1. CS 177 Week 3 Recitation Slides
Pictures, Pixels, Colors
and
for Loop
Look at 1-fridayMix slides

2. Announcements

3. ANY QUESTIONS?

4. 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. 4

5. 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 5

6. We already saw how to display a picture in JES
myFile = pickAFile()
myPicture = makePicture(myFile)
show(myPicture)

7. Now, let’s 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

8. 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.

9. 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

10. 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
We’re representing color in 24 (3 * 8) bits.

11. 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
Picture is a Python (JES) object
Picture is a matrix of 294 rows x 222 colums
Each element of a matrix contains a pxel

12. Exploring the pixels of a picture..
You can use the picture tool of JES to see the color of a picture pixel….

13. 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)
How to do it
STEP 1: write your function in the JES edit area 13 13

14. 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:
setRed, setGreen, and setBlue
Step 2: save your function in a file

15. 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
Step 2: save your function in a file 15 15

16. Examples: Manipulating Colors
Let’s see some examples… Demo
You must give a name to your file

17. 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.
Step 3: load your function in main memory so as JES can execute it

18. Example of a for Loop def decreaseRed(picture):
for p in getPixels(picture):
value=getRed(p)
setRed(p,value*0.5)
Let’s see an example… Demo

19. 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)

20. 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.

21. 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

22. 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

23. Final QUESTIONS???