CSC1401 Manipulating Pictures 3

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CSC1401Manipulating Pictures 3
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Recall last class

• We used a while loop to change each pixels color

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removeBlue() using while
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Another Picture Manipulation

• Lets write a method to modify all the pixels in
  a picture
• Reduce the blue value at each pixel by 50
• Get the array of pixels from the picture
• Loop through all the pixels in the picture
• Get the blue value at the current pixel
• Calculate the new blue value
• Set the blue value of the current pixel to the
  new value

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Decrease Blue Algorithm

• To decrease the blue value in a picture by 50
• Get the array of pixels from the picture
• We want to loop through all the pixels in a
  picture so we need to declare and initialize some
  variables
• Set up an index to start at 0
• Check if the index is less than the length of the
  array
• Get the pixel at the current index from the array
  of pixels
• Get the blue value at the pixel
• Divide the blue value by 2
• Set the blue value at the pixel to the reduced
  blue value
• Increment the index and go back to step 3

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Loop Algorithm to Code

• How to write (code) the loop?
• Use a while loop with a counter for the index
  starting at 0
• int index 0
• Loop while the index is less than the length of
  the array
• while (index
• Get the current pixel from the array of pixels
  for the current index
• Pixel pixelObj pixelArrayindex

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Loop Algorithm to Code - Continued

• Get the blue value at the pixel
• int valueBlue pixelObj.getBlue()
• Divide the blue value by 2
• valueBlue valueBlue / 2
• Set the pixel blue value
• pixel.setBlue(valueBlue)
• Add one to (increment) the index
• index index 1

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Decrease Blue Method

• public void decreaseBlue()
• Pixel pixelArray this.getPixels()
• Pixel pixelObj null
• int index 0
• // loop through all the pixels
• while(index
• // get the current pixel
• pixelObj pixelArrayindex

• // get the red value
• int value pixelObj.getBlue()
• // decrease the red value
• value value / 2
• // set the pixel red
• pixelObj.setBlue(value)
• // increment the index
• index index 1

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Using Multiplication by 0.5

• You could have also multiplied the blue value by
  0.5
• value value 0.5
• Change the line that divided by 2 and try to
  compile this
• In the decreaseBlue() method

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Casting to Solve Loss of Precision Error

• It will compile if we tell the compiler we know
  about the possible loss of precision
• And that it is intended
• By using a cast to int
• value (int) (value 0.5)
• Notice that we cast the result of the
  multiplication back to an integer
• Casting is forcing a value into a type
• (type) expression

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Move Declarations Outside Loops

• When you need a variable in a loop it is best to
  declare it before the loop
• Otherwise you are declaring a new variable each
  time through the loop
• Which is slower than just changing the value
  associated with the variable
• In Java you can declare variables anywhere in a
  method
• As long as you declare them before you use them

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A new problem

• Create a new method
• public void decreaseBlue (int percentage)
• We parameterize how much to decrease the blue by
• Note Java allows us to have two methods of the
  same name, as long as they have different numbers
  of parameters, or different parameter types
• If the user wishes to decrease the blue by 20,
  they can specify
• mypicture.decreaseBlue(20)

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What should the formula look like?

• What is the matter with
• blueValue pixelcount.getBlue()
• blueValue blueValue ((100-percentage)/100)
• Pixelcount.setBlue(blueValue)

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Exercise

• Create a similar method called
• public void decreaseBlue (int percentage)
• Now create a method called sunset()
• This method should decrease the blue and the
  green by 30 percent, and should call the
  decreaseBlue and decreaseGreen methods

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Another problem

• Lets convert a picture to Grayscale
• The book has a complex formula for converting to
  grayscale
• A simple formula is to set the amount of red,
  green and blue to each be
• (red green blue) / 3

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Assignment

• Review Media Computation Chapter 4