1. CS 101: Introduction to Computing
More referencing pixels directly by index number: Red eye and mirroring
Developed by Mark Guzdial, Georgia Institute of Technology, 2003–2004; modified by Robert H. Sloan, University of Illinois at Chicago, 2005, for educational use.

2. Removing “Red Eye”
When the flash of the camera catches the eye just right (especially with light colored eyes), we get bounce back from the back of the retina.
This results in “red eye”
We can replace the “red” with a color of our choosing.
First, we figure out where the eyes are (x,y) using MediaTools

3. Removing Red Eye
def removeRedEye(pic,startX,startY,endX,endY,replacementcolor):
red = makeColor(255,0,0)
for x in range(startX,endX):
for y in range(startY,endY):
currentPixel = getPixel(pic,x,y)
if (distance(red,getColor(currentPixel)) < 165):
setColor(currentPixel,replacementcolor)
Why use a range? Because we don’t want to replace her red dress!
What we’re doing here:
Within the rectangle of pixels (startX,startY) to (endX, endY)
Find pixels close to red, then replace them with a new color

4. “Fixing” it: Changing red to black
removeRedEye(jenny, 109, 91, 202, 107, makeColor(0,0,0))
Jenny’s eyes are actually not black—could fix that
Eye are also not mono-color
A better function would handle gradations of red and replace with gradations of the right eye color
Now ask for a student who knows their picture number and that they’ve got red eye. Pict 19 Spring 2005 will do.

5. If you know where the pixels are: Mirroring
Imagine a mirror horizontally across the picture, or vertically
What would we see?
How do generate that digitally?
We simply copy the colors of pixels from one place to another

6. Mirroring a picture
Slicing a picture down the middle and sticking a mirror on the slice
Do it by using a loop to measure a difference
The index variable is actually measuring distance from the mirrorpoint
Then reference to either side of the mirror point using the difference

7. Program for mirroring
converts a real number to an integer. e.g. if width=5, then 5/2 = 2.5 and int(5/2) = 2
def mirrorVertical(source):
mirrorpoint = int(getWidth(source)/2)
for y in range(1,getHeight(source)):
for xOffset in range(1,mirrorpoint):
pright = getPixel(source, xOffset+mirrorpoint,y)
pleft = getPixel(source, mirrorpoint-xOffset,y)
c = getColor(pleft)
setColor(pright,c)

8. How does that work? Compute the half-way horizontal index
The y value travels the height of the picture
The xOffset value is an offset
It’s not actually an index
It’s the amount to add or subtract
We copy the color at mirrorpoint-offset to mirrorpoint+offset
def mirrorVertical(source):
mirrorpoint = int(getWidth(source)/2)
for y in range(1,getHeight(source)):
for xOffset in range(1,mirrorpoint):
pright = getPixel(source, xOffset+mirrorpoint,y)
pleft = getPixel(source, mirrorpoint-xOffset,y)
c = getColor(pleft)
setColor(pright,c)

9. Can we do it with a horizontal mirror?
def mirrorHorizontal(source):
mirrorpoint = int(getHeight(source)/2)
for yOffset in range(1,mirrorpoint):
for x in range(1,getWidth(source)):
pbottom = getPixel(source,x,yOffset+mirrorpoint)
ptop = getPixel(source,x,mirrorpoint-yOffset)
setColor(pbottom,getColor(ptop))

10. Of course!

11. What if we wanted to copy bottom to top?
Very simple: Swap the order of pixels in the bottom line
def mirrorHorizontal(source):
mirrorpoint = int(getHeight(source)/2)
for yOffset in range(1,mirrorpoint):
for x in range(1,getWidth(source)):
pbottom = getPixel(source,x,yOffset+mirrorpoint)
ptop = getPixel(source,x,mirrorpoint-yOffset)
setColor(ptop,getColor(pbottom))

12. Messing with Santa some more

13. Doing something useful with mirroring
Mirroring can be used to create interesting effects, but it can also be used to create realistic effects.
Consider this image that from a trip to Athens, Greece.
Can we “repair” the temple by mirroring the complete part onto the broken part?

