1. 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,
by Nick Parlante

2. 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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2012 UCA

3. 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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4. 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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5. JES 4.3
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7. 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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2012 UCA

8. Pack a file to work with
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9. Let us pick the beach.jpg
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10. 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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11. 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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12. 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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13. pickAndShow()
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14. 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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15. Load program and use it Click “Load Program”
Then, in the command line area type
pict1 = pickAndShow()
pict2 = pickAndShow()
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17. 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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18. (0, 0)
(639, 0) x y
(639, 479)
(0, 479)
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19. 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
7/10/2012
2012 UCA

20. Get a pixel and find its color value
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21. Change color value of a pixel
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22. 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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23. 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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25. 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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26. 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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27. 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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28. Change values of Green and Blue
changeGreen( pict, alpha)
changeBlue ( pict, alpha)
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29. 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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2012 UCA

30. Solve a puzzle Load the image copper-puzzel.png
pic = pickAndShow()
A picture is hidden in.
Let’s find it?
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31. Solution Zero out redness Increase greenness 20 times for all pixels
Increase the blueness 20 times for all pixels
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32. Solve two more puzzles iron-puzzle.png, and canvas.png
Hint: Increase redness 15 times for all pixels
canvas.png
Hint: Increase blueness 16 times
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33. 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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34. 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
7/10/2012
2012 UCA

35. Hiding a message in a picture (steganography)
A scenic picture (original)
This picture with a hidden message
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37. 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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38. 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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39. 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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40. 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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41. 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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42. 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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43. 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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44. Recover the message Load the picture that you just saved
pic = pickAndShow()
The, decipher it with
deStega(pic)
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45. 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
7/10/2012
2012 UCA

46. 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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47. 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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48. Try to fix it with reflection
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49. Mirroring a picture -vertically
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50. 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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51. 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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52. 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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53. 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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54. 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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55. 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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56. 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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57. 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
7/10/2012
2012 UCA

58. Landing Dr. Mark Guzdial on the moon
From Mark Guzdial’s book: Introduction to computing and programming in Python
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59. Application specification
Provided two pictures: background and foreground
Replace the background through background subtraction
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60. 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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61. 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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62. 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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63. 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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64. Land Mark on the moon
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65. 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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