Introduction to Media Computation
Barb Ericson
Georgia Institute of Technology
June 2005
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Learning Goals
• Understand at a conceptual level
– What is media computation?
– How do digital pictures work?
– How do digital sounds work?
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What is Media Computation?
• Processing
– picture elements
– sound fragments
– movie frames
– Text files and HTML pages
• The speed and storage capacity of
modern computers makes this possible
– Even for beginning students just learning to
program
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How Does Color Vision Work?
• Our eyes and brain work
together to make sense
of what we see
• The cones in our eyes
are what allow us to see
in color
• The rods allow us to see
black, white, and shades
of gray
• Our cones are sensitive
to red, green, and blue
light
– All other colors are
combinations of these
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Red, Green and Blue Light
• White light is a combination of red, green,
and blue
– Full intensity red, green, and blue combined
• Black is the absence of all light
– No red, green or blue light
• All other colors are combinations
– Of red, green, and blue
– Of different intensities
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Color Exercise
• Start DrJava
– In the interactions
pane type
– ColorChooser.pickACo
lor();
– Click on the RGB tab
and move the sliders
to change the intensity
of red, green, and blue
– Make white, black,
red, blue, green,
yellow, violet, and
orange
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How do Digital Cameras Work?
• There are red, green, and blue
filters that capture the amount of
each color at a position
– A part of a grid
• There are many positions
– picture element or pixel
– 640 x 480 is low resolution
– 1600 x 1200 is high resolution
• The more pixels the better the
picture
– Can enlarge it without it looking
grainy
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How do Computer Displays Work?
• A display has pixels
(picture elements)
• Each pixel has a red,
green, and blue
component
• Combinations of red,
green, and blue give
the resulting color
– Black is 0 red, 0 green
and 0 blue
– White is 255 red, 255
green, 255 blue
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Pictures are made up of Pixels
• Digital cameras
record light at pixels
• Monitors display
pictures using pixels
• Our limited vision
acuity helps us to see
the discrete pixels as
a smooth picture
– If we blow up the
picture we can see the
pixels
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Digital Pictures
• Capture the intensity of the red,
green, and blue colors at each
pixel
• Stored as a bunch of numbers
– 8 bits for red, 8 bits for green, 8
bits for blue
– Need nearly 1 million bytes to
store a 640 x 480 picture
– Need 3 million bytes to store an
image from a 1 megapixel (million
pixel) camera
• Displayed as red, green, and blue
colors on the computer display
– Lots of them close together
– Our brain sees a smooth color
image
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Picking a File Name
• Use the class method pickAFile() on our
FileChooser class to pick a file
> System.out.println(FileChooser.pickAFile())
– Look for a file that holds a digital picture
• .jpg or .gif
– Look for a file that holds a digital sound
• .wav or .aif or .aiff or .au
– The result might look like
• C:\intro-prog-java\mediasources\cat.jpg
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Parts of a File Name
• Path separators ‘\’
• Path – where on the hard disk the file is
– C:\intro-prog-java\mediasources\
• Base file name
– cat.jpg
• File extension
– .jpg
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Creating Objects
• Remember that a
class is a factory that
creates objects of that
class
• We ask a class to
create an object by
using the keyword:
new ClassName
• We also ask the class
to initialize the object
– And pass data to help
initialize it
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Creating a Picture Object
• A picture needs data to use to create the
picture
– One way to create it is to give it the full path
name of the file to read the data from
– We know how to get the full path name
• Using FileChooser.pickAFile()
> System.out.println(new Picture(FileChooser.pickAFile()))
Picture, filename C:\intro-progjava\mediasources\partFlagSmall.jpg height 217 width 139
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Where is the Picture?
• When you printed the result of
– new Picture(FileChooser.pickAFile())
• You got the file name, width, and height
– So it looks like a picture object was created
• But, where is the picture?
– To get it to display send the picture object the
message “show()”
– new Picture(FileChooser.pickAFile()).show()
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How did that work?
