177 CS Week 4 Recitation Slides
for Loop
if statement
and
range
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Announcements
EXAM 1
Wednesday 09/29
6:30p - 7:30p
EE 129
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ANY QUESTIONS?
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Lets remember for Loop
def decreaseRed(picture):
for p in getPixels(picture):
value = getRed(p)
setRed(p,value*0.5)
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What is wrong here?
def decreaseRed(picture):
for p in 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.
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Clearing Blue
def clearBlue(picture):
for p in getPixels(picture):
setBlue(p,0)
Again, this will work for any picture.
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Lightening and darkening an image
def lighten(picture):
def darken(picture):
for px in getPixels(picture):
for px in getPixels(picture):
color = getColor(px)
color = getColor(px)
color = makeLighter(color)
color = makeDarker(color)
setColor(px ,color)
setColor(px ,color)
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Creating a negative
Lets think it through
R,G,B go from 0 to 255
Lets say Red is 10. Thats very light red.
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Whats 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)
negative of negative is
setColor(px,negColor)
the original picture
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Converting to greyscale
We know that if red=green=blue, we get grey
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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 simplesimply 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))
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Building a better greyscale
Well weight red, green,
def greyScaleNew(picture):
and blue based on how
for px in getPixels(picture):
light we perceive them
newRed = getRed(px) * 0.299
to be, based on
newGreen = getGreen(px) * 0.587
laboratory experiments.
newBlue = getBlue(px) * 0.114
luminance = newRed + newGreen + newBlue
setColor(px,makeColor(luminance,luminance,luminance))
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Comparing the two greyscales:
Average on left, weighted on right
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How to save the changes?
writePictureTo(picture,filename)
Windows:
MacOS
writePictureTo(picture,"E:/temp/output.jpg")
writePictureTo(picture,"/home/users/guzdial/med
iasources/output.jpg")
Writes the picture out as a JPEG
Be sure to end your filename as .jpg!
If you dont specify a full path,
will be saved in the same directory as JES.
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if statement
An statement if 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
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If a is 45, prints a is small
If a is 153, does nothing
if - else statement
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"
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If a is 45, prints a is small
If a is 153, prints a is large
Lets 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
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Lets 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
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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 parameter value
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]
>>> range(9)
[0, 1, 2, 3, 4, 5, 6, 7, 8]
[0]
>>> range(0)
[]
[]
>>> range(-5)
[]
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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)
[0, 1, 2]
>>> range(0, 10)
[4, 5, 6]
>>> range(7, 4)
>>> range(-2, 2)
[-2, -1, 0, 1]
>>> range(-2,
-5)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] []
[]
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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]
>>> range(0, 6, 3)
[0, 3]
[1, 3, 5]
>>> range(-7, -1, 2)
[-7, -5, -3]
[-5, -2, 1, 4]
>>> range(7, 1, -2)
[7, 5, 3]
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Final QUESTIONS???
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