1. Lecture 27: Image Processing

2. Images in MATLAB An image in MATLAB is treated as a matrix
Every pixel is a matrix element
All the operators in MATLAB defined on
matrices can be used on images: +, -, *, /, ^, sqrt, sin, cos etc

3. 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

4. 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)

5. Read and Display an Image
Read in an image
Img = imread(‘wholeimg.png’);
Display an image
imshow(Img)
Check the image storage information
whos
See how wholeimg.png is stored into the MATLAB workspace.
Check pixel values
Img(100, 150)
Img(50:60, 120:125)

6. Image Processing Histogram - display histogram of image data
imhist(Img)
Change a gray image into a binary image
level = graythresh(Img)
Computes a global threshold that can be used to convert an intensity image to a binary image.
BW = im2bw(Img, level);
bImg = Img >75;
Add noise
N = imnoise(Img, ‘gaussian’);

7. Image Processing Smoothing the image Invert an intensity image
K = wiener2(N);
Invert an intensity image
invImg = Img;
invbImg = 1 - bImg; % inverts the binary image.
Crop the image
cImg = Imcrop(Img);
cImg = Img(100:200, 150:240);

8. Image Processing Multiple-image display Write the image
subplot(1, 2, 1)
subimage(Img)
subplot(1, 2, 2)
subimage(bImg)
Write the image
imwrite(invImg, ‘newImg.jpg’);

9. Image Processing Color images rgbImg = imread(‘disc.png’);
%store in a 3D matrix
imshow(rgbImg)
imshow(rgbImg(:, :, 1)
gImg = (rgbImg(:,:,1)+rgbImg(:,:,2)+rgbImg(:,:,3))/3;
%convert to grey-scale

10. Practice Question