I) ans = 479 600 3 (RGB image) >> Igrey=rgb2gray(I); >> imshow(Igrey) >> imwrite(lgrey, 'cell_gray.tif', 'tiff') Alternatives to imshow >>imagesc(I) >>imtool(I) >>image(I)

18. Images and Matrices • How to build a matrix (or image)? >> A = [ 1 2 3; 4 5 6; 7 8 9 ]; A = 1 2 3 4 5 6 7 8 9 >> B = zeros(3,3) B =0 0 0 0 0 0 0 0 0 >> C = ones(3,3) C = 1 1 1 1 1 1 1 1 1 >>imshow(A) (imshow(A,[]) to get automatic pixel range)

19. Matrices

20. Matrices • Rows and columns are always numbered starting at 1 • Matlab matrices are of various types to hold different kinds of data (usually floats or integers) • A single number is really a 1 x 1 matrix in Matlab! • Matlab variables are not given a type, and do not need to be declared • Any matrix can be assigned to any variable

21. Matrices Building matrices with [ ]: A = [2 7 4] A = [2; 7; 4] A = [2 7 4; 3 8 9] B = [ A A ] 2 7 4 2 7 4 2 7 4 3 8 9 ?

22. Matrices Building matrices with [ ]: A = [2 7 4] A = [2; 7; 4] A = [2 7 4; 3 8 9] B = [ A A ] 2 7 4 2 7 4 2 7 4 3 8 9 2 7 4 2 7 4 3 8 9 3 8 9

23. Matrices

24. Matrices Some operators must be handled with care: A = [1 2 ; 4 5] B = A * A prints 9 12 24 33 B = A .* A prints 1 4 16 25 Element by element multiplication

25. Submatrices A matrix can be indexed using another matrix, to produce a subset of its elements: a = [100 200 300 400 500 600 700] b = [3 5 6] c = a(b): 300 500 600

26. Submatrices • To get a subsection of a matrix, we can produce the index matrix with the colon operator: • a(2:5) • prints • ans = 200 300 400 500 • This works in 2-D as well, e.g. c(2:3, 1:2) produces a • 2 x 2 submatrix. • The rows and columns of the submatrix are renumbered.

27. loops ‘for’ loops in MATLAB iterate over matrix elements: b = 0 for i = [ 3 9 17] b = b + i; end Result: 29 Note: The MATLAB way to write that program would have been: b = sum([ 3 9 17]); Avoid loops if possible !

28. loops The typical ‘for’ loop looks like: for i = 1:6 … end Which is the same as: for i = [1 2 3 4 5 6] … end

29. loops Once again: AVOID LOOPS

30. Images So why MATLAB and IMAGE PROCESSING ?

31. Images Images can be treated as matrices !

32. Matrix >> A=[1 2 3;3 2 1] A = 1 2 3 3 2 1 >>b=A(1,:) b = 1 2 3 >> B=A' B = 1 3 2 2 3 1

33. Images and Matrices X • Accesing image elements (row, column) >> A(2,1) ans = 4 • : can be used to extract a wholecolumn or row >> A(:,2) ans = 2 5 8 • or a part of a column or row >> A(1:2,2) ans = 2 5 Y • A = • 2 3 • 5 6 • 7 8 9

34. Image Arithmetic • Arithmetic operations such as addition, subtraction, multiplication and division can be applied to images in MATLAB • +, -, *, / performs matrix operations >> A+A ans = 2 4 6 8 10 12 14 16 18 >> A*A ans = 30 36 42 66 81 96 102 126 150 • To perform an elementwise operation use . (.*, ./, .*, .^ etc) >> A.*A ans = 1 4 9 16 25 36 49 64 81 • A = • 2 3 • 5 6 • 7 8 9

35. Logical Conditions • equal (==) , less than and greater than (< and >), not equal (~=) and not (~) • find(‘condition’) - Returns indexes of A’s elements that satisfies the condition. >> [row col]=find(A==7) row = 3 col = 1 >> [row col]=find(A>7) row = 3 3 col = 2 3 >> Indx=find(A<5) Indx = 1 2 4 7 • A = • 2 3 • 5 6 • 7 8 9

36. Flow Control • Flow control in MATLAB - if, else and elseif statements (row=1,2,3 col=1,2,3) if row==col A(row, col)=1; elseif abs(row-col)==1 A(row, col)=2; else A(row, col)=0; end A = 1 2 0 2 1 2 0 2 1

37. Flow Control • Flow control in MATLAB - for loops for row=1:3 for col=1:3 if row==col A(row, col)=1; elseif abs(row-col)==1 A(row, col)=2; else A(row, col)=0; end end end A = 1 2 0 2 1 2 0 2 1

38. Flow Control • while, expression, statements, end Indx=1; while A(Indx)<6 A(Indx)=0; Indx=Indx+1; end • A = • 2 3 • 5 6 • 7 8 9 A = 0 2 3 0 5 6 7 8 9

39. Working with M-Files • M-files can be scripts that simply execute a series of MATLAB statements, or they can be functions that also accept input arguments and produce output. • MATLAB functions: • Are useful for extending the MATLAB language for your application. • Can accept input arguments and return output arguments. • Store variables in a workspace internal to the function.

