01-introtomediacomp1 barb ericson georgia institute of technology feb 2010 introduction to computer...

01-IntroToMediaComp 1

Barb Ericson

Georgia Institute of Technology

Feb 2010

Introduction to Computer Science and Media Computation


Learning Goals

• What is Computer Science?• What is Media Computation?• What are the standard math operators?• What is a variable?• What is a function?• How do digital pictures work?• How can you manipulate a digital picture?• What is an array?• What is a loop?


What is Computer Science?

• The study of process, how to specify what is to be done and define the stuff being processed.

• You can say that is the study of computational recipes– Ones that can be executed on a computer– A recipe that runs on a computer is called a program


What do Computer Scientists do?

• Study if there are better ways to write recipes– Algorithms – textual description of how to solve a problem

• Study how to structure the data in the recipes– Data structures and databases

• Determine if there are recipes that can't be written? That make machines intelligent?– Theory, artificial intelligence

• Study how to make computers easier for people to use– Human-computer interface

• Study how computers communicate with each other– Networking

• Study how to create 3D models and simulations– Graphics and scientific computing


Multimedia CS1 in Python

• Focus: Learning programming and CS concepts within the context of media manipulation and creation

–Converting images to grayscale and negatives, splicing and reversing sounds, writing programs to generate HTML, creating movies out of Web-accessed content.

–Computing for communications, not calculation


We will program in JES• JES: Jython

Environment for Students

• A simple editor (for entering in our programs or recipes): the program area

• A command area for entering in commands for Python to execute.

Editor or Program Area

Command Area


Using JES – Try the following>>> print 3 + 5>>> print 23.2 / 3>>> print 1 / 3>>> print 1.0 / 3>>> print 10 % 3>>> print "Hello">>> print "Hello" + "Barb">>> print 10 > 3>>> print 3 > 10

Print will print out the result from the following expression

Type these expressions after the >>> in the command area


Did any answer surprise you?

• Integer division results in an integer answer– The values after the decimal point are thrown away– If you want a floating point result using a floating point

value in the expression (1.0 / 3)

• You can append strings one after the other, but this doesn't add any spaces>>> print "Hello" + "Barb"HelloBarb

• Python uses 0 for false and 1 for true>>> print 10 > 31>>> print 3 > 100


Command Area Editing

• Up/down arrows walk through command history

• You can edit the line at the bottom– and then hit Return/Enter– that makes that last line execute


Demonstrating JES for files and sounds

>>> print pickAFile()c:/ip-book/mediasources/barbara.jpg>>> print makePicture(pickAFile())Picture, filename c:/ip-bookmediasources/barbara.jpg height 294 width 222>>> show(makePicture(pickAFile()))None>>> print pickAFile()C:/ip-book/mediasources/hello.wav>>> print makeSound(pickAFile())Sound of length 54757>>> print play(makeSound(pickAFile()))None


Naming parts – declaring variables

• You can name the result from a function call– And then use the name as input to other functions

myFile = pickAFile() # name the picked file

myPict = makePicture(myFile) # name the picture

show(myPict)

• The value associated with that name is used= doesn't mean equals here but assign the value for

myFile to the result of pickAFile()

Try the following in JES

>>> x = 3

>>> y = 2

>>> z = x * y

>>> print z

>>> message = "Bye"

>>> print message

>>> message = "Go away"

>>> print message


Variables

• Can hold values like integers (3) and strings of characters "Bye"

• Can be printed– The value in them is printed

• Can be used in calculations (like x * y)– The values in them is used

• Can be changed to new values– The values in them can vary



Quick Calculation

• What if an item is 30% off and you also have a coupon for an additional 20% off the sale price?– If the original cost was $45.00, how much is the price

after the 30% and then how much do you pay with the additional 20% off?

