Intermediate C Programming

Image Processing 391

}33

// detect edges and save the edges in the image34

pxl = 0;35

for ( row = 0; row < height ; row ++)36

{37

pxl += 3; // skip the first pixel in each row38

for ( col = 1; col < width ; col ++)39

{40

int diff = twoDGray [ row ][ col ] -41

twoDGray [ row ][ col - 1];42

// take the absolute value43

i f ( diff < 0)44

{45

diff = - diff ;46

}47

i f ( diff > thrshd ) // an edge48

{49

// set color to white50

img -> data [ pxl + 2] = 255;51

img -> data [ pxl + 1] = 255;52

img -> data [ pxl ] = 255;53

}54

e l s e // not an edge55

{56

// set color to black57

img -> data [ pxl + 2] = 0;58

img -> data [ pxl + 1] = 0;59

img -> data [ pxl ] = 0;60

}61

pxl += 3;62

}63

}64

for ( row = 0; row < height ; row ++)65

{66

free ( twoDGray [ row ]) ;67

}68

free ( twoDGray );69

}70

23.2.6 Color Equalization

Sometimes an image is over exposed (too bright) or under exposed (too dark). The image

can usually be enhanced by using color equalization using the following steps:

1. Find the maximum and the minimum values of the colors.

2. If the maximum and the minimum values are diﬀerent, scale the maximum value to

255 and the minimum value to 0.

3. Scale the color based on a formula.

There are many ways to scale the pixel’s colors. One simple method is called linear

scaling: using a linear equation to express the relationship between the original color and

392 Intermediate C Programming

the new color. Let x and y be the old and the new colors, then a linear equation has two

coeﬃcients: a and b.

y = ax + b (23.1)

Suppose M and m are the original minimum and the maximum values. They should

become 0 and 255 after the scaling. The following two equations are used to determine the

correct values for a and b.

0 = am + b

255 = aM + b

(23.2)

a =

255

M − m

and b = −

255m

M − m

(23.3)

The code listing below implements this color equalization scheme.

// bm p equalize . c1

#in clude " bmpfunc . h"2

void BMP_eq ualize ( B MP_Image * img )3

int pxl ;5

unsigned char redmin = 255;6

unsigned char redmax = 0;7

unsigned char greenmin = 255;8

unsigned char greenmax = 0;9

unsigned char bluemin = 255;10

unsigned char bluemax = 0;11

// find the maximum and the minimum values of each color12

for ( pxl = 0; pxl < ( img -> data_ size ) ; pxl += 3)13

{14

unsigned char red = img -> data [ pxl + 2];15

unsigned char green = img -> data [ pxl + 1];16

unsigned char blue = img -> data [ pxl ];17

i f ( redmin > red ) { redmin = red ; }18

i f ( redmax < red ) { redmax = red ; }19

i f ( greenmi n > green ) { greenmin = green ; }20

i f ( greenma x < green ) { greenmax = green ; }21

i f ( bluemin > blue ) { bluemin = blue ; }22

i f ( bluemax < blue ) { bluemax = blue ; }23

}24

// calc ulate the scaling factors25

// max and min must be d ifferent to prevent26

// divided by zero error27

double r edscale = 1.0;28

double greensca l e = 1.0;29

double bluescale = 1.0;30

i f ( redmax > redmin )31

{32

redscale = 255.0 / ( redmax - redmin ) ;33

}34

i f ( greenma x > green m in )35

{36

Image Processing 393

greensc a le = 255.0 / ( greenmax - green m i n );37

}38

i f ( bluemax > bluemin )39

{40

bluescale = 255.0 / ( bluemax - bluemin );41

}42

// equaliz e the pixels44

for ( pxl = 0; pxl < ( img -> data_ size ) ; pxl += 3)45

{46

i f ( redmax > redmin )47

{48

img -> data [ pxl + 2] = ( in t ) ( redscale *49

( img -> data [ pxl + 2] - redmin ) );50

}51

i f ( greenma x > green m in )52

{53

img -> data [ pxl + 1] = ( in t ) ( greens cale *54

( img -> data [ pxl + 1] - greenm i n ) );55

}56

i f ( bluemax > bluemin )57

{58

img -> data [ pxl ] = ( i n t ) ( bluesca le *59

( img -> data [ pxl ] - bluemin )) ;60

}61

}62

}63

This chapter gives a starting point for image processing. Image processing is a rich

subject and there are many books on the topic and related topics.

This page intentionally left blankThis page intentionally left blank

Previous Chapter

23. Image Processing (2/3)

Next Chapter

24. Huffman Compression (1/10)

Table of Contents for Intermediate C Programming

Table of Contents for
Intermediate C Programming