Working with ranges in pictures
Barb Ericson
Georgia Institute of Technology
Oct 2010
02-RangesInPictures
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Learning Goals
• How do you create a range of values?
• What is a two-dimensional array?
• How do you loop through a two-dimensional
array?
– Nested loops
• How do you simplify a hard problem?
• How do you copy one picture to another?
• How do you make a general function?
02-RangesInPictures
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Try the following in JES
>>> print range (0, 3)
>>> print range (5, 7)
>>> print range (0,10)
>>> print range (3,1)
What do you think the range function does?
Creating ranges of values
You can create a range of values in python
>>> print range (0,3)
[0, 1, 2]
[0,1,2] is an array of values
>>> print range (0 ,10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print range (3,1)
[]
[] is an array with no values
Pictures are really two dimensional
• Pictures have a width and
a height
– getWidth(picture)
– getHeight(picture)
• You can access a pixel of a
picture by using the x and y
values for the pixel
– pixel = getPixel(picture, x, y)
Working with part of a picture
• What if you want to only modify part of a picture?
– Not every pixel in the picture
• You need to be able to say where you want to
start and stop
– Using ranges
for x in range(0, getWidth(picture)):
for y in range(0, getHeight(picture) / 2):
pixel=getPixel(picture, x, y)
Challenge
• Create a version of decrease red that only
changes the red in the top half of the picture
Mirroring a Picture
• If you put a vertical mirror in the middle of a
picture and looked in the mirror you would see
something strange
Mirroring from left to right
What is the Vertical Mirror for this?
• Try the solve the
problem for small
samples
• If you can’t solve it
on a small sample
– You can’t write a
program to solve it
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Mirror Vertical Algorithm
• Loop through all the rows (y
starts at 0 and is less than the
picture height)
– Loop with x starting at 0 and x
less than the midpoint (mirror
point) value
• Get the left pixel at x and y
• Get the right pixel at width – 1
-x
• Set the color for the right pixel
to be the color of the left pixel
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Algorithm to Code
def mirrorVertical(source ):
mirrorPoint = getWidth(source) / 2
width = getWidth(source)
for y in range(0, getHeight(source )):
for x in range(0, mirrorPoint ):
leftPixel = getPixel(source, x, y)
rightPixel = getPixel(source, width - x - 1,y)
color = getColor(leftPixel)
setColor(rightPixel ,color)
Challenge – right to left?
• Copy mirrorVertical and
modify it to mirror from
right to left instead
– What part of the function
needs to change?
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Mirror Horizontal
• What about mirroring
around a mirror held
horizontally in the vertical
center of the picture?
– Like a reflection in a lake?
Thinking Through Mirror Horizontal
• Again think of a number at
each x and y location
– Instead of a color
– And try it with a small sample
• How can we write a nested for
loop to do this?
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What is the horizontal mirror of this?
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• Try to solve the
problem for several
small samples
problems
• See if you can come
up with the algorithm
to solve it
– Test it more small
samples
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Mirror Horizontal Algorithm
• Get the height midpoint
– Picture height / 2
• Loop through all the x values
– Loop from y=0 to y < vertical
midpoint
• Get the top pixel
– At x and y
• Get the bottom pixel
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– Height - 1 - y
• Set the bottom pixel’s color to
the top pixel color
Challenge
• Write the function to mirror top to bottom
• Write the function to mirror from bottom to top
• How about diagonal mirroring?
– In just a square section
Mirror part of a picture
• Can we mirror just part of a picture?
Mirror part of the temple
def mirrorTemple ():
source = makePicture(getMediaPath("temple.jpg"))
mirrorPoint = 277
for x in range (14, mirrorPoint ):
for y in range (28 ,98):
print "Copying color from",x,y,"to",mirrorPoint+mirrorPoint -1-x,y
pleft = getPixel(source,x,y)
pright = getPixel(source ,mirrorPoint+mirrorPoint -1-x,y)
setColor(pright ,getColor(pleft ))
show(source)
return source
fixedTemple = mirrorTemple()
Copying Pixels to a New Picture
• Need to track the source
picture x and y
– And the target picture x
and y
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• We can use a blank
picture
– As the target picture
• Several blank pictures
are available
– 640x480.jpg
– 7inX95in.jpg
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Copy Picture Algorithm
• Copy a picture to the 7 by 9.5 inch blank picture
– Create the target picture object
– Invoke the method on the target picture
• Create the source picture object
• Loop through the source picture pixels
– Get the source and target pixels
– Set the color of the target pixel to the color of the source
pixel
Copying pixels
• In general, what we want to do is to keep
track of a sourceX and sourceY, and a
targetX and targetY.
