Nested Loops – part 2
Barb Ericson
Georgia Institute of Technology
Nov 2009
NestedLoops-part2
1
Learning Goals
• Understand at a conceptual and practical
level
– How to mirror part of a picture
– How to calculate the number of times a
nested loop will execute
– How to copy pixels from one picture to
another
– How to declare, initialize and change several
variables in a for loop
– How to copy part of a picture to another
picture
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Mirror part of a picture
• Use the explorer to figure out the part to mirror
NestedLoops-part2
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Mirror temple method
/**
* Method to mirror part of the temple picture around a
* vertical line at a mirror point
*/
public void mirrorTemple()
{
int mirrorPoint = 276;
Pixel leftPixel = null;
Pixel rightPixel = null;
// loop through the rows
for (int y = 27; y < 97; y++)
{
// loop from 13 to just before the mirror point
for (int x = 13; x < mirrorPoint; x++)
NestedLoops-part2
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Mirror temple - continued
{
leftPixel = getPixel(x, y);
rightPixel = getPixel(mirrorPoint +
(mirrorPoint - x), y);
right Pixel .setColor(leftPixel .getColor());
}
}
}
NestedLoops-part2
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Testing the mirrorTemple method
> String fileName =
"C:/intro-prog-java/mediasources/temple.jpg";
> Picture picture = new Picture(fileName);
> picture.explore();
> picture.mirrorTemple();
> picture.explore();
NestedLoops-part2
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How many pixels were changed?
• We could put print statements in the code
– to see the x and y values, but that would take a long
time
• We can also just keep a count
– And print the value of the count after the loop ends
• We can calculate the number of times a loop will
run using end – start + 1 = number of times
– numOuter = 96 – 27 + 1 = 70
– numInner = 275 – 13 + 1 = 263
– Total for a nested loop is numInner * numOuter
• 70 * 263 = 18,410
NestedLoops-part2
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Copying Pixels to a New Picture
• Need to track the
source picture x and y
– And the target picture
x and y
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2
3
4
• We can use a blank
picture
– As the target picture
• Several blank pictures
are available
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3
– 640x480.jpg
– 7inX95in.jpg
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4
Copy Picture Algorithm
• Copy a picture to the 7 by 9.5 inch blank
picture
– Create the target picture object
– Invoke the method on the target picture
• Create the source picture object
• Loop through the source picture pixels
– Get the source and target pixels
– Set the color of the target pixel to the color of
the source pixel
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Copy Algorithm to Code
• Loop through the source pixels
// loop through the columns
for (int sourceX = 0, targetX = 0;
sourceX < sourcePicture.getWidth();
sourceX++, targetX++)
{
// loop through the rows
for (int sourceY = 0, targetY = 0;
sourceY < sourcePicture.getHeight();
sourceY++, targetY++)
{
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Copy Algorithm to Code – Cont
• Get the source and target pixels
sourcePixel =
sourcePicture.getPixel(sourceX,sourceY);
targetPixel = this.getPixel(targetX,targetY);
• Set the color of the target pixel to the
color of the source pixel
targetPixel.setColor(sourcePixel.getColor());
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Copy Method
public void copyKatie()
{
String sourceFile =
FileChooser.getMediaPath("KatieFancy.jpg");
Picture sourcePicture = new Picture(sourceFile);
Pixel sourcePixel = null;
Pixel targetPixel = null;
// loop through the columns
for (int sourceX = 0, targetX = 0;
sourceX < sourcePicture.getWidth();
sourceX++, targetX++)
{
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Copy Method - Continued
// loop through the rows
for (int sourceY = 0, targetY = 0;
sourceY < sourcePicture.getHeight();
sourceY++, targetY++)
{
// set the target pixel color to the source pixel color
sourcePixel = sourcePicture.getPixel(sourceX,sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
}
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Trying the copyKatie Method
• Create a picture object using the 7inX95in.jpg
file in the mediasources directory
– Picture p1 = new
Picture(FileChooser.getMediaPath(“7inX95in.jpg”));
• Show the picture
– p1.show();
• Invoke the method on this picture object
– p1.copyKatie();
• Repaint the picture
– p1.repaint();
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Result of copyKatie Method
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Copying Pixels to a New Picture
• What if we want to
copy the target to a
different location in
the source
0
0
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2
1
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4
– Than 0,0
– Say startX and startY
• What is an algorithm
that will do this?
