CSC1401 Using Decisions in Java - 1

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CSC1401Using Decisions in Java - 1
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Recall from Alice

• We only wanted to shoot a lightning bolt at a
  philosopher
• So, we used the If statement

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Learning Goals

• Understand at a conceptual and practical level
• How to conditionally execute a statement or a
  block of statements
• How to remove red-eye from a picture
• How to use conditionals with two possibilities
• How to do simple edge detection

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Remove Red Eye

• Red eye is when the flash from the camera is
  reflected from the subjects eyes
• We want to change the red color in the eyes to
  another color
• But not change the red of her dress

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Red Eye Algorithm

• How do we know if the color of a pixel is
  mostly red?
• We can find the distance between the current
  color and our definition of red
• And change the color of the current pixel only if
  the current color is within some distance to the
  desired color

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Color Distance

• The distance between two points is computed as
• Square root of (( x1 x2)2 (y1 y2)2)
• The distance between two colors can be computed
• Square root of ((red1 red2)2 (green1-green2)2
  (blue1 blue2)2)
• There is a method in the Pixel class to do this
• double dist pixelObj.colorDistance(color1)

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Red Eye algorithm

• How do we only remove the red in her eyes,
  without removing the red from her sweater?
• We can find the area around the eyes to limit
  where we change the colors
• Using pictureObj.explore()

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Detailed Red Eye Algorithm

• Loop with x staring at some passed start value
  and while it is less than some passed end value
• Loop with y starting at some passed start value
  and while it is less than some passed end value
• Get the pixel at this x and y
• Get the distance between the pixel color and red
• If the distance is less than some value (167)
  change the color to some passed new color

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Conditional Execution

• Sometimes we want a statement or block of
  statements executed only if some expression is
  true
• We can use the if statement in Java
• if (colorDistance lt value)
• Statement or block to execute
• next statement

false
if (expression)
true
Statement or block
statement
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Remove Red Eye Method

• public void removeRedEye(int startX, int startY,
  int endX,
• int endY, Color
  newColor)
• Pixel pixelObj
• // loop through the pixels in the rectangle
  defined by the // startX, startY, and endX and
  endY
• for (int x startX x lt endX x)
• for (int y startY y lt endY y)
• // get the current pixel
• pixelObj getPixel(x,y)

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Remove Red Eye Method

• // if the color is near red then change
  it
• if (pixelObj.colorDistance(Color.red) lt
  167)
• pixelObj.setColor(newColor)

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Testing removeRedEye

• Use the picture explorer to find the values for
  start x, start y, end x, and end y

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Challenge

• Take a picture of a friend
• And try to change their eye color
• Try to change their hair color
• Try to change their clothing color
• Can you write one method to do this?
• And call it several times with different
  parameters?

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A 2nd example Edge Detection

• Loop through all the pixels in the picture
• Calculate the average color for the current pixel
  and the pixel at the same x but y1.
• Get the distance between the two averages
• If the absolute value of the distance is greater
  than some value turn the current pixel black
• Otherwise turn the current pixel white

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Edge Detection Algorithm

• To find areas of high contrast
• Try to loop from row 0 to row height 1
• Loop from x 0 to x width
• Get the pixel at the x and y (top pixel)
• Get the pixel at the x and (y 1) bottom pixel
• Get the average of the top pixel color values
• Get the average of the bottom pixel color values
• If the absolute value of the difference between
  the averages is over a passed limit
• Turn the pixel black
• Otherwise turn the pixel white

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How do we determine absolute value in Java?

• To test if the absolute value of a and b is less
  than 20, we can write
• if ((a b lt 20) (b a) lt 20))
• Or we can write
• if (Math.abs(a-b) lt 20)

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And, or, not in Java

• And
• Or
• Not !

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Use if and else for two possibilities

• Sometimes you want to do one thing if the
  expression is true
• and a different thing if it is false
• int x 200
• if (x lt 128)
• System.out.println(lt128)
• else
• System.out.println(gt128)

false
if (expression)
else
true
Statement or block
Statement or block
statement
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Edge Detection Exercise

• Write a method edgeDetection that takes an input
  limit
• And turns all pixels black where the absolute
  value of the difference between that pixel and
  the below pixel is greater than the passed limit
• And turns all pixels white where the absolute
  value of the difference between that pixel and
  the below pixel is less than or equal the passed
  limit
• Pixel has a getAverage() method that returns the
  average of the three colors at the pixel

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Testing Edge Detection

• String file
• FileChooser.getMediaPath(butterfly1.jpg)
• Picture p new Picture(file)
• p.explore()
• p.edgeDetection(10)
• p.explore()

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Challenge

• Create another method for simple edge detection
• This time compare the current pixel with the one
  to the right (x1)
• How do you need to change the nested loop?
• Do you get a different result?

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Truth Table
Conditional Operand 1 Operand 2 Result
And - true true true
And true false false
And false true false
And false false false
Or true true true
Or true false true
Or false true true
Or false false false
Not - ! true false
Not false true
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Summary

• Use if and else (the else is optional) if you
  have two possibilities to deal with
• if (test)
• // statements to execute when the test is true
• else
• // statements to execute when the test is false
• Complex conditionals
• Use to test for more than one thing being
  true
• Use to test if at least one thing is true
• Use ! to change the result from true to false
  and false to true

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Assignment

• Read Media Computation Chapter 6, Sections 1-2