1. 01-IntroToMediaComp1 Barb Ericson Georgia Institute of Technology Oct 2010 Introduction to Computer Science and Media Computation

2. 01-IntroToMediaComp2 Learning Goals What is Computer Science? What is Media Computation? What is a function? How do digital pictures work? How can you manipulate a digital picture? What is an array? What is a loop?

3. 01-IntroToMediaComp3 What is Computer Science? The study of process, how to specify what is to be done and define the stuff being processed. You can say that is the study of computational recipes –Ones that can be executed on a computer –A recipe that runs on a computer is called a program

4. 01-IntroToMediaComp4 What do Computer Scientists do? Study if there are better ways to write recipes –Algorithms – textual description of how to solve a problem Study how to structure the data in the recipes –Data structures and databases Determine if there are recipes that can't be written? That make machines intelligent? –Theory, artificial intelligence Study how to make computers easier for people to use –Human-computer interface Study how computers communicate with each other –Networking Study how to create 3D models and simulations –Graphics and scientific computing

5. 01-IntroToMediaComp5 Multimedia CS1 in Python Focus: Learning programming and CS concepts within the context of media manipulation and creation – Converting images to grayscale and negatives, splicing and reversing sounds, writing programs to generate HTML, creating movies out of Web-accessed content. – Computing for communications, not calculation

6. 01-IntroToMediaComp6 We will program in JES JES: Jython Environment for Students A simple editor (for entering in our programs or recipes): the program area A command area for entering in commands for Python to execute. Editor or Program Area Command Area

7. 01-IntroToMediaComp7 Naming parts – declaring variables You can name the result from a function call –And then use the name as input to other functions myFile = pickAFile() # name the picked file myPict = makePicture(myFile) # name the picture show(myPict) The value associated with that name is used = doesn't mean equals here but assign the value for myFile to the result of pickAFile()

8. Making our own functions To make a function, use the command def Then, the name of the function, and the names of the input values between parentheses (“(input1)”) End the line with a colon (“:”) The body of the recipe is indented (Hint: Use three spaces) –That’s called a block 01-IntroToMediaComp8

9. 9 Making functions the easy way Get something working by typing commands in the command area (bottom half of JES) –Use the up-arrow to bring up a statement again in the command area Enter the def command in the editing window (top part of JES) Copy-paste the tested commands up into the recipe

10. 01-IntroToMediaComp10 A recipe for showing a picture def pickAndShow(): myFile = pickAFile() myPict = makePicture(myFile) show(myPict) Note: myFile and myPict, inside pickAndShow(), are completely different from the same names in the command area. We say that they are in a different scope. Type this in the program area (editor)

11. 01-IntroToMediaComp11 Blocking is indicated for you in JES Statements that are indented the same, are in the same block. Statements in the same block as the cursor are enclosed in a blue box. Type pickAndShow in the command area and all statements in the block will be executed.

12. Image Processing Goals: –Give you the basic understanding of image processing, including psychophysics of sight, –Identify some interesting examples to use 01-IntroToMediaComp12

13. 01-IntroToMediaComp13 Light perception We perceive light with color sensors that peak around 425 nm (blue), 550 nm (green), and 560 nm (red). Our brain figures out which color is which by figuring out how much of each kind of sensor is responding One implication: We perceive two kinds of “orange” — one that’s spectral and one that’s red+yellow (hits our color sensors just right) Dogs and other simpler animals have only two kinds of sensors –They do see color. Just less color.

14. 01-IntroToMediaComp14 Luminance vs. Color We perceive borders of things, motion, depth via luminance –Luminance is not the amount of light, but our perception of the amount of light. –We see blue as “darker” than red, even if same amount of light. –Contrast also plays a role Luminance is actually color blind. Completely different part of the brain.

15. 01-IntroToMediaComp15 Digitizing 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 –Our background/depth acuity is particularly low Each picture element is referred to as a pixel Pixels are picture elements –Each pixel object knows its color –It also knows where it is in its picture

16. 01-IntroToMediaComp16 Exploring Pictures >>> file = pickAFile() >>> beachPict = makePicture(file) >>> explore(beachPict) Zoom in to see individual pixels Move the cursor to see the x and y values Look at the red, green, and blue values

17. 01-IntroToMediaComp17 Encoding color Each pixel encodes color at that position in the picture Lots of encodings for color –Printers use CMYK: Cyan, Magenta, Yellow, and blacK. –Others use HSB for Hue, Saturation, and Brightness (also called HSV for Hue, Saturation, and Value. We’ll use the most common for computers –RGB: Red, Green, Blue

18. 01-IntroToMediaComp18 Encoding Color: RGB In RGB, each color has three component colors: –Amount of redness –Amount of greenness –Amount of blueness Each does appear as a separate dot on most devices, but our eye blends them. In most computer-based models of RGB, a single byte (8 bits) is used for each –So a complete RGB color is 24 bits, 8 bits of each

