Images and MATLAB

1. Images and MATLAB

2. Matlab Advantages • Every variable in Matlab is a multidimensional matrix. • Highly modular. • No memory allocation is necessary. • Matlab enables its own garbage collection. • Simple interface for complex mathematical concepts.

3. Image Types • Intensity imagesscaled to represent intensities (uint8 – [0,255], double [0,1]) • Binary imageslogical array of 0s and 1s • Indexed imagesLook up table [x, map] • RGB imagestruecolor, array of (m*n*3) Checking the image type : isind, isbw, isgray, isrgbConverting image types: rgb2ind, rgb2gray, gray2ind, ind2gray,….

4. Data Classes Converting between types : B = data_class_name(A)for example: B = double(A)

5. Conversions • When converting between data classes and types it is important to keep the value range for each data class >> img = double(img)/255; >> img = im2double(img);

6. MATLAB supported image formats • JPEG: Joint Photographics Experts Group • TIFF: Tagged Image File Format • GIF: Graphics Interchange Format • BMP: Microsoft Bitmap Format • PNG: Portable Network Graphics • …

7. Displaying an image(cont.) • Spatial domain

8. Matlab Basics • Digital image representation : 2D function f(x,y) -> finite discrete quantities • Coordinate Conventions img(r,c)r – rows (height)c – cols (width) • The first pixel:img(1,1)

9. Matlab 內建影像 • C:\MATLAB7\toolbox\images\imdemos • 皆為Matlab Help中範例的原始影像。 • 使用時只需直接在指令中輸入檔名，即可使用。 • 適用於觀察影像處理結果

10. Different Image Types • Indexed images • Intensity (grayscale) images • Binary images • RGB (true-color) images

11. Reading an image • imread() • 功用:將影像載入並存成array格式備用 • 用法:[I,map] = imread(filename); I = imread(filename); ex: I = imread('pout.tif'); I為指向影像的變數 不指定變數，則為ans

12. Displaying an image • imshow() • 功用:開啟一個視窗顯示影像 用法: imshow(I) imshow(I,map) • Figure, imshow() 功用:開啟一個新視窗顯示影像 用法: figure,imshow(I)

13. Displaying an image(cont.) • imshow(I, [low, high]) • imshow(I, [ ]) 功用:displays I as a grayscale intensity image, specifying the data range for I. The minimum value in I is displayed as black, and the maximum value is displayed as white. • pixval : 功能:cursor on image to show pixel values 用法: imshow(I),pixval

14. Displaying an image(cont.) • colorbar • 功能:To display an image with a colorbar that indicates the range of intensity values. • 用法： imshow(I), colorbar ex: I = imread('pout.tif'); imshow(I) , colorbar

15. Writing an image • imwrite() • 功能:將影像寫入成檔案 • 用法： imwrite(I,filename,format) ex: imwrite(I,'pout.jpg','JPEG');

16. Image information • Image size: size() ex: I= imread('saturn.png'); size(I) [M,N] = size(I) M=影像I的高 N=影像I的寬

17. Image information • whos 功用:display information about an image . • ex: whos I • Imfinfo( filename ) 功用: display information about image file . • ex: info = imfinfo('saturn.png')

18. Digital Image processing • 影像二元化 • g = im2bw(I, T); • 功用:Convert intensity image I to binary image g using threshold T, where T must be in range [0, 1]. ex: I= imread('pout.tif'); g = im2bw(I, 0.4); imshow(g); colorbar

19. Digital Image processing(cont.) • 彩色轉灰階 • Rgb2gray() • 功用:將RBG彩色影像轉換成gray-level影像。ex: I2= imread ('onion.png'); figure,imshow(I2); colorbar g2 = rgb2gray(I2); figure,imshow(g2); colorbar

20. Digital Image processing(cont.) • 反相 • imcomplement( ) • 功用:The negative of an image. ex: I2= imread ('onion.png'); figure,imshow(I2); colorbar J2 = imcomplement(g2); figure, imshow(J2); colorbar

21. Grayscale images 灰階影像 • Matlab example: • w=imread('pout.tif'); • figure, imshow(w), pixval on • figure: create a window to place graphic object • imshow: display matrix • Data type of w? • 291x240 uint8 (unsigned integer 8 bits

22. Binary image 二元影像 • Matlab example: • w2=imread('circles.png'); • figure, imshow(w2), pixval on • Data type of w? • 256x256 logical • Pixel value is 0 or 1

23. RGB (true color) images 全彩影像 • Matlab example: • w3=imread('peppers.png'); • figure, imshow(w3), pixval on • size(w3) • w3(100,200,2) • w3(100,200,1:3) • w3(100,200,:)

24. RGB color model

25. Pixel depth • Pixel depth: the number of bits used to represent each pixel in RGB space • Full-color image: 24-bit RGB color image • (R, G, B) = (8 bits, 8 bits, 8 bits)

26. Indexed color image 彩色索引影像 • Matlab example: • wI=imread('trees.tif'); • figure, imshow(w), pixval on • What’s wrong?

