Fade in and Out
You can set the speed when rising different from the speed when falling; here, the opacity increases quickly (+10 per frame) but fades out slowly (-1 per frame). The same idea can apply to motion and to size animation, of course. Borrowing an idea from sound,we can call this the "Attack" and "Decay."
int opacity;
int speed=10;
void setup()
{
size(500,300);
}
void draw()
{
background(0);
opacity = opacity + speed;
if (opacity > 255)
{
opacity = 255;
speed = -1;
}
if (opacity < 0)
{
speed=10;
}
fill(255,0,0,opacity);
ellipse(250,150,250,250);
}