CSC1401 Using Decisions in Java 1

1
CSC1401Using Decisions in Java - 1
2
Recall from Alice

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

3
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

4
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

5
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

6
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)

7
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()

8
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

9
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
• Statement or block to execute
• next statement

false
if (expression)
true
Statement or block
statement
10
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
• for (int y startY y
• // get the current pixel
• pixelObj getPixel(x,y)

11
Remove Red Eye Method

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

12
Testing removeRedEye

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

13
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?

14
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

15
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

16
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
• Or we can write
• if (Math.abs(a-b)

17
And, or, not in Java

• And
• Or
• Not !

18
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
• System.out.println(
• else
• System.out.println(128)

false
if (expression)
else
true
Statement or block
Statement or block
statement
19
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

20
Testing Edge Detection

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

21
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?

22
Truth Table
23
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

24
Assignment

• Read Media Computation Chapter 6, Sections 1-2