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Using Decisions in Java - 1
Recall from Alice
We only wanted to
shoot a lightning bolt
at a philosopher
So, we used the If
statement
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
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
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
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 + (green1green2)2 + (blue1 – blue2)2)
There is a method in the Pixel class to do
this
double dist = pixelObj.colorDistance(color1);
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()
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
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 < value)
Statement or block to
execute
next statement
false
if (expression)
true
Statement
or block
statement
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 < endX; x++)
{
for (int y = startY; y < endY; y++)
{
// get the current pixel
pixelObj = getPixel(x,y);
Remove Red Eye Method
// if the color is near red then change it
if (pixelObj.colorDistance(Color.red) < 167)
{
pixelObj.setColor(newColor);
}
}
}
}
Testing removeRedEye
Use the picture explorer to find
the values for start x, start y, end
x, and end y
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?
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 y+1.
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
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
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 < 20) || (b – a) < 20))
Or we can write:
if (Math.abs(a-b) < 20)
And, or, not in Java
And
Or
Not
&&
||
!
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 < 128)
{System.out.println(“<128”);}
else
{System.out.println(“>=128”);}
false
if (expression)
else
true
Statement
or block
statement
Statement
or block
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
Testing Edge Detection
String file =
FileChooser.getMediaPath(“butterfly1.jpg”);
Picture p = new Picture(file);
p.explore();
p.edgeDetection(10);
p.explore();
Challenge
Create another method for simple edge
detection
This time compare the current pixel with the
one to the right (x+1)
How do you need to change the nested loop?
Do you get a different result?
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
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
Assignment
Read Media Computation Chapter 6,
Sections 1-2