Transcript Module Development Template

CS100: Into to Computer Science
Image Processing in MatLab
MatLab is a powerful tool to manipulate
graphics and images
Prepared by Fred Annexstein
University of Cincinnati
Some Rights Reserved
1
WORKING WITH IMAGES in MatLab
Basic Image functions:
Reading:
img1=imread('image1.jpg');
Writing:
imwrite(img1, 'image2.jpg');
Displaying:
imshow(img1);
Recall that Last time we did an Exercise: Find 2 images online and add
them together.
You only needed to crop them to the same size.
2
WORKING WITH IMAGES in MatLab
RESIZING
>> img=imread(‘my.jpg');
>> [rows, cols, colors]= size(img)
% Increase the number of rows
>> stretchfactor = 1.5
>> rowVec= linspace(1,rows,stretchfactor*rows);
>> newrows=round(rowVec);
% Decrease number of columns
>> stretchfactor = 0.75;
>> colVec= linspace(1,cols,stretchfactor*cols);
>> newcols=round(colVec);
>> newimag=img(newrows,:,:)
>> imshow(newimg);
>> newimag=newimg(:,newcols,:)
>>imshow(newimg)
3
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 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.
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v = imread(‘myimg.jpg’)
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image(v)
row= input(‘which row?’);
red = v(row,:,1);
green = v(row,:,2);
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blue = v(row,:,3);
plot(red,’r’);
hold on
plot(green,’g’);
plot(blue,’b’);
4
WORKING WITH IMAGES in MatLab
Suppose we want to replace those values whose intensities exceed a
threshold value of 160 in each color.
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v= imread(‘myimg.jpg’);
thresh= 160
layer = (v(:,:,1) > thresh) & (v(:,:,2) > thresh) (v(:,:,2) > thresh)
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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);
5
Let us try to do this with a blue screen…..