#title: greenScreen.py
#author: Ariel (arielb@bu.edu)
#Description: A program to create a green screen effect by transposing one picture over another.

def greenscreen():
  message1= "Pick a foreground picture..."
  showInformation(message1)
  #pick a foreground (fg) picture file (with green background)
  fg=pickAFile()
  message2= "Now pick a background picture..."
  showInformation(message2)
  #pick a background (bg) picture file 
  bg=pickAFile()
  #make a picture out of the fg
  makePicture(fg)
  picfg=makePicture(fg)
  #make a picture out of the bg
  makePicture(bg)
  picbg=makePicture(bg)
  #show the before pictures
  show(picfg)
  show(picbg)
  #for each pixel in the fg image:
  for pixel in getPixels(picfg):
    #    get the red value
    valueR= getRed(pixel)
    #    get the green value
    valueG=  getGreen(pixel)
    #    get the blue value
    valueB= getBlue(pixel)
    #    if the red plus blue is less than green 
    if valueR + valueB < valueG:
      #        get the pixel color from the bg image at the same location
      Xfg= getX(pixel)
      Yfg= getY(pixel)
      pixelfg= getPixel(picfg, Xfg, Yfg)
      pixelbg= getPixel(picbg, Xfg, Yfg)
      newPixel= getColor(pixelbg)
      #        set the fg pixel color to be the color obtained from the bg image
      setColor(pixelfg, newPixel)
  #show the resulting picture (repaint)
  repaint(picfg)