• The expression inside the parentheses is
evaluated first
– FileChooser.pickAFile()
– This returns the full path name of a file as a String object
• “C:\intro-prog-java\mediasources\cat.jpg”
• Next the Picture class was asked to create and
initialize a new object
– using the full path name of the file as the place to read the
picture data from
• new Picture(“C:\intro-prog-java\mediasources\cat.jpg”)
• Then we asked the new picture object to show
itself
– pictureObject.show()
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Naming the Results
• Another way to do the same thing is to
name each of the results
– Gives us a way to refer to the result again
– Makes it easier to understand
• In Java we must give a type when we
create a new name
– This is called declaring a variable
– Type name; or
– Type name = something;
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Assignment Exercise
• Try some integer calculations using the
assignment operator (‘=‘)
int x = 3;
int y =4;
int z;
z = x + y; // assign z to be x + y
System.out.println(z);
System.out.println(x);
System.out.println(y);
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Working with Variables
• You must declare a
variable before you
can use it
• Declare a variable
once and only once
– You will get an error if
you declare it more
than once
• You can reuse
variables
– That you have
declared
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Variables are Temporary
• Hold values until
– Reassigned
int x = 2;
x = 3;
– DrJava is exited
– The interactions pane is reset
• You can force this by clicking the “Reset” button
• Or by right click and choose “Reset Interactions”
• Happens automatically when you compile
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Allowed Types in Java
• The type for a variable declaration can be
– A primitive type: int, double, char, or boolean
– The name of a Class
• in the Java language
– String, JFrame, BufferedImage, etc
• or the name of a class created by you or someone
else
– Picture, Sound, FileChooser
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Object and Primitive Variables
• Primitive variables cause
space allocation for their
size
int a = 3
a
3
– The contents of the space
is set to the variable’s
value
• Object type variables
cause space allocation
for an address
– The contents of the space
is the address of the
referred to object
– Or null if it isn’t referring to
an object yet.
String str =“Hi”;
str
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reference
“Hi”
Memory Exercise
• Draw the memory used for the following:
int x = 2;
int y = 7;
int z = x + y;
• Draw the memory used for the following:
String fullName;
String firstName = “James”;
String lastName = “Clark”;
fullName = firstName + lastName;
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Naming each Piece
• First let’s pick the file name and save a
reference to the resulting String object
– String fileName = FileChooser.pickAFile();
• Next, let’s create the Picture object and
save a reference to it.
– Picture pictureObj = new Picture(fileName);
• Now send the show() message to the
picture object
– pictureObj.show();
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Naming Each Part
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Show Picture Exercise
• Try both ways of creating a picture object
and showing it.
– new Picture(FileChooser.pickAFile()).show()
– And do each piece one step at a time naming
the result of each method
•
•
•
•
•
String fileName = FileChooser.pickAFile();
System.out.println(fileName);
Picture picture = new Picture(fileName);
System.out.println(picture);
picture.show();
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Substitution and Evaluation
• In programming you can
– Use a literal
– Use a variable
– Use an expression
– Use the result of a method invocation
• Values get substituted for variable names
when expressions are evaluated
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What is a Sound?
• Any object that produces
vibrations in matter will
make a sound
• The vibrations produce a
sound wave
• The pitch is based on the
frequency of the cycles in
the sound wave
• The loudness is based on
the amplitude (height) of
the sound wave
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How does Hearing Work?
• The outer ear “catches” sounds
• The eardrum vibrates
• The inner ear translates the vibrations to
nerve impulses for the brain to interpret
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How does Recorded Sound Work?
• Phonograph
recordings capture
sound continuously,
as an analog signal
• CDs and DVDs
sample sounds and
record numbers that
represent the sound
at the time of the
sample
– 44,100 samples per
second
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Why Digitize Sound?
• High fidelity
– Reproduced sound is very similar to the
original
• Perfect reproduction
– Sounds the same every time
• Easy to transmit
– Download as data
• Easier to manipulate on a computer
– Even though there are billions of bits
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Playing a Sound
• We can create a Sound object just as we
created a Picture object
– Get a file name
• String fileName = FileChooser.pickAFile();
– Create the sound object by asking the class to
create a new Sound object and initialize it by
reading data from the given file name
• Sound sound1 = new Sound(fileName);
– Play the Sound
• sound1.play();
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Play Sound Exercise
• Try creating a Sound
object and playing it
by
– Specifying it all at
once
– Specifying it in steps
• How would you play
the same sound
twice?
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Summary
• Computers can do math and make logical
comparisons
• Computers can execute billions of instructions
per second
• Computers keep getting faster, smaller, and
cheaper
• Pictures, sounds, text and movies can be
digitized
• Media computation can mean processing
millions to billions of bytes
• The speed of modern computers makes media
computation possible even for beginners
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