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CS 101: Introduction to Computing

CS 101: Introduction to Computing. Programming picture manipulations. Developed by Mark Guzdial, Georgia Institute of Technology, 2003–2004; modified by Robert H. Sloan, University of Illinois at Chicago, 2005, for educational use. Demonstrating: Manipulating Colors. >>> print color

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CS 101: Introduction to Computing

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  1. CS 101:Introduction to Computing Programming picture manipulations 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. Demonstrating: Manipulating Colors >>> print color color r=81 g=63 b=51 >>> print newcolor color r=255 g=51 b=51 >>> print distance(color,newcolor) 174.41330224498358 >>> print color color r=168 g=131 b=105 >>> print makeDarker(color) color r=117 g=91 b=73 >>> print color color r=117 g=91 b=73 >>> newcolor=pickAColor() >>> print newcolor color r=255 g=51 b=51 >>> print getRed(pixel) 168 >>> setRed(pixel,255) >>> print getRed(pixel) 255 >>> color=getColor(pixel) >>> print color color r=255 g=131 b=105 >>> setColor(pixel,color) >>> newColor=makeColor(0,100,0) >>> print newColor color r=0 g=100 b=0 >>> setColor(pixel,newColor) >>> print getColor(pixel) color r=0 g=100 b=0

  3. We can change pixels directly… >>> file="/Users/sloan/mediasources/MauryBaby.jpg" >>> pict=makePicture(file) >>> show(pict) >>> setColor(getPixel(pict,10,160), blue) >>> setColor(getPixel(pict,11,160), blue) >>> setColor(getPixel(pict,12,160), blue) >>> setColor(getPixel(pict,13,160), blue) >>> repaint(pict) But that’s really tedious… Manipulating pictures more cleverly is our next topic!

  4. How do you find out what RGB values you have? And where? • Use the MediaTools! (especially useful when testing and debugging…)

  5. Use a loop!Our first picture recipe def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Note: the name picture takes on the value of pic Used like this: >>> file="/Users/sloan/Mediasources/Tonks.jpg" >>> pic=makePicture(file) >>> show(pic) >>> decreaseRed(pic) >>> repaint(pic)

  6. How loops are written • foris the name of the command • An index variable is used to represent the different values in the loop • The word in • A function that generates a sequence • The index variable will be the name for each value in the sequence, each time through the loop • A colon (“:”) • And, another block that is indented • e.g. for x in range(1,100): print x

  7. What happens when a loop is executed • The index variable is set to the next (or first, at the beginning) item in the sequence • The block is executed • The variable is often used inside the block • Then execution returns to the for statement, where the index variable gets set to the next item in the sequence • Repeat until the sequence is exhausted. for x in range(1,100): print x • output: 1 2 3 ...

  8. getPixels returns a sequence of pixels • Each pixel knows its color and its location in the picture • Change the pixel, you change the picture • So the loop below assigns the index variable p to each pixel in the picturepicture, one at a time. def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)

  9. Do we need value? • Not really: Remember that we can swap names for data or functions that are equivalent. def decreaseRed(picture): for p in getPixels(picture): setRed(p, getRed(p) *0.5)

  10. Let’s walk through that slowly… def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Here we get a picture object in as input and give it the variable name picture picture

  11. Now, get the pixels def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) We get all the pixels from the picture, then make p be the name of each one one at a time picture getPixels() Pixel, color r=168 g=131 b=105 Pixel, color r=160 g=131 b=105 Pixel, color r=168 g=132b=106 … p

  12. Get the red value from pixel def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) We get the red value of pixel p and name it value picture Pixel, color r=168 g=131 b=105 Pixel, color r=160 g=131 b=105 Pixel, color r=168 g=132b=106 … value = 168 p

  13. Now change the pixel def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Set the red value of pixel p to 0.5 (50%) of value picture Pixel, color r=84 g=131 b=105 Pixel, color r=160 g=131 b=105 Pixel, color r=168 g=132b=106 … value = 168 p

  14. Then move on to the next pixel def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Move on to the next pixel and name it p picture Pixel, color r=84 g=131 b=105 Pixel, color r=160 g=131 b=105 Pixel, color r=168 g=132b=106 … value = 168 p

  15. Get its red value Get its red value def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Change value to the new red value at the new p Change value to the new red value at the new p picture picture Pixel, color r=84 g=131 b=105 Pixel, color r=84 g=131 b=105 Pixel, color r=160 g=131 b=105 Pixel, color r=168 g=131 b=105 Pixel, color r=168 g=132b=106 Pixel, color r=168 g=132b=106 … … value = 160 p p

  16. And change this red value def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Change the red value at pixel p to 50% of value picture Pixel, color r=84 g=131 b=105 Pixel, color r=80 g=131 b=105 Pixel, color r=168 g=132b=106 … value = 160 p

  17. And eventually, we do all of the pixels... • We go from this… to this!

  18. “Tracing/Stepping/Walking through” the program • What we just did is called “stepping” or “walking through” the program • You consider each step of the program, in the order that the computer would execute it • You consider what would specifically happen there • You write down what values each variable (name) has at each point. • It’s one of the most important debugging skills you can have. • And everyone has to do a lot of debugging, especially at first.

  19. Did that really work?How can we be sure? • Sure, the picture looks different, but did we actually decrease the amount of red? By as much as we thought? • Let’s check it!

  20. >>> file = pickAFile() >>> print file /Users/sloan/Mediasources/Tonks.jpg >>> pict = makePicture(file) >>> pixel = getPixel(pict,145,17) >>> print pixel Pixel, color=color r=236 g=98 b=147 >>> decreaseRed(pict) >>> print pixel Pixel, color=color r=236 g=98 b=147  Still the old pixel >>> newPixel = getPixel(pict, 145,17) >>> print newPixel Pixel, color=color r=118 g=131 b=105 >>> print 236 * 0.5 118.0

  21. Checking it in the MediaTools

  22. If you make something you like… • writePictureTo(picture, “filename”) • Writes the picture out as a JPEG • Be sure to end your filename as “.jpg”! • e.g. myPic.jpg • If you don’t specify a full path,will be saved in the same directory as JES.

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