#this turns a picture into a sepia-ttinted version.
#note that this function alters the picture itself,
#rather than return a sepia-tinted picture
#as a value, so it is a little different from
#the suggested organization in the assignment.

#This also lets you set the percentage for
#altering red and blue components of dark, midtone,
#and bright pixels.

def sepia_tint(p,shadow,midtone,highlight):
  for px in getPixels(p):
    r=getRed(px)
    g=getGreen(px)
    b=getBlue(px)
    av=(r+g+b)/3
    setGreen(px,av)
    setBlue(px,av)
    if av<64:
        setRed(px,min(255,(1+shadow)*av))
        setBlue(px,(1-shadow)*av)
    elif av>=192:
        setRed(px,min(255,(1+highlight)*av))
        setBlue(px,(1-highlight)*av)
    else:
        setRed(px,min(255,(1+midtone)*av))
        setBlue(px,(1-midtone)*av)

#This lets the user pick an image file and
#returns a sepia-tinted version.

def sepia_demo():
  q=makePicture(pickAFile())
  sepia_tint(q,0.1,0.15,0.08)
  show(q)