19. 01-IntroToMediaComp19 Making Colors with Light Type >>> myColor = pickAColor() Try to create –White –Black –Yellow –Red –Brown –Purple

20. 01-IntroToMediaComp20 Basic Picture Functions makePicture(filename) creates and returns a picture object, from the JPEG file at the filename pict = makePicture("c:/ip-book/mediasources/barbara.jpg") show(picture) displays a picture in a window show(pict) explore(picture) makes a copy of the picture and shows it in the explorer window explore(pict) We’ll learn functions for manipulating pictures like getColor, setColor, and repaint

21. 01-IntroToMediaComp21 Making our own functions To make a function, use the command def Then, the name of the function, and the names of the input values between parentheses (“(input1)”) End the line with a colon (“:”) The body of the recipe is indented (Hint: Use two spaces) Your function does NOT exist for JES until you load it

22. 01-IntroToMediaComp22 Saving Functions in Files Click on File and then Save Program –Name it with some file name and.py at end You can define more than one function in a file –Maybe call these pictureFunctions.py You can later open these files up –And use the Load Program button to load all functions in the file You can build a library of python functions for working with pictures, sounds, movies, etc

23. 01-IntroToMediaComp23 Modifying a Pixel Color You can get the amount of red, green, or blue –redValue = getRed(pict) –greenValue = getGreen(pict) –blueValue = getBlue(pict) You can change the amount of red, green, or blue –setRed(pict,value) –setGreen(pict,value) –setBlue(pictValue)

24. 01-IntroToMediaComp24 Modifying Colors You can also get the color from a pixel –myColor = getColor(pixel) You can create a new color by giving values for red, green, and blue from 0 to 255 –newColor = makeColor(255,0,0) You can set a color using –setColor(pixel,newColor)

25. 01-IntroToMediaComp25 How to change lots of pixels? If we want to change all the pixels in a picture how can we do that? –In a 640 x 480 picture that is 307,200 pixels Computers are very fast and can process billions of instructions per second –But we wouldn't want to name each pixel or modify the color on each one by typing in the commands 307,200 times We can get an array of pixels to process –Using getPixels(picture) and loop through the pixels one at a time

26. 01-IntroToMediaComp26 What is an array? Space in memory for many values of the same type –Numbers, pictures, pixels, etc You can refer to the elements of an array using an index –Starting with 0 for the first element –And length – 1 for the last element You can also loop through all elements of an array using a forEach loop

27. 01-IntroToMediaComp27 Our first picture recipe def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5) Used like this: >>> file="c:/ip-book/mediasources/katie.jpg" >>> picture=makePicture(file) >>> explore(picture) >>> decreaseRed(picture) >>> explore(picture)

28. 01-IntroToMediaComp28 How do you make an omelet? Something to do with eggs… What do you do with each of the eggs? And then what do you do? All useful recipes involve repetition - Take four eggs and crack them…. - Beat the eggs until… We need these repetition (“iteration”) constructs in computer algorithms too - Today we will introduce one of them

29. 01-IntroToMediaComp29 Decreasing the red in a picture Recipe: To decrease the red Ingredients: One picture, name it picture and pass it to the function where we will call it pict Step 1: Get all the pixels of pict. For each pixel p in the set of pixels… Step 2: Get the value of the red of pixel p, and set it to 50% of its original value

30. 01-IntroToMediaComp30 More about for each loops for is the name of the command An index variable is used to hold each of the different values of a sequence The word in A 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) def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

31. 01-IntroToMediaComp31 What happens when a for loop is executed The index variable is set to an item in the sequence The block is executed –The variable is usually 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 is used.

32. 01-IntroToMediaComp32 getPixels returns a sequence of pixels Each pixel knows its color and place in the original picture Change the pixel and you change the picture So the loops here assign the index variable p to each pixel in the picture picture, one at a time. def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

33. 01-IntroToMediaComp33 Do we need the variable value? No –We can calculate the original red amount right when we are ready to change it. –It’s a matter of programming style. The meanings are the same. def decreaseRed(pict): for p in getPixels(pict): setRed(p, getRed(p) * 0.5) def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

34. 01-IntroToMediaComp34 Let’s walk that through slowly… Here we take a picture object in as a parameter to the function and call it pict pict def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

35. 01-IntroToMediaComp35 Now, get the pixels We get all the pixels from the picture, then make p be the name of each one one at a time Pixel, color r=135 g=131 b=105 Pixel, color r=133 g=114 b=46 Pixel, color r=134 g=114 b=45 … p getPixels() pict def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

36. 01-IntroToMediaComp36 Get the red value from pixel We get the red value of pixel p and name it value … value = 135 pict Pixel, color r=135 g=131 b=105 Pixel, color r=133 g=114 b=46 Pixel, color r=134 g=114 b=45 … p getPixels() def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

