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Conditionals in Pictures

Conditionals in Pictures. Barb Ericson Georgia Institute of Technology Oct 2010. Learning Goals. What are Boolean expressions? How does Python represent true and false? How to conditionally execute code? How to create a conditional with two options?

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Conditionals in Pictures

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  1. Conditionals in Pictures Barb Ericson Georgia Institute of Technology Oct 2010 03-ConditionalsInPictures

  2. Learning Goals • What are Boolean expressions? • How does Python represent true and false? • How to conditionally execute code? • How to create a conditional with two options? • How to create a conditional with more than two options? • How to create complex Boolean expressions joined with 'and' and 'or'? 03-ConditionalsInPictures

  3. Boolean Expressions • Try the following in JES: >>> 3 < 5 >>> 3 == 3 >>> 3 > 5 >>> 6 > 2 >>> 3 >= 2 >>> 2 <= 3 >>> 3 >= 3 What value is used for true and what value for false? 03-ConditionalsInPictures

  4. Expressions Can test equality with == = means "make them equal" == "are they equal?" Can also test <, >, >=, <=, <> (not equals) In general, 0 is false, 1 is true So you can have a function return a “true” or “false” value. 03-ConditionalsInPictures

  5. 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 the explorer explore(pict) 03-ConditionalsInPictures

  6. How an if (conditional) works if is the command name Next comes an expression: Some kind of true or false comparison Then a colon Then the body of the if which is the things that will happen if the expression is true if distance(color, brown) < 50.0: redness=getRed(px)*1.5 blueness=getBlue(px) greenness=getGreen(px) 03-ConditionalsInPictures

  7. 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 03-ConditionalsInPictures

  8. “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 03-ConditionalsInPictures

  9. Challenge • Try to change one color in a range to another color • Like change the hair in the picture of a person • Or the clothes 03-ConditionalsInPictures

  10. Generating sepia-toned prints • Pictures that are sepia-toned have a yellowish tint to them that we associate with older photographs. • It’s not just a matter of increasing the amount of yellow in the picture, because it’s not a one-to-one correspondence. • Instead, colors in different ranges get converted to other colors. • We can create such convertions using if 03-ConditionalsInPictures

  11. Example of sepia-toned prints 03-ConditionalsInPictures

  12. Here’s how we do it def sepiaTint(picture): #Convert image to grayscale grayscale(picture) #loop through picture to tint pixels for p ingetPixels(picture): red = getRed(p) blue = getBlue(p) #tint shadows if (red < 63): red = red*1.1 blue = blue*0.9 #tint midtones if (red > 62 and red < 192): red = red*1.15 blue = blue*0.85 #tint highlights if (red > 191): red = red*1.08 if (red > 255): red = 255 blue = blue*0.93 #set the new color values setBlue(p, blue) setRed(p, red) Bug alert! Make sure you indent the right amount 03-ConditionalsInPictures

  13. Chroma Key – Blue Screen • For TV and movie special effects they use a blue or green screen • Here just a blue sheet was used • Professionally you need an evenly lit, bright, pure blue background • With nothing blue in the scene 03-ConditionalsInPictures

  14. Chromakey Function def chromakey(source ,bg): # source should have something in front of blue # bg is the new background for x in range(0, getWidth(source )): for y in range(0, getHeight(source )): p = getPixel(source ,x,y) # if red + green < blue if (getRed(p) + getGreen(p) < getBlue(p)): setColor(p,getColor(getPixel(bg ,x,y))) return source 03-ConditionalsInPictures

  15. Testing chromakey markP = makePicture(getMediaPath("blue-mark.jpg")) newBack = makePicture(getMediaPath("moon-surface.jpg")) chromakey(markP,newBack) explore(markP) 03-ConditionalsInPictures

  16. Chromakey with Distance to a Color def chromakeyGreen(source ,bg, backColor, limit): # source should have something in front of green # bg is the new background for x in range(0, getWidth(source )): for y in range(0, getHeight(source )): p = getPixel(source ,x,y) # if distance to the color is less than limit if (p.colorDistance(backColor) < limit) setColor(p,getColor(getPixel(bg ,x,y))) return source 03-ConditionalsInPictures

  17. How many when there is an “And”? • I want you to get soup, milk, bread, and yogurt at the store. • How many items will you come home with? • I want you to clean your room and mop the floor in the kitchen and wash the dishes. • How many tasks do you need to do? • I want a scoop of chocolate scoop and a scoop of vanilla. • How many scoops of ice cream is this? 03-ConditionalsInPictures

  18. How many when there is an “Or” • You need to help clean the house • You can clean the bathroom or the kitchen or the living room • How many jobs do you have to do? • You want to get an ice cream • The flavors you can pick from are chocolate, vanilla, strawberry, or orange sherbet • How many flavors do you need to pick for a single scoop? 03-ConditionalsInPictures

  19. Truth Table 03-ConditionalsInPictures

  20. Conditional Exercise • When are the following true? When are they false? • You can go out if your room is clean and you did your homework • You can go out if your room is clean or you did your homework • You can go out if either your room is clean or you did your homework but not if both of these is true 03-ConditionalsInPictures

  21. Challenge • Modify the general copy function to check that the targetX and targetY are within the width and height of the target picture before trying to copy the color from the source pixel to the target pixel. 03-ConditionalsInPictures

  22. Summary • You can execute code when some condition is true • Using an if • Or if and else • You can have as many possibilities as you want • Add additional if's like on sepia tint • You can combine Boolean expressions with 'and' and 'or' 03-ConditionalsInPictures

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