Topic 5 Introduction to Pictures and Arrays

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Topic 5 Introduction to Picturesand Arrays
Notes adapted from Introduction to Computing and
Programming with Java A Multimedia Approach by
M. Guzdial and B. Ericson, andinstructor
materials prepared by B. Ericson.
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Learning Goals

• To understand at a conceptual and practical
  level
• How to manipulate pictures by changing pixels
• What is an array
• How to use an array

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Digital Pictures

• Recall that we can represent a digital picture
  as a grid of pixels
• With a red, green, and blue value stored for
  each pixel

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Picture as a Grid of Pixels
X

• A picture isorganized as a grid (matrix) of
  pixels
• The grid has columnsand rows
• Each pixel has an (x, y)position in the grid
• x specifies the column, starting at 0
• y specifies the row, starting at 0
• This is called column-major order

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Picture Objects

• Recall we can create a Picture object by
  String fileName FileChooser.pickAFile()
  Picture pictureObj new Picture (fileName)
• A Picture object has properties width and height
• We can get the picture width (in pixels) using
• pictureObj.getWidth()
• We can get the picture height (in pixels) using
• pictureObj.getHeight()

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Picture Exercise

• Create a picture in DrJava and report the
  pictures width, height, and number of pixels
• String fileName FileChooser.pickAFile()
• Picture pictureObj new Picture(fileName)
• pictureObj.show()
• int width pictureObj.getWidth()
• System.out.println("The picture width is "
  width)
• int height pictureObj.getHeight()
• System.out.println("The picture height is "
  height)
• System.out.println("The picture contains"
  _________________ " pixels")

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Manipulating Pictures

• A picture is made up of pixels
• So, to manipulate a picture, we manipulate the
  pixels that make up the picture
• We can change the red, green, blue values of a
  pixel

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The Pixel Class

• We have a class called Pixel that models
  individual pixels
• (also created at Georgia Tech Pixel.java is in
  the bookClasses folder)
• Each object of the Pixel class has
• An (x,y) position in a picture
• x specifies the column, starting at 0
• y specifies the row, starting at 0
• A red, green, and blue value
• Each is an integer between 0 and 255

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The Picture Explorer

• The Picture class has a method that lets us
  explore a picture by moving the cursor around
  in the picture
• We can see the x and y values and the color of
  the pixel at the cursor
• To use this on a Picture object
• pictureObj.explore()

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Creating Pixel Objects

• We can get a pixel at a specific location in a
  picture by using the getPixel method of the
  Picture class
• ExamplePixel pixel1 pictureObj.getPixel(0,0)
• This will create a Pixel object from the pixel at
  the top left-hand corner of the picture
• And store a reference to this Pixel object in the
  variable pixel1

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Manipulating Pixel Objects

• We can get and set the red, green, blue values of
  a Pixel object individually, using methods of the
  Pixel class
• Example of getting a pixels color values
  int red pixel1.getRed()
• int green pixel1.getGreen()
• int blue pixel1.getBlue()
• Example of setting a pixels color values
• pixel1.setRed(red10)
• pixel1.setGreen(0)
• pixel1.setBlue(blue-10)

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Pixel Location in a Picture

• We can get the pixels location in the grid of
  pixels that make up the Picture object
• getX()
• Returns its x position (the column)
• getY()
• Returns its y position (the row)
• Example what will be printed here?System.out.pri
  ntln(pixel1.getX() ,
  pixel1.getY())

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Pictures Can Have Lots of Pixels

• How could we refer to each of the Pixel objects
  from a picture?
• pixel1, pixel2, pixel3, pixel4, pixel5,
• But, in a 640 x 480 image, there are 307,200
  pixels!
• Do we really want to name each one?
• Computer languages have a way of efficiently
  dealing with lots of data of the same type
• We use a data structure called an array

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What is an Array?

