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lab41
creating images; mixing images

# swatch: int, int -> image
# Create a Picture (of size `width`x`height`)
# whose color varies left-to-right.
#
def swatch( width, height ):
  result = makeEmptyPicture(width,height)
  show(result)       # Just so we can watch progress
  for x in range(width):
    paintStripe(result,x,  0,0,x)
    repaint(result)  # watch the progress
  return result

# paintStripe: Picture, int,  int, int, int  ->  None
# Modify `aPict` so that column# `columnNum` has the color (`r`,`g,`b`).
# 
def paintStripe( aPict, columnNum,  r, g, b ):
  for rowNum in range(getHeight(aPict)):
      px = getPixel(aPict, columnNum, rowNum)
      setRed( px, r )
      setGreen( px, g )
      setBlue( px, b )
  return None

Try variations: in swatch, play with things like: paintStripe(result,x, x,x,x) or paintStripe(result,x, x,2*x,3*x).

Using “remainder” to cycle around (just like a clock's minute-hand): Divide by 256, but only use the remainder! That's what the % operator does: paintStripe(result,x, x % 256, (2*x) % 256, (3*x)%256 ).

We can make a pattern where each column isn't constant: (follow along in lecture)

# pattern: int,int -> Picture
# Return (and, show) a picture of a geometric pattern.
#
def pattern(width,height):
    rslt = makeEmptyPicture(width,height)
    show(rslt)
    for x in range(width):
        for y in range(height):
            px = getPixelAt(rslt,x,y)
            setRed(  px,   (x+y)/5%256)
            setGreen(px, 3*(x+y)/5%256)
            setBlue( px, 4*(x+y)/5%256)
        repaint(rslt)
    return rslt

Note that this uses a “nested loop” instead of having one loop (in swatch) that calls a function which does its own “inner” loop (in paintStripe). In general, either approach can be taken — a nested loop avoids making a second function, but can also make it significantly harder to debug if something isn't quite right. (Nested loops also requires keeping track of more things all at once, in your head.) In general, prefer having separate functions for each loop, but realize that you'll see people write nested loops elsewhere.

TASK SETUP: Find two images that are both 640x480. (You can include "640x480" in the google-images search, or you can crop an larger photo using, say, Windows Photo Manager or OS X's Preview.) Download them to your H: drive, and start a JES program which puts each in a variable.
Use variable-names that describe each picture. Example: kittenz = makePicture( 'H:\\itec109\\frolicking-tiger-cubs.jpg' )

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©2014, Ian Barland, Radford University
Last modified 2014.Apr.19 (Sat) Please mail any suggestions
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lab41 creating images; mixing images

lab41
creating images; mixing images