14. Figuring out where to mirror
Use MediaTools to find the mirror point and the range that we want to copy

15. Writing functions for specific files…generally
The function to mirror the temple needs to work for one and only one file.
But we still don’t want to write out the whole path.
setMediaPath() allows us to pick a directory where our media will be stored.
getMediaPath(filename) will generate the entire path for us to the filename in the media directory
THIS ONLY WORKS WHEN WE’RE ACCESSING FILES IN THE MEDIA DIRECTORY AND WHERE WE HAVE SET THE PATH FIRST!

16. Program to mirror the temple
def mirrorTemple():
source = makePicture(getMediaPath("temple.jpg"))
mirrorpoint = 277
lengthToCopy = mirrorpoint - 14
for x in range(1,lengthToCopy):
for y in range(28,98):
p = getPixel(source,mirrorpoint-x,y)
p2 = getPixel(source,mirrorpoint+x,y)
setColor(p2,getColor(p))
show(source)
return source

17. Did it really work?
It clearly did the mirroring, but that doesn’t create a 100% realistic image.
Check out the shadows: Which direction is the sun coming from?
Consider other reasons why this worked as well as it did. Clouds in the sky?

18. Understanding the Temple Fix
What is the very first transfer of pixels from and to? Which (x,y) pixel from? Which (x,y) pixel to?
What is the second set of pixels?
How many pixels get copied?
Try to get people to make guesses here, first, before going on.

19. Adding print statements to see what’s happening
def mirrorTemple():
source = makePicture(getMediaPath("temple.jpg"))
mirrorpoint = 277
lengthToCopy = mirrorpoint - 14
for x in range(1,lengthToCopy):
for y in range(28,98):
print "Copying color from",mirrorpoint-x,y
print "to",mirrorpoint+x,y
p = getPixel(source,mirrorpoint-x,y)
p2 = getPixel(source,mirrorpoint+x,y)
setColor(p2,getColor(p))
show(source)
return source

20. First pixels are either side of the mirrorpoint, then moving down
>>> p2=mirrorTemple()
Copying color from to
Copying color from to
Copying color from to

21. Counting pixels def mirrorTemple():
source = makePicture(getMediaPath("temple.jpg"))
mirrorpoint = 277
lengthToCopy = mirrorpoint - 14
count = 0
for x in range(1,lengthToCopy):
for y in range(28,98):
p = getPixel(source,mirrorpoint-x,y)
p2 = getPixel(source,mirrorpoint+x,y)
setColor(p2,getColor(p))
count = count + 1
show(source)
print "We copied",count,"pixels"
return source

22. Counting pixels >>> p2=mirrorTemple() We copied 18340 pixels
Where did that come from?
How many rows? Y goes from 28 to 98
= 70 rows of pixels
How many columns? X goes from 1 to =263
= 262 columns of pixels
70 * 262 = 18340

23. Some Utility Functions and other random stuff about JES
If you know the name of the file, searching for it with pickAFile() feels tedious
You can set and get a media folder (path) for remembering a place where your media will be coming from (or going to)
setMediaPath() lets you pick a file in your media folder
getMediaPath(basefilename) lets you generate a complete filename out of only the last part
This is useful if all your images are in a single directory

24. Example >>> setMediaPath()
#And then I get a file picker to make the choice with
New media folder: /Users/sloan/Mediasources/
>>> getMediaPath("tonks.jpg")
'/Users/sloan/Mediasources/tonks.jpg'
>>> tonks = makePicture("tonks.jpg")
>>> print tonks
Picture, filename /Users/sloan/Mediasources/tonks.jpg height 296 width 224

25. “Driver” utility functions
def hw(picture):
for eggs in spam spam spam spam:
def driver():
file = pickAFile()
show(makePicture(file)) # Show the “before” picture
picture = makePicture(file)
hw(picture)
show(picture) # Show the “after” picture
writePictureTo(picture, r“myFunkyPicture.jpg”)
return(picture)
Sometimes you want to break up your code into multiple functions like this to make it easier to read and understand.

26. Comments – You should know what these are by now, but if not...
Python ignores from “#” through the rest of the line
If you start a line with “#”, the whole line is ignored
Why do we want lines to be ignored?
To be able to leave notes to ourselves or someone else about how the program works