1. Workshop for Programming And Systems Management Teachers
Chapter 7
Conditionals
Georgia Institute of Technology

2. Georgia Institute of Technology
Learning Goals
Understand at a conceptual and practical level
How to use simple conditionals
How to use complex conditionals
How to remove red-eye
How to posterize a picture
How to do chromakey or blue screen
Georgia Institute of Technology

3. Georgia Institute of Technology
Remove Red Eye
Red eye is when the flash from the camera is reflected from the subject’s eyes
We want to change the red color in the eyes to another color
But not change the red of her dress
See for more information on red-eye.
Georgia Institute of Technology

4. Georgia Institute of Technology
Red Eye Algorithm
We can find the area around the eyes to limit where we change the colors
Using picture.explore()
But we still just want to change the pixels that are “close to” 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
Georgia Institute of Technology

5. 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
Georgia Institute of Technology

6. Conditional Execution
Sometimes we want a statement executed only if some expression is true
We can use the “if” statement in Java
if (colorDistance < value)
Statement or block to execute
next statement
false
if (expression)
true
Statement or block
Use indentation to make your code easier for other people to read. It doesn’t matter to the compiler.
Remember that a block of statements in Java is inside {}.
statement
Georgia Institute of Technology

7. Georgia Institute of Technology
Using if Exercise
Open DrJava and try this in the interactions pane
int x = 2;
if (x > 1) System.out.println(“X is > 1”);
System.out.println(“X is “ + x);
x = 0;
Georgia Institute of Technology

8. Georgia Institute of Technology
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 Picture class to do this
public double getColorDistance(color1,color2)
Georgia Institute of Technology

9. Georgia Institute of Technology
Remove Red Eye Method
/**
* Method to remove red eye from the current picture object in the rectange
* define by startX, startY, endX, endY. The red will be replaced with the
* passed newColor
startX the top left corner x value of a rectangle
startY the top left corner y value of a rectangle
endX the bottom right corner x value of a rectangle
endY the bottom right corner y value of a rectangle
newColor the new color to use */
public void removeRedEye(int startX, int startY, int endX,
int endY, Color newColor) {
Pixel pixel = null;
// loop through the pixels in the rectangle defined by the startX, startY, and
// endX and endY
for (int x = startX; x < endX; x++)
for (int y = startY; y < endY; y++)
// get the current pixel
pixel = getPixel(x,y);
// if the color is near red then change it
if (pixel.colorDistance(Color.red) < 167)
pixel.setColor(newColor); }
Georgia Institute of Technology

10. Georgia Institute of Technology
Edge Detection
Find the areas of high contrast and turn pixels is this area black
Turn all other pixels white
The image is from foal.jpg in mediasources after doing edge detection with a limit of 10.
Georgia Institute of Technology

11. 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
Georgia Institute of Technology

12. 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 (like x > 128 and x <= 128)
int x = 24;
if (x > 128)
System.out.println(“x is > 128”);
else
System.out.println(“x <= 128”);
Georgia Institute of Technology

13. 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
Georgia Institute of Technology

14. Georgia Institute of Technology
Sepia-Toned Pictures
Have a yellowish tint, used to make things look old and western
Georgia Institute of Technology

15. Sepia-toned Algorithm
First make the picture grayscale.
Change the shadows (darkest grays) to be even darker (0 <= red < 60)
Make the middle grays a brown color (60 <= red < 190)
Make the highlights (lightest grays) a bit yellow (190 <= red)
Increase red and green
Or decrease blue
Georgia Institute of Technology

16. Conditional Operators
We can check if several things are true - And
Using && (evaluation stops if the first item is false)
Using & (to always evaluate both operands)
We can check if at least one of several things are true - Or
Using || (evaluation stops if the first item is true)
Using | (to always evaluate both operands)
We can check if only one and only one of the things is true – Exclusive Or
Using ^
The conditional operators && and || are ‘short circuit’ operators. They stop evaluation early when the result is obvious from evaluating the first operand.
See for Sun’s tutorial on conditional operators.
Georgia Institute of Technology

17. Georgia Institute of Technology
Conditional Examples
Check that a value is in a range
Is some value between 0 and 255 (inclusive)
for valid pixel color values
0 <= 13 <= is written as
0 <= 13 && 13 <= 255 // in Java
Check if one of several things is true
Is this black or white?
True if either it is black or it is white
Georgia Institute of Technology

18. Count White Pixels Exercise
Write a method that counts the number of white pixels (red = 255, blue = 255, green = 255) in a picture
The method should return the number of white pixels
Use it to count the number of white pixels in catapillarClipart.jpg
Georgia Institute of Technology

19. Georgia Institute of Technology
Conditional Exercise
When are the following true? When are they false?
You can go out if your room is clean and you did your homework
You can go out if your room is clean or you did your homework
You can go out if either your room is clean or you did your homework but not if both of these is true
Georgia Institute of Technology

20. How many when there is an “And”?
I want you to get soup, milk, bread, and yogurt at the store.
How many items will you come home with?
I want you to clean your room and mop the floor in the kitchen and wash the dishes.
How many tasks do you need to do?
I want a scoop of chocolate scoop and a scoop of vanilla.
How many scoops of ice cream is this?
Georgia Institute of Technology

21. How many when there is an “Or”
You need to help clean the house
You can clean the bathroom or the kitchen or the living room
How many jobs do you have to do?
You want to get an ice cream
The flavors you can pick from are chocolate, vanilla, strawberry, or orange sherbet
How many flavors do you need to pick for a single scoop?
Georgia Institute of Technology