– Use the python command area to figure it out• Name the result of each calculation

Making our own functions• To make a function, use

the command def• Then, the name of the

function, and the names of the input values between parentheses (“(input1)”)

• End the line with a colon (“:”)

• The body of the recipe is indented (Hint: Use three spaces)– That’s called a block



Making functions the easy way

• Get something working by typing commands in the command area (bottom half of JES)

• Enter the def command in the editing window (top part of JES)

• Copy-paste the tested commands up into the recipe


A recipe for showing a picture

def pickAndShow(): myFile = pickAFile() myPict = makePicture(myFile) show(myPict)

Note: myFile and myPict, inside pickAndShow(), are completely different from the same names in the command area. We say that they are in a different scope.

Type this in the program area (editor)


Blocking is indicated for you in JES

• Statements that are indented the same, are in the same block.

• Statements in the same block as the cursor are enclosed in a blue box.

• Type pickAndShow in the command area and all statements in the block will be executed.

Image Processing

• Goals:– Give you the basic understanding of image

processing, including psychophysics of sight,– Identify some interesting examples to use



Light perception

• We perceive light with color sensors that peak around 425 nm (blue), 550 nm (green), and 560 nm (red).

• Our brain figures out which color is which by figuring out how much of each kind of sensor is responding

• One implication: We perceive two kinds of “orange” — one that’s spectral and one that’s red+yellow (hits our color sensors just right)

• Dogs and other simpler animals have only two kinds of sensors

– They do see color. Just less color.


Luminance vs. Color

• We perceive borders of things, motion, depth via luminance– Luminance is not the

amount of light, but our perception of the amount of light.

– We see blue as “darker” than red, even if same amount of light.

– Contrast also plays a role

Luminance is actually color blind. Completely different part of the brain.


Digitizing pictures

• We digitize pictures into lots of little dots• Enough dots and it looks like a continuous

whole to our eye– Our eye has limited resolution– Our background/depth acuity is particularly low

• Each picture element is referred to as a pixel• Pixels are picture elements

– Each pixel object knows its color– It also knows where it is in its picture


Exploring Pictures

>>> file = pickAFile()

>>> beachPict = makePicture(file)

>>> explore(beachPict)

• Zoom in to see individual pixels

• Move the cursor to see the x and y values

• Look at the red, green, and blue values


Encoding color

• Each pixel encodes color at that position in the picture

• Lots of encodings for color– Printers use CMYK: Cyan, Magenta, Yellow, and

blacK.– Others use HSB for Hue, Saturation, and Brightness

(also called HSV for Hue, Saturation, and Value.

• We’ll use the most common for computers– RGB: Red, Green, Blue


Encoding Color: RGB• In RGB, each color has

three component colors:– Amount of redness– Amount of greenness– Amount of blueness

• Each does appear as a separate dot on most devices, but our eye blends them.

• In most computer-based models of RGB, a single byte (8 bits) is used for each– So a complete RGB color is

24 bits, 8 bits of each


Making Colors with Light

• Type>>> myColor = pickAColor()

• Try to create – White– Black– Yellow– Red– Brown– Purple


Basic Picture Functions

• makePicture(filename) creates and returns a picture object, from the JPEG file at the filenamepict = makePicture("c:/ip-book/mediasources/barbara.jpg")

• show(picture) displays a picture in a windowshow(pict)

• explore(picture) makes a copy of the picture and shows it in the explorer windowexplore(pict)

• We’ll learn functions for manipulating pictures like getColor, setColor, and repaint


Making our own functions• To make a function, use the

command def• Then, the name of the

function, and the names of the input values between parentheses (“(input1)”)

• End the line with a colon (“:”)• The body of the recipe is

indented (Hint: Use two spaces)

• Your function does NOT exist for JES until you load it


Saving Functions in Files

• Click on File and then Save Program– Name it with some file name and .py at end

• You can define more than one function in a file– Maybe call these pictureFunctions.py

• You can later open these files up– And use the Load Program button to load all functions

in the file

• You can build a library of python functions for working with pictures, sounds, movies, etc


Modifying a Pixel Color

• You can get the amount of red, green, or blue– redValue = getRed(pict)– greenValue = getGreen(pict)– blueValue = getBlue(pict)

• You can change the amount of red, green, or blue– setRed(pict,value)– setGreen(pict,value)– setBlue(pictValue)


Modifying Colors

• You can also get the color from a pixel– myColor = getColor(pixel)

• You can create a new color by giving values for red, green, and blue from 0 to 255– newColor = makeColor(255,0,0)

• You can set a color using – setColor(pixel,newColor)


How to change lots of pixels?