37. 01-IntroToMediaComp37 Now change the pixel Set the red value of pixel p to 0.5 (50%) of value pict Pixel, color r=67 g=131 b=105 … p value = 135 getPixels() Pixel, color r=133 g=114 b=46 Pixel, color r=134 g=114 b=45 def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

38. 01-IntroToMediaComp38 Then move on to the next pixel Move on to the next pixel and name it p pict … p value = 135 getPixels() Pixel, color r=67 g=131 b=105 Pixel, color r=133 g=114 b=46 Pixel, color r=134 g=114 b=45 def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

39. 01-IntroToMediaComp39 Get its red value p Set value to the red value at the new p, then change the red at that new pixel. p pict … p value = 133 getPixels() Pixel, color r=67 g=131 b=105 Pixel, color r=133 g=114 b=46 Pixel, color r=134 g=114 b=45 def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

40. 01-IntroToMediaComp40 And change this red value Change the red value at pixel p to 50% of value pp pict … p value = 133 getPixels() Pixel, color r=67 g=131 b=105 Pixel, color r=66 g=114 b=46 Pixel, color r=134 g=114 b=45 def decreaseRed(pict): for p in getPixels(pict): value=getRed(p) setRed(p,value*0.5)

41. 01-IntroToMediaComp41 And eventually, we do all pixels We go from this…to this!

42. 01-IntroToMediaComp42 Tracing/Stepping/Walking 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 exactly would happen –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.

43. 01-IntroToMediaComp43 Challenges Create an increaseRed function –Copy the decreaseRed function and rename it –Modify it to change the red value to 2 * the original red value Create a clearBlue function –Copy the decreaseRed function and rename it –Modify it to change the blue value to 0

44. 01-IntroToMediaComp44 Can we modify more than one value? How do we turn this beach scene into a sunset? What happens at sunset? –At first, we tried increasing the red, but that didn't work very well –New Theory: As the sun sets, less blue and green is visible, which makes things look more red.

45. 01-IntroToMediaComp45 A Sunset-generation Function def makeSunset(picture): for p in getPixels(picture): value = getBlue(p) setBlue(p, value * 0.7) value = getGreen(p) setGreen(p, value * 0.7)

46. 01-IntroToMediaComp46 Creating a negative Let’s think it through –R, G, B go from 0 to 255 –Let’s say Red is 10. That’s very light red. What’s the opposite? LOTS of Red! –The negative of that would be 245: 255-10 So, for each pixel, if we negate each color component in creating a new color, we negate the whole picture.

47. 01-IntroToMediaComp47 Creating a negative def negative(picture): for px in getPixels(picture): red = getRed(px) green = getGreen(px) blue = getBlue(px) negColor = makeColor( 255-red, 255-green, 255-blue) setColor(px, negColor)

48. 01-IntroToMediaComp48 Original, negative, double negative (This gives us a quick way to test our function: Call it twice and see if the result is equivalent to the original) We call this a lossless transformation.

49. 01-IntroToMediaComp49 Converting to grayscale We know that if red=green=blue, we get gray –But what value do we set all three to? What we need is a value representing the darkness of the color, the luminance There are many ways, but one way that works reasonably well is dirt simple—simply take the average:

50. 01-IntroToMediaComp50 Converting to grayscale def grayscale(picture): for p in getPixels(picture): sum = getRed(p) + getGreen(p) + getBlue(p) intensity = sum / 3 setColor(p, makeColor(intensity, intensity, intensity)) Does this make sense?

51. 01-IntroToMediaComp51 Why can’t we get back again? Converting to grayscale is different from computing a negative. –A negative transformation retains information. With grayscale, we’ve lost information –We no longer know what the ratios are between the reds, the greens, and the blues –We no longer know any particular value. Media compressions are one kind of transformation. Some are lossless (like negative); Others are lossy (like grayscale)

52. 01-IntroToMediaComp52 But that’s not really the best grayscale In reality, we don’t perceive red, green, and blue as equal in their amount of luminance: How bright (or non-bright) something is. –We tend to see blue as “darker” and red as “brighter” –Even if, physically, the same amount of light is coming off of each Photoshop’s grayscale is very nice: Very similar to the way that our eye sees it –B&W TV’s are also pretty good

53. 01-IntroToMediaComp53 Building a better grayscale We’ll weigh red, green, and blue based on how light we perceive them to be, based on laboratory experiments. def grayscaleNew(picture): for px in getPixels(picture): newRed = getRed(px) * 0.299 newGreen = getGreen(px) * 0.587 newBlue = getBlue(px) * 0.114 luminance = newRed + newGreen + newBlue setColor(px, makeColor(luminance, luminance, luminance))

54. 01-IntroToMediaComp54 Summary Name = expression creates a name (a variable) that has a value You can create your own functions in python You can execute functions using the function name and passing in any required values –decreaseRed(picture) You can modify pictures by modifying the pixels red, green, and blue values You can use an array to hold many values of the same type You can loop through all the values in an array using a for variable in array: Blocks in Python are shown by indention