22. Georgia Institute of Technology
Truth Table
Conditional
Operand 1
Operand 2
Result
And
true
false Or
Exclusive Or
An and conditional is only true when both operands are true.
An or conditional is true as long as either operand is true.
An exclusive or conditional is true only if one, but not both, operands are true.
Georgia Institute of Technology

23. Not Conditional Operator
Use ! To change the value to the opposite
!true = false
!false = true
A not conditional operator applied to a complex conditional changes it
!(op1 && op2) = !op1 || !op2
!(op1 || op2) = !op1 && !op2
This is known as De Morgan’s Law
See for information on DeMorgan’s Law
Georgia Institute of Technology

24. Using Multiple If Statements
If we are doing different things based on a set of ranges
0 <= x <= 5
5 < x <= 10
10 < x
if (0 <= x && x <= 5)
Statement or block
if (5 < x && x <= 10)
if (10 < x)
Georgia Institute of Technology

25. Using “else if” for > 2 Options
If we are doing different things based on a set of ranges
0 <= x <= 5
5 < x <= 10
10 < x
if (0 <= x && x <= 5)
Statement or block
else if (x <= 10)
else
Georgia Institute of Technology

26. Georgia Institute of Technology
Sepia-toned Method
public void sepiaTint() {
Pixel pixel = null;
double redValue = 0;
double greenValue = 0;
double blueValue = 0;
// first change the current picture to grayscale
this.grayscale();
// loop through the pixels
for (int x = 0; x < this.getWidth(); x++)
for (int y = 0; y < this.getHeight(); y++)
// get the current pixel and color values
pixel = this.getPixel(x,y);
redValue = pixel.getRed();
greenValue = pixel.getGreen();
blueValue = pixel.getBlue();
// tint the shadows darker
if (redValue < 60)
redValue = redValue * 0.9;
greenValue = greenValue * 0.9;
blueValue = blueValue * 0.9; }
Georgia Institute of Technology

27. Sepia-toned Method - Continued
// tint the midtones a light brown
// by reducing the blue
else if (redValue < 190) {
blueValue = blueValue * 0.8; }
// tint the highlights a light yellow
else
blueValue = blueValue * 0.9;
// set the color
pixel.setColor (new Color((int) redValue,
(int) greenValue,
(int) blueValue));
Georgia Institute of Technology

28. Georgia Institute of Technology
Posterize
Reducing the number of different colors in an image
Set all values in a range to one value (the midpoint of the range)
Set all 0 to 63 to 31
Set all 64 to 127 to 95
Set all 128 to 191 to 159
Set all 192 to 255 to 223
The image here is part of market.jpg
Georgia Institute of Technology

29. Georgia Institute of Technology
Posterize Algorithm
Loop through all the pixels in an image
Get the red value for the pixel
Find the right range and set the new red value
Get the green value for the pixel
Find the right range and set the new green value
Get the blue value for the pixel
Find the right range and set the new blue value
Georgia Institute of Technology

30. Georgia Institute of Technology
Posterize Exercise
Write the method posterize() in the Picture.java class
that will reduce the number of colors by changing color values to 1 of 4 values
Set all 0 to 63 to 31
Set all 64 to 127 to 95
Set all 128 to 191 to 159
Set all 192 to 255 to 223
Georgia Institute of Technology

31. Background Replacement
If you have a picture of a person in front of some background
And a picture of the background itself
Can you replace the pixel colors for the background to be from another image?
Georgia Institute of Technology

32. Georgia Institute of Technology
Replace Background
Replace the colors at all the pixels in the source image
That are within some range of the background color
Use pixels from another background image
Georgia Institute of Technology

33. Replace Background Algorithm
Works on the source picture
Pass in the original background and the new background pictures
Loop through all the pixels in the source image
Check if the distance from the source pixel color is within 15.0 of the background picture pixel color
If so replace it with the color at the new background pixel
Georgia Institute of Technology

34. Replace Background Method
public void swapBackgroundForThreshold(Picture oldBackground,
Picture newBackground,
double threshold) {
Pixel currPixel = null;
Pixel oldPixel = null;
Pixel newPixel = null;
// loop through the columns
for (int x=0; x<getWidth(); x++)
// loop through the rows
for (int y=0; y<getHeight(); y++)
// get the current pixel and old background pixel
currPixel = this.getPixel(x,y);
oldPixel = oldBackground.getPixel(x,y);
/* if the color at the current pixel is within 15.0 of the old background pixel
* then swap in the new background pixel */
if (currPixel.colorDistance(oldPixel.getColor()) < threshold)
newPixel = newBackground.getPixel(x,y);
currPixel.setColor(newPixel.getColor()); }
Georgia Institute of Technology

35. Replace Background Result
The background color was too close to the shirt color
The source picture was kid-in-frame.jpg in mediasources. The old background was in bgframe.jpg. The new background was in moon-surface.jpg.
Georgia Institute of Technology

36. Chromakey – Blue Screen
For TV and movie special effects they use a blue or green screen
Here just a blue sheet was used
Professionally you need an evenly lit, bright, pure blue background
With nothing blue in the scene
See for more information
Georgia Institute of Technology

37. Georgia Institute of Technology
Chromakey Exercise
Write the method chromakey that takes a new background picture as an input parameter
It will loop through all the pixels
If the pixel color is blue (red + green < blue)
Replace the pixel color with the color from the new background pixel (at the same location)
The source picture is blue-mark.jpg. The blue background is blue-empty.jpg. The moon picture is moon-surface.jpg.
Georgia Institute of Technology

38. Georgia Institute of Technology
Summary
Use if, if-else, or if, else if, else for conditional execution
Conditional Operators
Use && to require two expressions to both be true for the result to be true
Use || to allow either (or both) to be true
Use ^ to allow one and only one to be true
Georgia Institute of Technology