What's the best way to write fade in out function?

Question

I want to create a continuous fade in and out.

I'd like to have some function F(time) that returns values increasing from 0 to 255, then from 255 back down to -255, and repeats, as time progresses.

Answer 1

I know the question has been answered but just for old times sake here is the optimized classic way (works only with fixed steps but takes only 2 bytes ;-) )

//init:
unsigned char value=0;
unsigned char d=1;


//in loop:
value+=d;
if((value==0) || (value==255))
 d=-d;

Answer 2

What you're looking for is a periodic function like Sine. Here's how you can use Sine to achieve what you're looking for:

Create a loop counter
Begin your fade loop
    Increment your loop counter
    fade_amount = 255 * sin(loop_counter)

It's as simple as that. Since sin(loop_counter) will cycle from -1.0 through 0 to 1.0 and back again, you just multiply it by the maximum value you want.

Answer 3

You can also use a tweening library to smoothly fade in and out such as cpptweener.

http://code.google.com/p/cpptweener/