Fading
You can also use variables to control transparency. With the fill() command, you can add a 4th parameter for opacity (alpha). Like the Red, Green, and Blue values, the Alpha value goes from 0 to 255.
int opacity=0;
void setup()
{
size(500,300);
background(0);
}
void draw()
{
background(0);
// draw a blue circle
fill(0,0,255);
ellipse(250,100,150,150);
// draw red circle over it
opacity++;
if (opacity>255)
{
opacity=0;
}
fill(255,0,0,opacity);
ellipse(250,200,150,150);
}