40. Working with M-Files • Create a new empty m-file function B=test(I) [row col]=size(I) for r=1:row for c=1:col if r==c A(r, c)=1; elseif abs(r-c)==1 A(r, c)=2; else A(r, c)=0; end end end B=A;

42. Examples Try these MATRIX AND VECTOR OPERATIONS This is how we can define a vector >> v=[1, 2, 3] Matlab prints out the following v =     1     2     3 Similarly we can define a matrix >> M= [ 1 2 3; 4 5 6; 7 8 9] The result is: M =     1     2     3     4     5     6     7     8     9 If you want to suppress the MatLab output then you need to finish the line with semicolon as follows. >>M= [ 1 2 3; 4 5 6; 7 8 9];

43. Projection Say you want to extract some rows and columns of a matrix. This is called a projection. We simply give the subset of rows and columns as parameters, as follows >> M11=M(2:3 , 2:3) M11 =      5     6      8     9 To specify all elements in a given dimension one can use ':‘ So to get all rows but just columns 1 and 2, we type >> A= M( :, 1:2) A =      1     2      4     5      7     8

44. WORKING WITH IMAGES in MatLab Let’s talk about image files and their formats….. Color vs GrayScale Basic Image Processing functions: Reading in an image: >> img1=imread('Water lilies.jpg'); Displaying an image: >> imshow(img1); Finding out size of an image: >> size(img1); >> size(img1) ans = 600 800 3 imread imshow size

45. WORKING WITH IMAGES in MatLab • Cropping an image: • >> imgsmall=img1(200:300,300:400,1:3); • >> imshow(imgsmall) • >> imgsmall=img1(150:250,350:450,1:3); • >> imshow(imgsmall) • >> size(imgsmall) • ans = • 101 101 3 • Exercise: 1. Find 2 images online • 2. Crop them to the same size • 3. Add the two images together. • 4. Display resulting image • Advanced Exercise: • Rescale images to same size then add them • See next slide to see HOWTO rescale variable imgsmall

46. ReScaling We can rescale by changing the number of rows and columns, yet preserve the information in the image >> [rows, cols, colors]= size(img1) rows = 600 cols = 800 colors = 3 % Increase the number of rows >> stretchfactor = 1.5 >> rowVec= linspace(1,rows,stretchfactor*rows); >> newrows=round(rowVec); >> newimag=img1(newrows,:,:) >> imshow(newimg); % Decrease number of columns >> stretchfactor = 0.75; >> colVec= linspace(1,cols,stretchfactor*cols); >> newcols=round(colVec); >> newimag=newimg(:,newcols,:) >>imshow(newimg) linspace round

47. Example Program: Inverting an image To invert or to add two images we need to convert to double and then rescale the result back so that it looks like an image InvImg= 1 - double(IMG1)/255; NewImg = uint8(round(InvImg*255))) Imshow(NewImg); uint8

48. WORKING WITH IMAGES in MatLab • Color Masking • Sometimes we want to replace pixels of an image of one or more colors with pixels from another image. It is useful to use a “blue or green screen” in some instances. • Find an image with a big plot of one color. First we will replace that color. And then we will find another image for pixel replacement. • Let us plot the color values of one chosen row…This will tell us the pixel values of the color we want to replace. • v = imread(‘myimg.jpg’) • image(v) • row= input(‘which row?’); • red = v(row,:,1); • green = v(row,:,2); • blue = v(row,:,3); • plot(red,’r’); • hold on • plot(green,’g’); • plot(blue,’b’); input

49. WORKING WITH IMAGES in MatLab • Suppose we want to replace those values whose intensities exceed a threshold value of 160 in each color. • v= imread(‘myimg.jpg’); • thresh= 160 • layer = (v(:,:,1) > thresh) & (v(:,:,2) > thresh) (v(:,:,2) > thresh) • mask(:,:,1) = layer; • mask(:,:,2) = layer; • mask(:,:,3) = layer; • If you want to only mask a portion of the image you can use something like… • >> mask(700:end,:,:)= false; • Which sets the mask so that we do not affect rows 700 and above • To reset the color to red • >>newv = v; • >>newv(mask)(1) = 255 • Or to replace pixels from a different image w use…>> newv(mask) = w(mask); • Let us try to do this with a blue screen…..

50. Histograms