CSC1401 Manipulating Pictures

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CSC1401Manipulating Pictures
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What we have done to date

• We have modified pictures by writing on top of
  them
• Using Turtles and using Graphics
• Drawing lines, shapes and text, as well as
  inserting a picture into another picture

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What would be nice to do

• Changing colors on the pictures
• Red-eye reduction
• Blending pictures
• Doing other fancy Adobe Photoshop-like
  transformations
• But this would be almost impossible to do by
  simply drawing on top of the existing picture

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The Goal

• Develop an easier approach to modifying our
  pictures by changing the pictures themselves
  rather than writing on top of them

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Digital Pictures

• Represented by pixels
• With a red, green, and blue value stored for each
  pixel
• Stored in .jpg (JPEG) files
• International standard
• With lossy compression
• Lossy means not all data is stored
• But what is lost isnt that important
• Compression means made smaller
• Other formats for storing digital pictures are
  GIF and BMP

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Manipulating a Picture

• To manipulate a picture we need to manipulate the
  pixels that make up the picture
• Change the red, green, or blue values at the
  pixel
• Pixel is a class created at Georgia Tech
• Each pixel object has a red, green, and blue
  value

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Pictures have lots of Pixels

• How can we refer to each pixel?
• pixel1, pixel2, pixel3, pixel4, pixel5,
• Do we really want to name each one?
• On a 640 x 480 picture, there are 640 x 480
  307,200 pixels

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How do we change a Pixel?

• Pixel first
• first stevepicture.getPixel(10,120)
• // 10 refers to the row and 120 to the
  columnfirst.setColor(Color.black)
• / The book writes the code as
• stevepicture.getPixel(10,100).setColor(Color.black
  )
• /
• Note that if you have already shown the
  picture, by invoking
• stevepicture.show()
• You can cause the picture to be updated by
• stevepicture.repaint()

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Pictures have lots of Pixels

• How do we deal with lots of data of the same
  type?
• Use an array

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What is an Array?
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• Storage for a sequence of items
• Of the same type
• You can access 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)
• Arrays know their length (have a public length
  field)
• arrayObj.length

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But how does an array help us?

• If we wish to change lots of pictures, we can use
  a loop!

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Recall from Alice
In this example, each time through the loop, the
bunny hopped Note that the index variable changes
from 0 to 1 to 2 to each time through the loop.
And we can take advantage of that when working
with arrays!
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In Java

• Use a loop to access an array of pixels
• The first time through the loop we access
  pixelArray0
• The second time through the loop we access
  pixelArray1

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What Data does a Picture Object Have?

• A picture object has an array of pixel objects
• That it read from the .JPG file
• It knows the picture width
• pictureObj.getWidth()
• It knows the picture height
• pictureObj.getHeight()
• It knows how to return an array of pixels
• Pixel pixelArray pictureObj.getPixels()

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Pixel Objects

• Each pixel has a red, green, and blue value
• getRed(), getGreen(), getBlue()
• setRed(v), setGreen(v), setBlue(v)
• Each pixel knows the location it was in the
  picture object
• getX(), getY()
• You can also get and set the color at the pixel
• getColor(), setColor(color)

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Turning all of the pixels in our picture to red
using a loop

• Notes
• We have created a counter that starts at 0, and
  goes up to the number of pixels in the picture
• Each pixel has its color set to red!
• How would this code have looked had it been
  written as a method inside of the Picture class?

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A note on Color

• You can either set the red, green and blue
  amounts individually, or all together using
  mypixel.setColor(someColor)
• You can create a color object by giving the red,
  green, and blue values for it
• Color colorObj new Color(255,10,125)

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Predefined Colors

• The Color class has defined class constants for
  many 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,

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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
• red 20
• green 30
• blue 100
• Color color2 new Color(red,green,blue)
• pixelObj.setColor(color2)

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Changing Pixel Colors

• There are two ways to change the color of a pixel
  in a picture
• Set the red, green, and blue values individually
• pixelObj.setRed(value),
• pixelObj.setGreen(value),
• pixelObj.setBlue(value),
• Or set the color
• pixelObj.setColor(colorObj)
• But, you wont see any change in the picture
• Until you ask it to repaint pictureObj.repaint()
  
• Or you invoke the show method

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Summary

• Pictures have pixels
• You can change the picture by changing the color
  of the pixels
• Arrays let you store and retrieve values of the
  same type using an index
• You can ask a picture for its width, height, and
  an array of pixels
• You can get and set the color of a pixel

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Assignment

• Read Media Computation Chapter 4 well be
  covering while loops and for-each loops next week