177 CS Week 3 Recitation Slides
Pictures, Pixels, Colors
and
for Loop
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Announcements
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ANY QUESTIONS?
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Reminder: Why digitize media?
Real media is analogue (continuous).
To digitize it, we break it into parts and encode them into
numbers.
By encoding them, we can more easily manipulate them,
store them, transmit them without error, etc.
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Digitization of 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
Each little dot (picture element) is referred to as a pixel
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We already saw how to display a
picture in JES
myFile = pickAFile()
myPicture = makePicture(myFile)
show(myPicture)
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Now, lets see how a picture is
represented in JES
A Picture is a matrix of pixels
Row: width
Column: height
Pixels are picture elements
Each pixel object knows its color
It also knows where it is in its picture
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Encoding Colors with RGB Model
Each pixel encodes the color at that position
in the picture
How we represent a color?
We
use RGB model to encode colors in computers.
RGB: Red, Green, Blue.
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RGB Color Model
Each component color (red, green, and blue) is encoded
as a single byte
Colors go from (0,0,0) black to (255,255,255) white
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Encoding RGB: Bits and Patterns
General rule: In n bits, we can have 2n patterns
In
8 bits, we can have 28 patterns, or 256
If we make the first pattern 0, then the highest value
we can represent is 28-1, or 255
Were representing color in 24 (3 * 8) bits.
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Reminder: how to show pictures
>>> file=pickAFile()
>>> print file
/Users/guzdial/mediasources/barbara.jpg
>>> picture=makePicture(file)
>>> show(picture)
>>> print picture
Picture, filename /Users/guzdial/mediasources/barbara.jpg
height 294 width 222
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Exploring the pixels of a picture..
You can use the picture tool of JES to see the
color of a picture pixel.
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Manipulating Pixels
or you can use a Python command to:
get a single pixel at given x,y position
getPixel(picture,x,y)
get all the pixels into an array
getPixels(picture)
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What can we do with a pixel?
Once you got a pixel, you can get its color
components:
getRed, getGreen, and getBlue
Input:
a pixel
Output: a value between 0 and 255
Or you can change one of its color
components:
setGreen, setRed, and setBlue
Input:
a pixel
Output: a value between 0 and 255
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What can we do with a color?
getColor:
Input: a pixel
Output: a Color object with the color at that pixel
setColor:
Input: a pixel and a Color
Output: a pixel with the color set
makeColor:
Input: red, green, and blue values (in that order) between 0 and 255
Output: a Color object
pickAColor lets you use a color chooser and returns the
chosen color
We also have functions that can makeLighter and
makeDarker an input color
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Examples: Manipulating Colors
Lets see some examples Demo
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Change Pixels Directly
>>> setColor(getPixel(pict,10,100),yellow)
>>> setColor(getPixel(pict,11,100),yellow)
>>> setColor(getPixel(pict,12,100),yellow)
>>> setColor(getPixel(pict,13,100),yellow)
>>>
What if we want to change millions of pixels?
Better way? Use a loop.
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Example of a for Loop
def decreaseRed(picture):
for p in getPixels(picture):
value=getRed(p)
setRed(p,value*0.5)
Lets see an example Demo
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Rules of for Loops
Command name: for
An index variable p is used to hold each of the different
values of a sequence
The word in
A Python 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)
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How a for Loop Works
The index variable is set to an item in the sequence
The block is executed
The variable is often 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 was used.
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Using the loop in our Egg
Omelette recipe
One Egg Omelette recipe:
Put oil in pan
Pre-heat pan
Crack an egg into bowl
Add Salt
Wisk the contents of bowl
Pour contents of bowl into pan
Wait for 3 minutes
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Using the loop in our Egg
Omelette recipe
Four Egg Omelettes recipe:
Put oil in pan
Pre-heat pan
Crack an egg into bowl
Add Salt
Wisk the contents of bowl
Pour contents of bowl into pan
Wait for 3 minutes
Repeat the above procedure again for the second egg omelette
Repeat the above procedure again for the third egg omelette
Repeat the above procedure again for the fourth egg omelette
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Final QUESTIONS???
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