27. Indices Color Map

28. Indexed color image • Matlab example: • [wI,wmap]=imread('trees.tif'); • figure, imshow(wI, wmap) • How do we know it’s an indexed image?

29. Indexed color image 彩色索引影像 • Matlab example: • w=imread(‘emu.tif’); • figure, imshow(w), pixval on • What’s wrong?

30. 0.1211 0.1211 0.1416 0.1807 0.2549 0.1729 0.2197 0.3447 0.1807 0.1611 0.1768 0.1924 0.2432 0.2471 0.1924 0.2119 0.1963 0.2002 6 10 15 12 5 11 20 10 4 6 10 7 … indices Color map Indexed color image

31. Indexed color image • Matlab example: • [w,wmap]=imread(‘emu.tif’); • imshow(w, wmap) • How do we know it’s an indexed image?

32. Get information about image • imfinfo('emu.tif'); Filename: 'emu.tif' FileModDate: '12-Jul-2004 11:40:00' FileSize: 119804 Format: 'tif' FormatVersion: [] Width: 331 Height: 384 BitDepth: 8 ColorType: 'indexed' ByteOrder: 'little-endian' NewSubfileType: 0 BitsPerSample: 8 Colormap: [256x3 double]

33. Get information about image • imfinfo(‘emu.tif’); Filename: 'emu.tif' FileModDate: '12-Jul-2004 11:40:00' FileSize: 119804 Format: 'tif' FormatVersion: [] Width: 331 Height: 384 BitDepth: 8 ColorType: 'indexed' ByteOrder: 'little-endian' NewSubfileType: 0 BitsPerSample: 8 Colormap: [256x3 double]

34. Data types and conversion • uint8 image must be converted to double before any arithmetic operation • w=imread('pout.tif'); • w=w+1; % fail • w=double(w); % data type is also conversion func. • w=w+1; % ok

35. Write image matrix to file • Matlab code • w=imread('pout.tif'); • imwrite(w, 'pout.jpg','jpg'); • General form • imwrite(X, map, ‘filename’, ‘format’);

36. Zooming and Shrinking Digital Images • Zooming: • Create a new pixel location. • Assign a gray-level to those new locations • Nearest neighbor interpolation • Pixel replication: a checkboard effect • Bilinear interpolation using four nearest neighbors • v(x’, y’)=ax’+by’+cx’y’+d • where a, b, c, and d are determined from the gray-level of the four neighbors. • Shrinking: • Direct shrinking causes aliasing • Expansion then Shrinking: blurring the image before shrinking it and reduce aliasing.

37. Sampling Methods of Inverse Mapping • If transformed pixel ‘X’ locates among • P1, P2, P3 and P4 • Nearest neighbor method: • I(‘X’) = I(P3 ) • where I(p) is the intensity value of pixel p • Bi-linear interpolation: I(‘X’) = (1-a)(1-b)I( P1 ) + a(1-b)I(P2 ) + (1-a)bI(P3 )+ abI(P4 ) where a, b are the fractional parts of ‘X’ • Bi-cubic interpolation: based on cubic splines P2 P1 P3 P4

38. Nearest neighbor interpolation • The closest neighbor is chosen , by rounding the “new” indexes to original image’s coordinates .

47. Digital Image processing(cont.) • 變更影像大小 • imresize(I,scale,method); • 功用:To change the size of an image. • interpolation Method: -'nearest‘ :Nearest-neighbor interpolation -'bilinear‘ :Bilinear (the default) -'bicubic‘ :Bicubic interpolation

48. Digital Image processing(cont.) • 變更影像大小 • imresize(I,scale,method); • 功用:To change the size of an image. • interpolation Method: -'nearest‘ :Nearest-neighbor interpolation -'bilinear‘ :Bilinear (the default) -'bicubic‘ :Bicubic interpolation

49. Digital Image processing(cont.) • ex: I3 = imread('circuit.tif'); J3 = imresize(I3,1.25); figure, imshow(I3) figure, imshow(J3) • ex: I3 = imread('circuit.tif'); J4= imresize(I3,[100 150], 'bilinear'); figure, imshow(I3) figure, imshow(J4)

50. Digital Image processing(cont.) • 旋轉影像 • imrotate(I, angle); • 功用:To rotate an image. • ex: I = imread('pout.tif'); J5 = imrotate(I,35); figure, imshow(J5)