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Introduction to MatLab: Image Processing

Into to Computer Science. Introduction to MatLab: Image Processing. - MatLab stands for Matrix Laboratory. - Most of the programming operations have as input or output a matrix or a vector. - Images are often represented a matrices.

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Introduction to MatLab: Image Processing

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  1. Into to Computer Science Introduction to MatLab: Image Processing - MatLab stands for Matrix Laboratory. - 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. Prepared by Fred Annexstein University of Cincinnati Some Rights Reserved

  2. Overview Like Scratch, MatLab is an interpreted programming language. Unlike Scratch, Matlab is a language in which you type commands at the prompt. 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];

  3. 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

  4. 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

  5. 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

  6. 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)

  7. 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);

  8. 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’);

  9. 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…..

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