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CS 177 Week 4 Recitation SlidesPowerPoint Presentation

CS 177 Week 4 Recitation Slides

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Let’s remember for Loop

def decreaseRed(picture):

forpin getPixels(picture):

value = getRed(p)

setRed(p,value*0.5)

4

def decreaseRed(picture):

forpin getPixels(picture):

value = getRed(p)

setRed(p,value*0.5)

Indentation is wrong!

This statement is not inside the for loop.

Only the last pixel is changed.

5

Clearing Blue

def clearBlue(picture):

for p in getPixels(picture):

setBlue(p,0)

Again, this will work for any picture.

Lightening and darkening an image

def darken(picture):

for px in getPixels(picture):

color = getColor(px)

color = makeDarker(color)

setColor(px ,color)

def lighten(picture):

for px in getPixels(picture):

color = getColor(px)

color = makeLighter(color)

setColor(px ,color)

Creating a negative

- Let’s think it through
- R,G,B go from 0 to 255
- Let’s say Red is 10. That’s very light red.
- What’s the opposite? LOTS of Red!

- The negative of that would be 255 – 10 = 245

- So, for each pixel, if we negate each color component in creating a new color, we negate the whole picture.

Creating a negative

def negative(picture):

for px in getPixels(picture):

red = getRed(px)

green = getGreen(px)

blue = getBlue(px)

negColor=makeColor( 255-red, 255-green, 255-blue)

setColor(px,negColor)

negative of negative is the original picture

Converting to greyscale

- We know that if red=green=blue, we get grey
- But what value do we set all three to?

- What we need is a value representing the darkness of the color, the luminance
- There are lots of ways of getting it, but one way that works reasonably well is really simple—simply take the average:

Converting to greyscale

def greyScale(picture):

for p in getPixels(picture):

intensity = (getRed(p)+getGreen(p)+getBlue(p))/3

setColor(p,makeColor(intensity,intensity,intensity))

Building a better greyscale

We’ll weight red, green, and blue based on how light we perceive them to be, based on laboratory experiments.

def greyScaleNew(picture):

for px in getPixels(picture):

newRed = getRed(px) * 0.299

newGreen = getGreen(px) * 0.587

newBlue = getBlue(px) * 0.114

luminance = newRed + newGreen + newBlue

setColor(px,makeColor(luminance,luminance,luminance))

Comparing the two greyscales:Average on left, weighted on right

How to save the changes?

- writePictureTo(picture,”filename”)
- Windows:
- writePictureTo(picture,"E:/temp/output.jpg")

- MacOS
- writePictureTo(picture,"/home/users/guzdial/mediasources/output.jpg")

- Writes the picture out as a JPEG
- Be sure to end your filename as “.jpg”!
- If you don’t specify a full path,will be saved in the same directory as JES.

if statement

If a is 45, prints “a is small”

If a is 153, does nothing

An if statement takes a logical expression and evaluates it.

If it is true, the statements in if block are executed,

otherwise, they are not executed.

if a < 100:

print "a is small“

if - else statement

If a is 45, prints “a is small”

If a is 153, prints “a is large”

Similarly, the logical expression is evaluated.

If it is true, the statements in if block are executed,

otherwise, the statements in else block are executed.

if a < 100:

print "a is small"

else:

print "a is large"

Let’s count the red pixels in a picture

def countRedPixels(picture):

redCount = 0

for p in getPixels(picture):

color = getColor(p)

if(color == red):

redCount = redCount + 1

print redCount

Let’s count the the non-red pixels too

def countPixels(picture):

redCount = 0

nonRedCount = 0

for p in getPixels(picture):

color = getColor(p)

if(color == red):

redCount = redCount + 1

else:

nonRedCount = nonRedCount + 1

print redCount

print nonRedCount

function range

- Range is a function that returns a sequence
- If range has only one input parameter: (i.e range(input))
- It generates the sequence of all the non-negative integers that are less than the input parametervalue
- the generated sequence starts with 0
- increment is 1
- the last element of the sequence is the value of input parameter – 1
>>> range(3) >>> range(1) >>> range(-1)

[0,1,2] [0] []

>>> range(9) >>> range(0) >>> range(-5)

[0, 1, 2, 3, 4, 5, 6, 7, 8] [] []

function range

- If two inputs (i.e range(first_input, second_input)):
- It generated the sequence of all the integers that are greater than or equal to the first_input value and less than the second_input value
- the first element of the sequence is the value of first_input
- increment is 1
- the last element of the sequence is the value of second_input – 1
>>> range(0, 3) >>> range(4, 7) >>> range(-2, 2)

[0, 1, 2] [4, 5, 6] [-2, -1, 0, 1]

>>> range(0, 10) >>> range(7, 4) >>> range(-2, -5)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [] []

function range

- If three inputs (i.e. range(first_input, second_input, third_input)):
- the sequence starts with the first_input value
- increment is third-input
- If increment is positive the sequence ends with the largest value less than second_input
- If increment is negative the sequence ends with the smallest value greater than second_input
>>> range(0, 3, 1) >>> range(1, 7, 2) >>> range(-5, 5, 3)

[0, 1, 2] [1, 3, 5] [-5, -2, 1, 4]

>>> range(0, 6, 3) >>> range(-7, -1, 2) >>> range(7, 1, -2)

[0, 3] [-7, -5, -3] [7, 5, 3]

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