1
0
1
2
3
0
1
2
3
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Copy to Position Exercise
• Write a method copyRobot
to copy
– robot.jpg
– To location
• 100, 100 in 7inx95in.jpg
• Test with
String file =
FileChooser.getMediaPath(“7inx95in.jpg”);
Picture p = new Picture(file);
p.copyRobot();
p.show();
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Cropping
• We can copy just part of a picture to a new
picture
– Just change the start and end source x and y
values to the desired values
– Use pictureObj.explore() to find the x and y
values
– What are the x and y values to get the face of
the girl in KatieFancy.jpg?
Copy Face Method
public void copyKatiesFace()
{
String sourceFile =
FileChooser.getMediaPath("KatieFancy.jpg");
Picture sourcePicture = new Picture(sourceFile);
Pixel sourcePixel = null;
Pixel targetPixel = null;
// loop through the columns
for (int sourceX = 70, targetX = 100;
sourceX < 135; sourceX++, targetX++)
{
// loop through the rows
for (int sourceY = 3, targetY = 100;
sourceY < 80; sourceY++, targetY++)
{
Copy Face Method - Continued
// set the target pixel color to the source pixel color
sourcePixel =
sourcePicture.getPixel(sourceX,sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
}
Testing Copy Katie’s Face
• Create a picture object
– Picture p1 = new
Picture(FileChooser.getMediaPath(
“7inX95in.jpg”));
• Show the picture
– p1.show();
• Invoke the method
– p1.copyKatiesFace();
• Repaint the picture
– p1.repaint();
Creating a Collage
• You can create a
collage by copying
several pictures to a
blank picture
– You can use the
general copy method
on flower1.jpg and
flower2.jpg
Create Collage Method
/**
* Method to copy flower pictures to create a
* collage.
* All the flower pictures will be lined up near the
* top of the current picture
*/
public void copyFlowersTop()
{
// create the flower pictures
Picture flower1Picture = new
Picture(FileChooser.getMediaPath(
"flower1.jpg"));
Picture flower2Picture = new
Picture(FileChooser.getMediaPath(
"flower2.jpg"));
// declare the source and target pixel variables
Pixel sourcePixel = null;
Pixel targetPixel = null;
// copy the first flower picture to the top left
// corner
for (int sourceX = 0, targetX = 0;
sourceX < flower1Picture.getWidth();
sourceX++, targetX++)
{
for (int sourceY = 0, targetY = 0;
sourceY < flower1Picture.getHeight();
sourceY++, targetY++)
{
sourcePixel =
flower1Picture.getPixel(sourceX,sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
Create Collage Method - cont
// copy the flower2 picture starting with x =
// 100
for (int sourceX = 0, targetX = 100;
sourceX < flower2Picture.getWidth();
sourceX++, targetX++)
{
for (int sourceY = 0, targetY = 0;
sourceY < flower2Picture.getHeight();
sourceY++, targetY++)
{
sourcePixel =
flower2Picture.getPixel(sourceX,
sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
// copy the flower1 negated to x = 200
flower1Picture.negate();
for (int sourceX = 0, targetX = 200;
sourceX < flower1Picture.getWidth();
sourceX++, targetX++)
{
for (int sourceY = 0, targetY = 0;
sourceY < flower1Picture.getHeight();
sourceY++, targetY++)
{
sourcePixel =
flower1Picture.getPixel(sourceX,
sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
Create Collage Method - cont
// clear the blue in flower 2 picture and add at
// x=300
flower2Picture.clearBlue();
for (int sourceX = 0, targetX = 300;
sourceX < flower2Picture.getWidth();
sourceX++, targetX++)
{
for (int sourceY = 0, targetY = 0;
sourceY < flower2Picture.getHeight();
sourceY++, targetY++)
{
sourcePixel =
flower2Picture.getPixel(sourceX,sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
// copy the negated flower 1 to x=400
for (int sourceX = 0, targetX = 400;
sourceX < flower1Picture.getWidth();
sourceX++, targetX++)
{
for (int sourceY = 0, targetY = 0;
sourceY < flower1Picture.getHeight();
sourceY++, targetY++)
{
sourcePixel =
flower1Picture.getPixel(sourceX,
sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor());
}
}
}
Challenge
• Create your own collage
– Copy at least two different pictures to the
collage
– Do at least 3 different picture manipulations to
the pictures
• Reduce red
• Negate
• Clear blue
– Mirror the collage
Summary
• To mirror part of a picture
– Set the starting and ending values in the nested loop
• To copy pixels from one picture to another
– Keep track of the sourceX, sourceY and targetX and
targetY
• You can declare, initialize, and change more
than one variable in a for loop
for (int sourceX = 0, targetX = 0; sourceX <
sourcePicture.getWidth();
sourceX++, targetX++)
• You can copy just part of a picture to another
picture
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