Copyright 2008-2010 Curt Hill Further Picture Manipulation Considering position.

Copyright © 2008-2010 Curt Hill

Further Picture Manipulation

Considering position

Previously• We have manipulated pictures with

a one dimensional array of pixels• This if fine for changing each pixel

in some specified way• It ignores lines boundaries and

other important information• It would be impossible to do many

interesting manipulations


What sort of manipulations?

• There are many manipulations that require some knowledge of relative position

• Mirroring the image• Finding points of high contrast• Altering the size of an image


How to obtain a pixel in context?

• Getting the pixel is not enough• We need to know where it is from

as well• This is done with the getPixel

method• getPixel is a method of Picture• It takes two parameters

– The x and y locations


Processing a picture• This also changes how we process a

picture• Up to this point we have used a

single counting for or a single for all• What we have to do now is use two

nested counting fors• One for processes the rows

– Count from zero to height• The second the position within a row


Example


public class PictureDemo5{ static void process(Picture p){ for(int y=0;y<p.getHeight();y++) for(int x=0;x<p.getWidth();x++){ Pixel px = p.getPixel(x,y); Color color = px.getColor(). brighter(); px.setColor(color); } }

Discussion• Two for loops here• First one went from zero to

p.getHeight()-1– This is the outer loop– It processes rows

• The inner loop processes one row– Counts from zero to p.getWidth()-1

• The pixel is obtained with:p.getPixel(x,y);


More discussion• Once pixel is obtained we can do

anything we want to the pixel• In this case we used getColor to obtain

the color• Then we made it brighter using the

brighter method• What we did this time was possible

with simpler processing of pixels• Lets do something we could not do

beforeCopyright © 2008-2010 Curt Hill

Mirroring• The idea is to reflect half of an

image across a line• The line can be horizontal or

vertical or diagonal• Here we take the obvious one

reflect across a vertical line that is in the middle


Mirror


static void process(Picture p){ int mirror = p.getWidth()/2; int width = p.getWidth()-1; for(int y=0;y<p.getHeight();y++) for(int x=0;x<mirror;x++){ // Get left half pixel Pixel px = p.getPixel(x,y); int c = px.getColor().getRGB(); // Set into right half p.setBasicPixel(width-x,y,c); } }

Applied to the beach


Applied to Barbara


One more time


static void process(Picture p){ int mirror = p.getWidth()/2; int width = p.getWidth()-1; for(int y=0;y<p.getHeight();y++) for(int x=0;x<mirror;x++){ // Get left half pixel Pixel px = p.getPixel(x,y); int c = px.getColor().getRGB(); // Set into right half p.setBasicPixel(width-x,y,c); } }

Discussion• The variable mirror is the midpoint• The variable width is the width

minus one• The method getRGB converts a

pixel’s colors into an integer– This is what is needed for setBasicPixel

• This is an application that is impossible without understanding of the line boundaries


Reminder on Arrays• The getPixel takes two parameters• These are essentially array

subscripts• Recall the valid rows are zero to

Height – 1• The valid positions in a row are

zero to Width – 1• Notice the getWidth() – 1 in in the

assignment Copyright © 2008-2010 Curt Hill

Subscript errors• If the integers of getPixel are not in

proper range a subscript error occurs

• x range is zero to Width – 1• y range is zero to Height – 1


Changing picture size• There are several ways• One is to use a specialized

program such as Adobe Photoshop• If you can afford it• Lets try doing it with a program


Reduction Strategy• In this approach we will reduce

each dimension by a factor of 2• Thus if the picture is 800 by 600

we will reduce to 400 by 300• How will we get rid of ¾ of the

pixels?• We will create a new pixel by

averaging 4 pixels into one


Processing a picture


0 1 2 3 4

6

012345

7

0 1 2 3 4

6

012345

7

General Technique• Read in an existing picture• Create a new picture with reduced

dimensions• Copy the average of groups of four

pixels of the old into the new• Write out the new picture


The copying of old to new• Count x by twos horizontally• Count y by twos vertically• Average the four pixels that are at

(x,y), (x+1,y), (x,y+1) and (x+1,y+1)

• Set a new picture pixel with this average


Picture Constructor• We have already seen this picture

constructor:Picture p;p = new Picture(fileName);

• The fileName was obtained using FileChooser

• There are other constructors as well

• The one we need is based on sizeCopyright © 2008-2010 Curt Hill

Sized Constructor• To create a new Picture

– Not based on an existing file• It takes two integer:

Picture p;p = new Picture(400,300);

• This creates a blank image of this size

• We then have to fill in the pixels


The main program


public static void main(String [] a){ String fileName; FileChooser.setMediaPath( “…/mediasources"); fileName = FileChooser.pickAFile(); Picture p1 = new Picture(fileName); Picture p2 = new Picture( p1.getWidth()/2,p1.getHeight()/2); p1.explore(); process(p1,p2); fileName = FileChooser.pickAFile(); System.out.println(fileName); p2.write(fileName); p2.show();}

A Problem with Explore• We had to use p2.show(); rather than p2.explore();

• The explore will give an error with this kind of construction


Process


Take four adjacent pixels.Average colors.Make into one new pixel.

The process method


static void process(Picture p, Picture n){ int mirror = p.getWidth(); int width = p.getWidth()-1; for(int y=0;y<p.getHeight();y+=2) for(int x=0;x<width;x+=2){ int red = 0, green = 0, blue = 0; for(int x2 = 0; x2<2; x2++) for(int y2 = 0;y2<2; y2++){ Pixel px = p.getPixel(x+x2,y+y2); red += px.getRed(); green += px.getGreen(); blue += px.getBlue(); } // More on next page

Rest of process method


int newx = x/2, newy = y/2; Pixel pix = n.getPixel(newx,newy); pix.setRed(red/4); pix.setGreen(green/4); pix.setBlue(blue/4); } // end of for }

// This is within the outer two for loops// Set the averaged pixel into// the new picture

Now some examples• Lets try the following:• Other reflections• Reducing the width and height by

factor of two• Blurring the picture


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Copyright 2008-2010 Curt Hill Further Picture Manipulation Considering position.

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