Looping through pixels. Pictures Looping through pixels.

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

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

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 */

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.

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

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

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;

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]

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

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, …)

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

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

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

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!!

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?

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 { }

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); }

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

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; }

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

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); }

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.