Debrief PA02

1
Session 15

• Debrief PA02
• More work with Conditionals

2
General Things

• Start Early
• Ask Questions!! (Harald has hours on Wed)
• You MUST follow directions
• Follow my names EXACTLY
• Provide the expected parameters EXACTLY
• Pay attention to return types (if not listed,
  there probably isnt one)
• Do NOT show/repaint in your methods unless
  instructed to do so
• These were to work on ANY image

3
Task 1

• Many of you dont have BOTH methods required.
• Those who do often didnt name correctly
• You were told this was a looping lab
• Watch that your borders actually MEET (some of
  you wrote code that didnt produce a complete
  rectangle)

4
Task 2

• Takes an image
• Returns a NEW image exactly the correct size
  (dont use the blank canvas)

5
Task 3

• Takes an image parameter
• Returns a new image exactly the correct size
• While it wasnt clear in my directions, if I give
  you a start X of 100 and a finishX of 200 the
  width of this should be 101 (ditto the height)

6
How would a computer see
One way computers see is to attempt to
determine the boundaries between items in an
image.
7
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 distance is greater than some value turn
  the current pixel black
• Otherwise turn the current pixel white

8
Conditionals

• What is different about the conditional statement
  in this problem versus the conditional statement
  in the removeRedEye() problem from Friday?
• This has two situations

9
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

false
if (expression)
else
true
Statement or block
Statement or block
statement
10
Edge Detection Algorithm

• To find areas of high contrast
• Try to loop from row 1 to row height
• Loop from x 1 to x width 1
• 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

11
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

12
Edge detection (an alternative to Program 40)

• def topDownEdge(source,limit)
• for y in range(1,getHeight(source))
• for x in range(1,getWidth(source)1)
• top getPixel(source,x,y)
• bottom getPixel(source,x,y1)
• if (distance(getColor(top),getColor(bottom
  )) lt limit)
• setColor(top,white)
• else
• setColor(top,black)

13
What about the other way?

• 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?

14
Comparing left/right instead of up/down

• def leftRightEdge(source,limit)
• for x in range(1,getWidth(source))
• for y in range(1,getHeight(source)1)
• left getPixel(source,x,y)
• right getPixel(source,x1,y)
• if (distance(getColor(left),getColor(right
  )) lt limit)
• setColor(left,white)
• else
• setColor(left,black)