Digital Image Manipulation
-2012 CS4HS @ UCA
References:
• Introduction to computing and programming in Python – a
multimedia approach by Mark Guzdial and Barbara Ericson
• JES: Jython Environment for Students,
http://code.google.com/p/mediacomp-jes/
• http://nifty.stanford.edu/2011/parlante-image-puzzle/ by Nick
Parlante
Outline of tasks
0. JES Introduction
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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JES: Jython Environment for Students
• JES has a set of functions to manipulate media
data (digital image and sounds)
• Let us open the environment
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JES 4.3
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Pack a file to work with
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Let us pick the beach.jpg
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Let the computer remember it
• Assign a name to the file we can then refer
and use it later.
myFile = pickAFile()
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View the picture
• Let us make myFile an picture object
myPict = makePicture(myFile)
• Does print myPict work?
print(myPict)
• View myPict with
show(myPict)
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Make a function: pick and show
• We do not want to repeatedly type the same
commands to load and show a picture every
time
• Let us create it once and be able to use it
whenever needed
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pickAndShow()
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Let us save it
• File -> Save Program
• Specify the location (your USB drive), and give
it a name (say prog1)
• Exit JES
• Open JES again
• File -> Open Program
• Select prog1
• It comes back
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Load program and use it
• Click “Load Program”
• Then, in the command line area type
pict1 = pickAndShow()
pict2 = pickAndShow()
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Investigate the details
• Now let us investigate the details of a digital
picture
• Pick “MediaTools” from the menu bar, then
select “Picture Tool …”
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(0, 0)
(639, 0)
x
y
(639, 479)
(0, 479)
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Get a pixel and find its color value
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Change color value of a pixel
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Image manipulation
• It is all about changing color values of pixels
• The process should be:
– get a specific pixel first, and
– then reset its color.
• Do we want to type all the command lines above
for every pixel again and again?
– The picture has 640 x 480 = 307,200 pixels.
• How should we do it?
• For example, how do we zero out red values for
all pixels?
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JES Functions
• JES Functions -> Pictures -> getPixels
• By using the function getPixels, we can get all
pixels at once.
• We then loop through each pixel to zero out
its red value.
• Let us open our prog1 and create another
function named ZeroRed
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Now let us use it
•
•
•
•
Load Program
Pick a picture
Zero red out for all pixels in the picture
View the changes with Picture Tool …
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Work with the pictureModule
• We provide you a collection of functions in the
pictureModule.py
• Open and load the module
• Here are some available functions:
– pickAndShow()
– changeRed( pict, alpha )
– changeGreen( pict, alpha )
– changeBlue( pict, alpha )
– ……
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chageRed(pict, alpha)
def changeRed( pict, alpha ):
allPixels = getPixels( pict )
for p in allPixels:
value = alpha * getRed( p )
if value > 255:
value = 255
setRed( p, value)
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Change values of Green and Blue
• changeGreen( pict, alpha)
• changeBlue ( pict, alpha)
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Solve a puzzle
• Load the image copper-puzzel.png
pic = pickAndShow()
• A picture is hidden in.
• Let’s find it?
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Solution
1. Zero out redness
2. Increase greenness 20 times for all pixels
3. Increase the blueness 20 times for all pixels
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Solve two more puzzles
• iron-puzzle.png, and
Hint: Increase redness 15 times for all pixels
• canvas.png
Hint: Increase blueness 16 times
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How do we create a puzzle?
• Zero out R, G, or B
• Weak the remaining color
• Add randomly generated values for the zero
out color
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Hiding a message in a picture
(steganography)
A scenic picture (original)
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This picture with a hidden message
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Problem specification
• Inputs:
– A cover image
– A message
• Outputs:
– An image seems exactly as the original but with
the message embedded
– From the image, one can recover the embedded
message
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Solution design
• To embed the message, we need to identify the
position of each pixel of the message and
remember them.
• To remember the position of a pixel, we only
need to change the original very little such that
the altered image seem exactly as the original
• When we need to reveal the message, we only
need to print the pixels at the positions
remembered.
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Solution design continue
• To identify the position of each pixel of the message,
we check if a pixel is close to black (or whatever color
the text is).
• To remember the position of a pixel, we do the
followings:
– Preparation: Make the red (or green, blue) value of each
pixel even.
– Memorization: For each message pixel, add one to the red
value (become odd now).
– Note: These two steps change the original very little
• To reveal the message, we only need to print the pixels
whose red value is odd.
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Implementation: encode
def stegano(msgPic, coverPic):
for pxl in getPixels(coverPic):
r = getRed(pxl)
if r%2 ==1:
setRed(pxl, r-1)
for x in range(1, getWidth(coverPic)):
for y in range(1, getHeight(coverPic)):
msgPxl = getPixel(msgPic, x, y)
coverPxl = getPixel(coverPic, x, y)
if distance(getColor(msgPxl), black) < 100.0:
setRed(coverPxl, getRed(coverPxl) +1)
return coverPic
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Implementation: decode
def deStega(segaPic):
w = getWidth(segaPic)
h = getHeight(segaPic)
msg = makeEmptyPicture(w, h)
for x in range(0,w):
for y in range(0, h):
encPxl = getPixel(segaPic, x, y)
msgPxl = getPixel(msg, x, y)
if (getRed(encPxl)%2) ==1:
setColor(msgPxl, black)
show(msg)
return msg
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Integration and maintenance
• The application should be either to encode a
message in a cover image or discover the
message from an encoded image.
