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

2012 CS4HS @ UCA 2

Outline of tasks

0. JES Introduction 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 3

Outline of tasks

0. JES Introduction: 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 5

JES 4.3

7/10/2012

2012 CS4HS @ UCA 67/10/2012

2012 CS4HS @ UCA 7

Outline of tasks

0. JES Introduction:1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 8

Pack a file to work with

7/10/2012

2012 CS4HS @ UCA 97/10/2012

Let us pick the beach.jpg

2012 CS4HS @ UCA 10

Let the computer remember it

• Assign a name to the file we can then refer and use it later.myFile = pickAFile()

7/10/2012

2012 CS4HS @ UCA 11

View the picture

• Let us make myFile an picture objectmyPict = makePicture(myFile)

• Does print myPict work?print(myPict)

• View myPict withshow(myPict)

7/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 13

pickAndShow()

7/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 15

Load program and use it

• Click “Load Program”• Then, in the command line area type

pict1 = pickAndShow()pict2 = pickAndShow()

7/10/2012

2012 CS4HS @ UCA 167/10/2012

2012 CS4HS @ UCA 17

Investigate the details

• Now let us investigate the details of a digital picture

• Pick “MediaTools” from the menu bar, then select “Picture Tool …”

7/10/2012

2012 CS4HS @ UCA 187/10/2012

(0, 0) (639, 0)

(0, 479)

(639, 479)

x

y

2012 CS4HS @ UCA 19

Outline of tasks

0. JES Introduction: 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 20

Get a pixel and find its color value

7/10/2012

2012 CS4HS @ UCA 21

Change color value of a pixel

7/10/2012

2012 CS4HS @ UCA 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?7/10/2012

2012 CS4HS @ UCA 23

JES Functions

7/10/2012

• 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

2012 CS4HS @ UCA 247/10/2012

2012 CS4HS @ UCA 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 …

7/10/2012

2012 CS4HS @ UCA 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 )– ……

7/10/2012

2012 CS4HS @ UCA 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)

7/10/2012

2012 CS4HS @ UCA 28

Change values of Green and Blue

• changeGreen( pict, alpha)• changeBlue ( pict, alpha)

7/10/2012

2012 CS4HS @ UCA 29

Outline of tasks

0. JES Introduction: 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 30

Solve a puzzle

• Load the image copper-puzzel.png pic = pickAndShow()

• A picture is hidden in.• Let’s find it?

7/10/2012

2012 CS4HS @ UCA 31

Solution

1. Zero out redness 2. Increase greenness 20 times for all pixels3. Increase the blueness 20 times for all pixels

7/10/2012

2012 CS4HS @ UCA 32

Solve two more puzzles

• iron-puzzle.png, andHint: Increase redness 15 times for all pixels

• canvas.pngHint: Increase blueness 16 times

7/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 34

Outline of tasks

0. JES Introduction: 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 35

Hiding a message in a picture (steganography)

7/10/2012

A scenic picture (original) This picture with a hidden message

2012 CS4HS @ UCA 367/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 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.

7/10/2012

2012 CS4HS @ UCA 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.

7/10/2012

2012 CS4HS @ UCA 40

Implementation: encodedef 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

7/10/2012

2012 CS4HS @ UCA 41

Implementation: decodedef 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

7/10/2012

2012 CS4HS @ UCA 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.

7/10/2012

2012 CS4HS @ UCA 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 imagesecret = stegano (coverPic, msg)

• Save it with JES Functions -> Pictures -> writePictureTo

• Note: use the .png extension not .jpg7/10/2012

2012 CS4HS @ UCA 44

Recover the message

• Load the picture that you just savedpic = pickAndShow()

• The, decipher it with deStega(pic)

7/10/2012

2012 CS4HS @ UCA 45

Outline of tasks

0. JES Introduction: 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ 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

7/10/2012

2012 CS4HS @ UCA 47

How do we fix it?

• Due the symmetry, we may flip the left half to the right

• How do we do it?

7/10/2012

2012 CS4HS @ UCA 48

Try to fix it with reflection

7/10/2012

2012 CS4HS @ UCA 49

Mirroring a picture -vertically

7/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 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)

7/10/2012

2012 CS4HS @ UCA 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)

7/10/2012

2012 CS4HS @ UCA 53

Implementationdef 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?

7/10/2012

2012 CS4HS @ UCA 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)

7/10/2012

2012 CS4HS @ UCA 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

7/10/2012

2012 CS4HS @ UCA 56

A sample main functiondef 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.

7/10/2012

2012 CS4HS @ UCA 57

Outline of tasks

0. JES Introduction: 1. Load and investigate details of an image 2. Alternate pixel RGB values 3. Solve image puzzles4. Embed information in a picture5. Fix the Temple of Hephaestus virtually6. Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 58

Landing Dr. Mark Guzdial on the moon

7/10/2012

From Mark Guzdial’s book: Introduction to computing and programming in Python

2012 CS4HS @ UCA 59

Application specification

• Provided two pictures: background and foreground

• Replace the background through background subtraction

7/10/2012

2012 CS4HS @ UCA 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.

7/10/2012

2012 CS4HS @ UCA 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).

7/10/2012

2012 CS4HS @ UCA 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.

7/10/2012

2012 CS4HS @ UCA 63

Implementationdef 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

7/10/2012

2012 CS4HS @ UCA 64

Land Mark on the moon

7/10/2012

2012 CS4HS @ UCA 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) .

7/10/2012

2012 CS4HS @ UCA 667/10/2012