Fading a picture gradually

Question

The idea of this function is to fade the top half only of the picture (make it gradually darker). Here is what I have but it seems to be making all of the top half solid black.

def fadeDownFromBlack(pic1):

w=getWidth(pic1)
h=getHeight(pic1)

for y in range(0,h/2):
     for x in range(0,w):
        px=getPixel(pic1,x,y) 
        setBlue(px,y*(2.0/h)) 
        setRed(px,y*(2.0/h)) 
        setGreen(px,y*(2.0/h))

Answer 1

To darken a pixel you multiply the red, green and blue levels by an appropriate fraction.

What you are doing:

setBlue(px,y*(2.0/h))

What you are being told to do:

setBlue(px,y*(2.0/h) * getBlue(px))

Answer 2

Let's look at just one line here:

setBlue(px,y*(2.0/h))

and key part here is

y*(2.0/h)

y changes, as you go down. Let's try some simple values for y and h. Let's say h is 100 and we will examine when y is both 0 and 50 (h/2). For y = 0, we get 0. For y = 50, we get 1. If your range of values for the colors is 256 with 0 being the darkest, then no wonder this is black. What you have is a range of values from 0. to 1., but I'm guessing what you want is to take that number and times it by the old color value.

What you want is:

setBlue(px,y*(2.0/h)*getBlue(px))

and similar things for the other colors.