02 Images and Matlab

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Fall 2007 EN 74-ECE Image Processing Lecture 2-1

Images and Matlab

Prof. Eric Miller

[email protected]


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Representation of Images

All images in this class are going to bediscrete

Represented as information defined onan array (pixels)

Array indexed by rows, columns, or

lexicographically Different types of images have differentstuff at each pixel


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Images

Rows

Columns

1

2

3

MM-1

1 2 3 N

1

2

3

M-1

M

M+1

M+2

MN

2M+1

Mostly work with row and

column indexing

Occasionally, want to use

single index

X(2,2) = information

in pixel on row 2

and column 2 inimage X

X(M+2) = info. in pixelon row 2 and column

2 in image X


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Types of Images x(m,n) = 0 or 1: binary image

x(m,n) = 0, 1, 2, P-1: grayscale image P typically something like 256, 512 or some power of 2.

256 is really the most common

0 = black, P = white x(m,n) = [0 to P-1, 0 to P-1, 0 to P-1]

Red, green, blue color image

Like three grayscale images x(m,n) = index into color map.

Color map = three column table with all possible colors Image index says which row in that table gives RBG values

for that pixel

So long as table is known, very easy to store indices


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Examples

Binary

Grayscale (256 levels)

RGB

Indexed

How to tell

difference?


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Matlab

Wonderful tool for doing all sorts ofcomputational things

Well use it here for image processing

Resources Formal tutorial on the class web site.

See especially sections 1, 2, 3, 4, 9, 12

Appendix to the text

Code used in class posted to the web site

Can always type help command


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Matlab Concepts Covered Reading images with imread

Plotting images with imshow

Getting pixel values with impixelinfo NOT pixval as in the text Making a figure window

with figure

Getting image file information with imfinfo

Multiple plots using subplot

Printing information to screen using disp

Indexing arrays using colon :

Use of semicolon to suppress output


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Image File Formats

Standards for storing images on

computers

Some better than others for different

things (e.g. compression)

imread will deal with all of the ones we

care about


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Common Formats

BMP

Microsoft format

Header followed by pixel information

Lossless compression GIF, PNG

Compuserve

Color only

Lossless compression More than one image per file


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Formats

JPEG (Joint Photographicc Experts

Group)

Lossy compression so much smaller files

TIFF (Tagged Image File Format)

All sorts of compression

All sorts of storage schemes

All sorts of color schemes


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JPEG HeaderField Size (bytes) DescriptionAPP0 Marker 2 Always equals 0xFFE0

Length 2 Length of segment excluding APP0 marker

Identifier 5 Always equals "JFIF\x00" (0x4A46494600)

Version 2 First byte is major version (currently 0x01),

Second byte is minor version (currently 0x02)

Density Units 1 Units for pixel density fields

* 0 - No units, aspect ratio only specified

* 1 - Pixels per Inch

* 2 - Pixels per CentimetreX Density 2 Integer horizontal pixel density

Y Density 2 Integer vertical pixel density

Thumbnail Width 1 Horizontal size of embedded JFIF thumbnail in pixels

Thumbnail Height 1 Vertical size of embedded JFIF thumbnail in pixels

Thumbnail Data 3*W*H Uncompressed 24 bit RGB raster thumbnail


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BMP Header


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Matlab Data Types Different means of representing numbers

depending on what you want to do

Examples: Floating point numbers for scientific applications

2.3543, -7.8956

Integers: 1, 2, -7, 6354, -2333430948

Unsigned integers for positive things (like pixelvalues): 255, 7, 0

Question: Can computers represent numberslike ore?

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02 Images and Matlab