• For the former, we need to pick a cover image,
then a message as an image. Then, encode it.
• For the later, we need to recover the message
from its cover image.
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Let us do it
• Open and load the pictureModule
• Load two pictures, one is the cover image and the
other is the text image, with
coverPic = pickAndShow()
msg = pickAndShow()
• Embed the message in the cover image
secret = stegano (coverPic, msg)
• Save it with JES Functions -> Pictures ->
writePictureTo
• Note: use the .png extension not .jpg
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Recover the message
• Load the picture that you just saved
pic = pickAndShow()
• The, decipher it with
deStega(pic)
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Let us fix the temple in Athens
• What is the problem
specification?
• How do we design a
solution?
• Implement it
• Test and refine the
computational solution
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How do we fix it?
• Due the symmetry, we may flip the left half to
the right
• How do we do it?
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Try to fix it with reflection
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Mirroring a picture -vertically
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Specification
• The center line is x = midpoint = width/2.
• The x-coordinate of a pixel on the left half is:
width/2 - offset
• The x-coordinate of a pixel on the right half is:
width/2 + offset
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Further specification
- symmetrically reflecting from left to right
• Stay on the same row, i. e., the value of y does
not change
• The x-distance to the center should be the
same
• We need to get the color value of a pixel at
(midpoint – offset, y) and assign it to the pixel
located at (midpoint + offset, y)
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Design:
• For a given row, fixed y, loop with offset from 1 to
width/2:
– Get the left pixel at (midpoint – offset, y)
lpixel = getPixel(pic, midpoint – offset, y)
– Get the color value of left pixel color = getColor(lpixel)
– Get the mirror (right) pixel (midpoint + offset, y)
rpixel = getPixel(pic, midpoint + offset, y)
– Make the color of right pixel the same as the color of the
left pixel
setColor(rpixel, color)
• The above operation mirrors one row, we need to do it
for every row. Hence, we should loop y from 1 to
height)
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Implementation
def mirrorLR(pic): #mirror a picture from left to right
midpoint = getWidth(pic)/2
for y in range(0, getHeight(pic)):
for offset in range(1, midpoint):
lpixel = getPixel(pic, midpoint - offset, y)
color = getColor(lpixel)
rpixel = getPixel(pic, midpoint + offset, y)
setColor(rpixel, color)
Note: This is a nested loop, i.e., loop of loop. Why do we need it?
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To test the function, we
• Load a picture object in computer memory
>>> mypict = pickAndShow( )
• Apply the function to it
>>> mirrorLR(mypict)
• Does it work?
>>> repaint(mypict)
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A function named main
• We have created functions to perform specific
tasks.
• These functions can be assembled in solving
applications.
• By writing a function named main, we can
call it without typing in command line
• In fact, in C and C++, the main function is
required and the only one can be executed
directly
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A sample main function
def main():
pict = pickAndShow( )
mirrorLtoR(pict)
repaint(pict)
Note: Using a simple main function, one may easily test
design ideas and subtasks. In computer science, this
kind of purpose is also called a driver.
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Outline of tasks
0. JES Introduction:
1. Load and investigate details of an image
2. Alternate pixel RGB values
3. Solve image puzzles
4. Embed information in a picture
5. Fix the Temple of Hephaestus virtually
6. Land Mark on the moon
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Landing Dr. Mark Guzdial on the moon
From Mark Guzdial’s book: Introduction to
computing and programming in Python
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Application specification
• Provided two pictures: background and
foreground
• Replace the background through background
subtraction
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Applications of chromakey
• Weatherman makes forecasts on TV with a
green (or blue) background
• In TV broadcast, they use background
subtraction to combine the map
• It is also very popular in making movies.
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Further specification
• Background subtraction is all about replacing
some pixels in foreground with those pixels in
background (without the person).
• In general, it is still hard for a computer to
distinguish if a pixel belongs to the foreground
or background.
• Let us simplify the background with a single
color (say green or blue).
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Where does it belong?
• Provide a single colored background
• Let a person take a picture with that
background
• To determine if a pixel belongs to the
background or not, we only need to compare
color distance.
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Implementation
def chromakey(foreground, background ):
for p in getPixels( foreground ):
# Assume a pixel blue if the sum of its R + G < B
if getRed(p) + getGreen(p) < getBlue(p):
bgPixel = getPixel(background, getX(p), getY(p))
newColor = getColor( bgPixel )
setColor( p, newColor )
show( foreground )
Note: We assume that the size of background is no less than that of foreground
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Land Mark on the moon
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Land yourself on the moon (or
anywhere else you want) virtually?
• What do you need (input data)?
• Modify the chromakey function
• Create a program that can virtually land
yourself on the moon (or anywhere else you
want to) .
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