1. Looping through pixels.
Pictures
Looping through pixels.

2. Lab Review (1) Objects Instantiated from Class
Turtle myTut = new Turtle(myWorld);
new operator creates an instance of the class
Constructor
Constructs (initializes) object
Has same name as class
May need to supply arguments as parameters

3. Lab Review (2) Methods Send message to object by calling method
myTut.turnLeft();
Argument can be supplied as a parameter
myTut.forward(45);

4. Lab Review (3)
/**
* Draws a square with side width pixels at the current position
* in the world. *
* @param width length of the side */
public void drawSquare(int width) {
//draw side one
this.forward(width);
// turn 90 deg counterclockwise
this.turnLeft();
// draw side two … }
For main program:
/**
This program draws uses the
Turtle to draw nested squares.
@author S. Sigman
@version 2/4/2007 */

5. Lab Review (4)
Using the Java packages require that the class or classes be made available to the compiler.
import statement used to do this
import java.awt.Color;
Put at top of the file
import java.awt.*; makes all classes in java.awt available.

6. Problem Manipulate a digital image
make a picture brighter/darker
alter the color in a picture
In computer science we must often start by learning about the domain in which we are writing programs.
Often requires us to learn more about our language

7. Digital Images How are images encoded?
Basic Idea: Represent image as a 2-dimensional matrix of colored dots.
Human Visual Perception
Humans have low visual acuity
We see color and luminance (black and white) using different neural pathways in our brain
Luminance is perceived with our peripheral vision

8. Digital Images - Arrays
Arrays are a contiguous collections of cells homogeneous data where each cell is identified by an index.
int [] score = {23, 25, 22, 25};
score[0] is 23; score[1] is 25;
score[3] is ?
Arrays are objects. The assignment above is a shorthand that hides the new operator.
int [] grades = new int [10]; // declares and // allocates
grades[0] = 35;

9. Digital Images – Matrices
Matrices are 2-dimensional arrays
In Java a matrix is an array of arrays
An image has type: Pixel [][] pictData; 1 2 3 1 2 3 1
Row Major Order
pictData [r] [c] 1
Column Major Order
pictData [c] [r]

10. Digital Images - Color Visible light is continuous
Wavelength: 370 nm – 730 nm
We have red, green, blue receptors
red peak: 560 nm
green peak: 550 nm
blue peak: 425 nm
Color we see is based upon signals from recptors

11. Digital Images – Color Models
RGB – Red Green Blue
HSV – Hue Saturation and Value
CMYK – Cyan, Magenta, Yellow, Black
Used by printers (ie. Those who print books, newsletters, pamplets, …)

12. Digital Image - RGP Three Channels Red, Green, Blue
Value Range: 0 – 255
24 bit color
16 million colors
Does not cover range of colors we perceive
32 bit color
alpha channel – transparency

13. Creating an Image String fileName = FileChooser.pickAFile();
Picture pict = new Picture(fileName);
// show the image
pict.show();
// explore the image
pict.expore();

14. Looking at Pixels Pixel pix = pict.getPixel(0,0);
int xCord = pix.getX(); int yCord = pix.getY();
int redVal = pix.getRed();
pix.setRed(255); pix.setGreen(0); pix.setBlue(0);
pict.repaint(); // must repaint to show // change

15. Looking at Pixels (2) Pixel [] imgData = pict.getPixels();
pict.getPixel(10,100).setColor(Color.black);
pict.getPixel(11,100).setColor(Color.black);
pict.getPixel(12,100).setColor(Color.black);
pict.getPixel(13,100).setColor(Color.black);
pict.repaint();
This process is a little laborious!!

16. Changing The Green Component For All Pixels
Push the green component to 255 for all pixels.
Algorithm
Get an array of pixels in image
For each pixel in the image
set the green component to 255
How do we do the for each?

17. The For Each Loop
Allows us to repeat a set of Java commands for each element in an array.
Syntax
for (type variableName: array)
statement;
for multiple statements enclose in { }

18. Method to Maximize Green
public void maxGreen() {
// get the pixels
Pixel [] pixels = this.getPixels();
// set the pixels to max green
for(Pixel curPix : pixels) {
curPix.setGreen(255); }

19. While Loops Java provides other ways to construct loops While loop
loop while some condition is true
syntax
while (test)
statement;
Advice: always use braces

20. Maximize Green With a While Loop
public void maxGreen() {
// get the pixels
Pixel [] pixels = this.getPixels();
// set the pixels to max green
int curPos = 0;
while (curPos < pixels.length) {
pixels[curPos].setGreen(255);
curPos++; // same as curPos = curPos + 1; }

21. For Loop Loop with a built-in counter – also called a counting loop
Syntax
for (init; limit test; change)
statement;
Advice: always use braces

22. Maximize Green With a For Loop
public void maxGreen() {
// get the pixels
Pixel [] pixels = this.getPixels();
// set the pixels to max green
for (int i=0;i < pixels.length; i++) {
pixels[i].setGreen(255); }

23. Scope For loop from previous slide Where is i’s value know?
for (int i=0;i < pixels.length; i++) {
pixels[i].setGreen(255); }
Where is i’s value know?
Just inside the block the defines the body of the loop. This is i’s scope.
Look at scope in Picture.
Principle: Variables scope should be as small as possible. Called localization.