• An array is storage for a collection of items of
  the same type
• You can access the individual items by using an
  index
• The index starts at 0
• The first item is at index 0
• The last item is at index (length 1)

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Exampleint array of length 6 What value is
stored at index 0? at index 4?
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What is an Array?

• In Java, an array is an object
• As with any other object
• We need to declare a reference variable
• We need to create the array object
• We declare array reference variables by
• type name
• Example int numbers
• This creates a reference variable called numbers
  that can be used to refer to an array of integers
• But this does not actually create the array

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Creating Arrays

• Creating an array is much like creating other
  objects in Java we use the Java keyword new
• Example numbers new int6
• This creates an array of 6 integers, and has our
  reference variable numbers refer to it
• The reference variable numbers refers to the
  entire collection of integers
• We can declare and create in one statementint
  numbers new int6
• Once we create an array, its size is fixed and we
  cannot change it

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Creating Arrays

• int numbers new int6
• numbers
• numbers is a reference variable
• What object does it refer to?
• What is stored in each element of the array now?
• Why?

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Array Indexing

• Each item of an array can be accessed
  individually, using indexing
• Examples
• numbers0 refers to the first element of the
  array (the element at position 0)
• numbers1 refers to the second element
  (the element at
  position 1)
• numbersi refers to the (i1)th element
  (the element at
  position i)

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Storing in Arrays

• numbers0 3
• numbers1 7
• numbers2 9
• numbers3 2
• numbers4 1
• numbers5 5
• index 0 1 2 3
  4 5
• numbers

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Array Indexing

• index
• numbers 0 1 2 3 4 5

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What would be printed by System.out.println(numb
ers0) How would we print the last item in
the array?
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Initializing Arrays

• We can declare, create, and initialize an array
  to specific values, in one statement
• Example
• int numbers 3, 7, 9, 2, 1, 5
• This creates an array of 6 integers, and sets the
  initial values of the integers in the array
  according to the values specified
• The length of the array is determined by the
  number of items listed

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Initializing Arrays

• int numbers 3, 7, 9, 2, 1, 5
• numbers

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Array Size

• Java remembers the size of arrays for you
• An array object has a special public property
  (attribute) called length, which stores the size
  of the array
• Public attributes are accessed using the dot
  notation, just as with method calls
• Note length is a variable, not a method!
• So there are no parentheses after length
• Example int arraySize numbers.length
• Useful to get the last item in an array, for
  example
• int lastNumber numbersnumbers.length 1

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Array of Pixels

• A Picture object has an array of pixels, i.e an
  array of Pixel objects
• This is the color data it read from the image
  file when the Picture object was created
• This array has
• the pixels from the first row of the grid
• followed by the pixels from the second row
• etc.
• To get the array of pixels to use to manipulate
  the picture
• Pixel pixelArray pictureObj.getPixels()

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Array of Pixels

• Pixel pixelArray pictureObj.getPixels()
• pixelArray 1st row
  2nd row etc.

pixel at (0,0)
pixel at (1,0)

• This creates an array of Pixel objects
• Note that each element of the array is
  actually a reference to a Pixel object

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Picture Exercise

• Create a picture in DrJava and report the number
  of pixels
• String fileName FileChooser.pickAFile()
• Picture pictureObj new Picture(fileName)
• pictureObj.show()
• Pixel pixelArray pictureObj.getPixels()
• System.out.println("The picture contains"
  pixelArray.length " pixels")

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Pixel Objects from a Pixel Array

• How do you get an individual Pixel object from
  the pixel array?
• How do you access an individual elementfrom an
  array normally?
• You use indexing!
• So, we use indexing to get Pixel objectsfrom the
  pixel array
• For example, to get the first pixel
• Pixel pixelObj pixelArray0

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Pixel Objects from a Pixel Array

• Pixel pixelObj pixelArray0
• pixelArray 1st row
  2nd row etc.

pixel at (0,0)
pixel at (1,0)
pixelObj

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More on Manipulating Pixel Objects

