CMPS1371Introduction to

Computing for Engineers

IMAGES

Images

Images are considered as matrices whose elements are the pixel values of the image

M rows

N columns

RED BLUE

GREEN

Images

Matlab has two main ways to represent an image that consists of M x N pixels:

RGB or “true color” image: uses an MxNx3 array where the third index contains the RGB components

Indexed color image: uses an MxN array containing integers that are the row entries in a “colormap” that defines the actual color.

RGB Color Image

• RGB: MxNx3 8-bit integer array

M r

ows

N columns

RED

BLUEGREEN Each in range 0→255

[215, 43, 62]

Divide by 255 => [0.842, 0.175, 0.246]

Indexed Color Image

• Indexed Color: MxN array with values that index into an NCx3 colormap array that contains the actual RGB colors.

M r

ows

N columns 3

256

row

s

143RED

BLUE

GREEN

[0.142, 0.375, 0.846]

Reading Image Files

Matlab can read and write data in most common image file formats:

jpeg, tif, bmp, png, etc…

A = imread(filename, fmt); reads file into an RGB image

[A,map] = imread(filename, fmt); reads file into an indexed image

Loading an Image

>> a = imread(‘picture.jpg’);

>> imshow(a);

Image Characteristics

Image (=matrix) size:size(a): 384 512 3

R G B

384

512

RED plane

A(:,:,[2 3]) = 0;

imshow(a);

GREEN plane

A(:,:,[1 3]) = 0;

imshow(a);

BLUE plane

A(:,:,[1 2]) = 0;

imshow(a);

Geometric Manipulation of Images

Let’s rearrange the pixels in the image:

<< Left/right = a(:end:-1:1,:);

<< image(Left/right)

Reversing the rows

Original

Geometric Manipulation of Images

Let’s rearrange the pixels to be upside down:

<< updown = a(end:-1:1,:,:);

<< image(updown)

Original

Reversing the columns

Geometric Manipulation of Images

Suppose we split an image into left and right halves and then replace one half with a mirror image of the other

Original

Splitting Code

function Anew = splitMirror(A)% ANEW = splitMirror(A)% Splits an image in half and % creates a mirror of the left half[Nr,Nc]=size(A);Anew=A;c2=round(Nc/2); % find midpointfor row=1:Nr for col=(c2+1):Nc cx=Nc+1-col; Anew(row,col)=A(row,cx); endend

Cute Dog Picture

Lake Scene

Collage Design

Cute Dog cropped to fitLake picture shrunk to

match

800

600

800

600

All borders 40 pixels wide

2 * 800 + 5 * 40

600

+ 4

* 4

0

R = 150G = 100B = 50

R = 255G = 255B = 200

Collage

Image Arithmetic

Adding ‘imadd’: Brighten an image

Subtracting ‘imsubtract’

Multiplying ‘immultiply’:

Multiply two images multiply image by constant:

brightens > 1, darkens < 1 Dividing

‘imdivide’

Spatial Transformations

Resizing ‘imresize’: changes the size of the

image Rotation

‘imrotate’: rotates the image by given angle

Cropping ‘imcrop’: Extract a rectangular part of

the image