– We increment (add to them) in pairs
• sourceX and targetX get incremented together
• sourceY and targetY get incremented together
– The tricky parts are:
• Setting values inside the body of loops
• Incrementing at the bottom of loops
Copy picture code
def copyBarb ():
# Set up the source and target pictures
barbf=getMediaPath("barbara.jpg")
barb = makePicture(barbf)
canvasf = getMediaPath("7inX95in.jpg")
canvas = makePicture(canvasf)
# Now , do the actual copying
targetX = 0
for sourceX in range(0, getWidth(barb )):
targetY = 0
for sourceY in range(0, getHeight(barb )):
color = getColor(getPixel(barb ,sourceX ,sourceY ))
setColor(getPixel(canvas ,targetX ,targetY), color)
targetY = targetY + 1
targetX = targetX + 1
show(barb)
show(canvas)
return canvas
Challenge
• How do we change where we copy the picture
to?
– Can we start the copy somewhere other than 0,0?
Create a Collage
Collage Code
def createCollage ():
flower1=makePicture(getMediaPath("flower1.jpg"))
flower2=makePicture(getMediaPath("flower2.jpg"))
canvas=makePicture(getMediaPath("640 x480.jpg"))
#First picture , at left edge
targetX =0
for sourceX in range(0, getWidth(flower1 )):
targetY=getHeight(canvas)-getHeight(flower1 )-5
for sourceY in range(0, getHeight(flower1 )):
px=getPixel(flower1 ,sourceX ,sourceY)
cx=getPixel(canvas ,targetX ,targetY)
setColor(cx ,getColor(px))
targetY=targetY + 1
targetX=targetX + 1
Collage Code - continued
#Second picture , 100 pixels over
targetX =100
for sourceX in range(0, getWidth(flower2 )):
targetY=getHeight(canvas)-getHeight(flower2 )-5
for sourceY in range(0, getHeight(flower2 )):
px=getPixel(flower2 ,sourceX ,sourceY)
cx=getPixel(canvas ,targetX ,targetY)
setColor(cx ,getColor(px))
targetY=targetY + 1
targetX=targetX + 1
Collage Code - continued
#Third picture , flower1 negated
negative(flower1)
targetX =200
for sourceX in range(0, getWidth(flower1 )):
targetY=getHeight(canvas)-getHeight(flower1 )-5
for sourceY in range(0, getHeight(flower1 )):
px=getPixel(flower1 ,sourceX ,sourceY)
cx=getPixel(canvas ,targetX ,targetY)
setColor(cx ,getColor(px))
targetY=targetY + 1
targetX=targetX + 1
Collage Code - continued
#Fourth picture , flower2 with no blue
clearBlue(flower2)
targetX =300
for sourceX in range(0, getWidth(flower2 )):
targetY=getHeight(canvas)-getHeight(flower2 )-5
for sourceY in range(0, getHeight(flower2 )):
px=getPixel(flower2 ,sourceX ,sourceY)
cx=getPixel(canvas ,targetX ,targetY)
setColor(cx ,getColor(px))
targetY=targetY + 1
targetX=targetX + 1
Collage code - continued
#Fifth picture , flower1 , negated with decreased red
decreaseRed(flower1)
targetX =400
for sourceX in range(0, getWidth(flower1 )):
targetY=getHeight(canvas)-getHeight(flower1 )-5
for sourceY in range(0, getHeight(flower1 )):
px=getPixel(flower1 ,sourceX ,source
cx=getPixel(canvas ,targetX ,target)
setColor(cx ,getColor(px))
targetY=targetY + 1
targetX=targetX + 1
show(canvas)
return(canvas)
General Copy Function
def copy(source , target , targX , targY ):
targetX = targX
for sourceX in range(0, getWidth(source )):
targetY = targY
for sourceY in range(0, getHeight(source )):
px=getPixel(source ,sourceX ,sourceY)
tx=getPixel(target ,targetX ,targetY)
setColor(tx ,getColor(px))
targetY=targetY + 1
targetX=targetX + 1
Better Collage Function
def createCollage2 ():
flower1=makePicture(getMediaPath("flower1.jpg"))
flower2=makePicture(getMediaPath("flower2.jpg"))
canvas=makePicture(getMediaPath("640 x480.jpg"))
h1 = getHeight(flower1)
h2 = getHeight(flower2)
hc = getHeight(canvas)
#First picture , at left edge
copy(flower1 ,canvas ,0, hc - h1 – 5)
#Second picture , 100 pixels over
copy(flower2 ,canvas ,100 , hc – h2 – 5)
Better Collage - continued
#Third picture , flower1 negated
negative(flower1)
copy(flower1 ,canvas ,200 , hc – h1 – 5)
#Fourth picture , flower2 with no blue
clearBlue(flower2)
copy(flower2 ,canvas ,300 , hc – h2 – 5)
#Fifth picture , flower1 , negated with decreased red
decreaseRed(flower1)
copy(flower1 ,canvas ,400 , hc – h1 – 5)
return canvas
Challenge
• Create your own collage
– Use at least 1 picture
– Use at least 3 image filters
– Mirror the results
Summary
• You can create ranges of values
– range(10) = 0 – 9
– range(0,10)
• You can loop through a picture
– Using x and y ranges
• You can loop through part of a picture
– By starting and ending at different values than normal
• You can break long methods into shorter parts
– Pull out common code and put it in a method