Pictures

1. 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; 0 1 2 3 0 1 2 3 0 1 0 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

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 • 16 million colors • 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 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.