CS 101: Introduction to Computing

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

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?

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?

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 276 28 to 278 28 Copying color from 276 29 to 278 29 Copying color from 276 30 to 278 30

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 277-14=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