• We have already seen how to get and set the red,
  green and blue values of a pixel individually
• Or, we can get and set the color of the pixel as
  a whole using the methods
• getColor()
• setColor(colorObj) - where colorObj refers to
  an object from the class Color

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The Color class

• Recall the class defined in Java that represents
  color
• The Color class in the package java.awt
• Recall that a package is a group of related
  classes
• To use the class, you must either use
• The full name java.awt.Color
• Or, much easier, use the import statement import
  java.awt.Color
• Then you can just use the class name Color
  (without needing the name of the package as well)
• In a Java program, import statements go at the
  beginning of the source file

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Predefined Colors

• The Color class has defined class constants for
  many different colors
• Color.red, Color.green, Color.blue, Color.black,
  Color.white, Color.yellow, Color.gray,
  Color.orange, Color.pink, Color.cyan,
  Color.magenta
• Or you can use all uppercase names
• Color.RED, Color.BLUE, Color.BLACK,
• (We saw these with our turtles)

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Color Objects

• You can create a Color object by giving the red,
  green, and blue values
• Example
• Color colorObj new Color(255,10,125)

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Making Colors Lighter or Darker

• The Color class has methods for making a Color
  object lighter or darker colorObj.brighter()co
  lorObj.darker()
• Example in Interactions pane
• gt import java.awt.Color
• gt Color testColor new Color(168,131,105)
• gt System.out.println(testColor)
• java.awt.Colorr168,g131,b105
• gt Color darkColor testColor.darker()
• gt System.out.println(darkColor)
• java.awt.Colorr117,g91,b73
• gt Color brightColor testColor.brighter()
• gt System.out.println(brightColor)
• java.awt.Colorr240,g187,b150

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Getting and Setting Pixel Colors

• To get a Pixels color as a Color object
• Color color1 pixelObj.getColor()
• int red color1.getRed()
• int green color1.getGreen()
• int blue color1.getBlue()
• To set a Pixels color using a new Color object
• Color color2 new Color(red10, 0, blue-10)
• pixelObj.setColor(color2)

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Choosing a Color

• You can also get a color by using thefollowing
  methodColorChooser.pickAColor()
• You can use this anywhere you would have used a
  Color object
• Example
• Color pickedColor ColorChooser.pickAColor()
• pixelObj.setColor(pickedColor)

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Pixel Color Review

• What is the difference between this code segment
  and the one on the next slide?
• int red pixelObj.getRed()
• int green pixelObj.getGreen()
• int blue pixelObj.getBlue()
• System.out.println("r " red ", g "
  green
• 
  ", b " blue)

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Pixel Color Review

• Color colorObj pixelObj.getColor()
• int red colorObj.getRed()
• int green colorObj.getGreen()
• int blue colorObj.getBlue()
• System.out.println("r " red ", g "
  green
• 
  ", b " blue)
• In what class are these methods getRed, getGreen,
  getBlue defined?
• In what class are the methods getRed, getGreen,
  getBlue on the previous slide defined?
• Will both code segments print the same thing?

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Changing Colors in a Picture

• We have seen how to change the color of a pixel
  in a picture
• But, you wont see any change in the picture
  until you repaint the picture by
  pictureObj.repaint()
• Another way to do this is by
  pictureObj.show()

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Saving Changes to Pictures

• After manipulating a picture, we can save our
  results to a file pictureObj.write("newPicture.j
  pg")
• You can specify a full path so you know exactly
  where it is saved, for example
  pictureObj.write(Z/jane/MyPictures/newPicture.jp
  g)
• Or, you can use the FileChooser here too
• String fileName FileChooser.pickAFile()
• pictureObj.write(fileName)

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New Concepts in this Section

• Java concepts
• Arrays
• Picture concepts
• Picture as an array of Pixel objects
• Pixel concepts
• Getting/setting red, green, blue values of pixel
• Getting/setting color of pixel as a Color object

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