• If we want to change all the pixels in a picture how can we do that?– In a 640 x 480 picture that is 307,200 pixels

• Computers are very fast and can process billions of instructions per second– But we wouldn't want to name each pixel or modify

the color on each one by typing in the commands 307,200 times

• We can get an array of pixels to process– Using getPixels(picture) and loop through the pixels

one at a time


What is an array?

• Space in memory for many values of the same type– Numbers, pictures, pixels, etc

• You can refer to the elements of an array using an index– Starting with 0 for the first element– And length – 1 for the last element


Processing Pixels in an Array

>>> file="C:/ip -book/mediasources/barbara.jpg">>> pict=makePicture(file)>>> show(pict)>>> pixels = getPixels(pict)>>> setRed(pixels [0], getRed(pixels [0]) * 0.5)>>> setRed(pixels [1], getRed(pixels [1]) * 0.5)>>> setRed(pixels [2], getRed(pixels [2]) * 0.5)>>> setRed(pixels [3], getRed(pixels [3]) * 0.5)>>> setRed(pixels [4], getRed(pixels [4]) * 0.5)>>> setRed(pixels [5], getRed(pixels [5]) * 0.5)>>> repaint(pict)


Our first picture recipe def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

Used like this:>>> file="c:/ip-book/mediasources/katie.jpg">>> picture=makePicture(file)>>> explore(picture)>>> decreaseRed(picture)>>> explore(picture)

It’s not iteration—it’s a set operation

• Research in the 1970’s found that people are better at set operations than iteration.– For all records, get the last name, and if it starts with

“G” then… => HARD!– For all records where the last name starts with “G”…

=> Reasonable!

• Because the Python for loop is a forEach, we can start out with treating it as a set operation:– “For all pixels in the picture…”



How do you make an omelet?

• Something to do with eggs…

• What do you do with each of the eggs?

• And then what do you do?

All useful recipes involve repetition- Take four eggs and crack them….- Beat the eggs until…

We need these repetition (“iteration”) constructs in computer algorithms too

- Today we will introduce one of them


Decreasing the red in a picture

• Recipe: To decrease the red• Ingredients: One picture, name it picture and pass it to

the function where we will call it pict• Step 1: Get all the pixels of pict. For each pixel p in the

set of pixels…• Step 2: Get the value of the red of pixel p, and set it to

50% of its original value


More about for each loops

• for is the name of the command• An index variable is used to hold each of the different

values of a sequence• The word in• A function that generates a sequence

– The index variable will be the name for one value in the sequence, each time through the loop

• A colon (“:”)• And a block (the indented lines of code)

def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)


What happens when a for loop is executed

• The index variable is set to an item in the sequence

• The block is executed– The variable is usually used inside the block

• Then execution loops to the for statement, where the index variable gets set to the next item in the sequence

• Repeat until every value in the sequence is used.


getPixels returns a sequence of pixels• Each pixel knows its

color and place in the original picture

• Change the pixel and you change the picture

• So the loops here assign the index variable p to each pixel in the picture picture, one at a time.



Do we need the variable value?

