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lab25
a function processing a picture

We finished last time, seeing how to modify a row of 5 (specific) pixels in an image:

pict0 = makePicture( "pathToSomeFile.jpg" )   # you can call pickAFile(), to find the full path

show(pict0)
explore(pict0)

px = getPixelAt(pict0, 20, 10)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 11)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 12)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 13)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 14)
setRed(px,255)
setBlue(px,255)

repaint(pict0)

We'll say that we “enpurple” a pixel, when we max out its red and blue components as above. (Enpurpling a black pixel makes it purple; if there is any green in a pixel then enpurpling it makes it a lighter purple; enpurpling a white pixel leaves it white — the lightest purple!)

Today's tasks: We won't check these off today, but will check them off in the next class.

Let's generalize the above steps into a function stripeAt20x10, which takes in any picture-object ¹, and changes (enpurples) pixels (20,10) up to but not including (20,15) as above, and then repaints it.
Test your function on two different pictures. It starts as:

# stripeAt20x10 : Picture -> None
# Modify `aPict` by enpurpling pixels (20,10), (20,11), (20,12), ..., (20,14).
#
def stripeAt20x10( aPict ):
  px = getPixelAt( aPict,20, 10)
  setRed(px,255)
  ⋮
  repaint( aPict )
  return

pitfall: A common problem is to end your function just with “repaint” rather than “repaint(aPict)” . I wish JES reported an error if you just mention a function's name without calling it ², but it doesn't.

Write a function stripeAtX10 which takes in a Picture-object and an x-coordinate, and changes (enpurples) the pixels 10 to 15 of that particular column X (and then repaints the Picture). Test your function by drawing strips at different columns.
That is stripeAtX10(pict0,20) should do the same thing that stripeAt20x10(pict0) did, and stripeAtX10( pict0, 21 ) should make a stripe starting at x=21,y=10, and stripeAtX10( pict0, 99 ) should make a stripe starting at x=99,y=10.
Write a function squareAtX10 which takes in a Picture-object and an x-coordinate, and enpurples a solid 5x5 square whose northwest corner is at (x,10) (and then repaints the Picture). This seems like it require 25 lines of code for changing the 25 pixels, but don't do that: instead, just call stripeAtX10 five times!
BONUS: Write a function bigSquareAtX10 which takes in a picture-object and an x-coordinate, and enpurples a 20x5 rectangle whose northwest corner is at (x,10). Call your previous function!
BONUS: Generalize the above functions so that instead of always enpurpling starting at y=10, the y-coordinate gets passed in.

We will see soon, how to use a loop to avoid the yucky repeated code we had back in stripeAtX10.

¹Well, it will take in any picture-object which is at least 21x15, if we are changing pixel (20,14). ↩

² In python, it can be technically legal to talk about a function without calling it — after all, really “repaint” is just a variable-name which happens to hold a function, and you can have a variable-name on a line by itself, even if that's not very helpful. For advanced programmers, having functions be values (which can be stored in a variable, like any value) is incredibly useful. But as beginners, mentioning a function without calling it is pretty much never what we want to do! ↩

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©2014, Ian Barland, Radford University
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lab25 a function processing a picture

lab25
a function processing a picture