• No– We can calculate the original red amount right when we are

ready to change it.– It’s a matter of programming style. The meanings are the same.

def decreaseRed(pict): for p in getPixels(pict): setRed(p, getRed(p) * 0.5)



Let’s walk that through slowly…

Here we take a picture object in as a parameter to the function and call it pict

pict



Now, get the pixels

We get all the pixels from the picture, then make p be the name of each one one at a time

Pixel, color r=135 g=131b=105

Pixel, color r=133g=114 b=46


…

p

getPixels()

pict



Get the red value from pixel

We get the red value of pixel p and name it value

…

value = 135 pict




…

p

getPixels()



Now change the pixel

Set the red value of pixel p to 0.5 (50%) of value

pict

Pixel, color r=67 g=131 b=105

…

p

value = 135

getPixels()Pixel, color r=133g=114 b=46




Then move on to the next pixel

Move on to the next pixel and name it p

pict

…

p

value = 135

getPixels()Pixel, color r=67 g=131 b=105





Get its red value

p

Set value to the red value at the new p, then change the red at that new pixel.

p

pict

…

p

value = 133






And change this red value

Change the red value at pixel p to 50% of value

pp

pict

…

p

value = 133






And eventually, we do all pixels

• We go from this… to this!


Tracing/Stepping/Walking• What we just did is called “stepping” or “walking

through” the program– You consider each step of the program, in the order

that the computer would execute it– You consider what exactly would happen– You write down what values each variable (name) has

at each point.

• It’s one of the most important debugging skills you can have.– And everyone has to do a lot of debugging, especially

at first.


Challenges

• Create an increaseRed function– Copy the decreaseRed function and rename it– Modify it to change the red value to 2 * the original

red value

• Create a clearBlue function– Copy the decreaseRed function and rename it– Modify it to change the blue value to 0


Can we modify more than one value?

• How do we turn this beach scene into a sunset?

• What happens at sunset?– At first, we tried

increasing the red, but that didn't work very well

– New Theory: As the sun sets, less blue and green is visible, which makes things look more red.


A Sunset-generation Function

def makeSunset(picture): for p in getPixels(picture): value = getBlue(p) setBlue(p, value * 0.7) value = getGreen(p) setGreen(p, value * 0.7)


Creating a negative

• Let’s think it through– R, G, B go from 0 to 255– Let’s say Red is 10. That’s very light red.

• What’s the opposite? LOTS of Red!

– The negative of that would be 245: 255-10

• 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) setColor(px, negColor)


Original, negative, double negative

(This gives us a quick way to test our function:Call it twice and see if the result is equivalentto the original)

We call this a lossless transformation.


Converting to grayscale

• We know that if red=green=blue, we get gray– 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 many ways, but one way that works reasonably well is dirt simple—simply take the average:


Converting to grayscale

def grayscale(picture): for p in getPixels(picture): sum = getRed(p) + getGreen(p) + getBlue(p) intensity = sum / 3 setColor(p, makeColor(intensity, intensity, intensity))

Does this makesense?


Why can’t we get back again?

• Converting to grayscale is different from computing a negative.– A negative transformation retains information.

• With grayscale, we’ve lost information– We no longer know what the ratios are

between the reds, the greens, and the blues– We no longer know any particular value.

Media compressions are one kind of transformation. Some are lossless (like negative);Others are lossy (like grayscale)


But that’s not really the best grayscale

• In reality, we don’t perceive red, green, and blue as equal in their amount of luminance: How bright (or non-bright) something is.– We tend to see blue as “darker” and red as

“brighter”– Even if, physically, the same amount of light is

coming off of each

• Photoshop’s grayscale is very nice: Very similar to the way that our eye sees it– B&W TV’s are also pretty good


Building a better grayscale

• We’ll weigh red, green, and blue based on how light we perceive them to be, based on laboratory experiments.

def grayscaleNew(picture): for px in getPixels(picture): newRed = getRed(px) * 0.299 newGreen = getGreen(px) * 0.587 newBlue = getBlue(px) * 0.114 luminance = newRed + newGreen + newBlue setColor(px, makeColor(luminance, luminance, luminance))


Summary• Name = expression creates a name (a variable)

that has a value• You can create your own functions in python• You can execute functions using the function

name and passing in any required values– decreaseRed(picture)

• You can modify pictures by modifying the pixels red, green, and blue values

• You can use an array to hold many values of the same type

• You can loop through all the values in an array using a for variable in array:

• Blocks in Python are shown by indention

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