In a number of programs that I have been writing recently I've wanted to draw a semi-opaque "background" using the rect() function in order to fade out everything that I have been drawing in a program. Here's a simple example:

1. int l = 20;
2. void setup()
3. {
4.   size(800, 400);
5.   background(255);
6. }
7. void draw()
8. {
9.   fill(255, 7);
10.   rect(0, 0, width, height);
11.   pushMatrix();
12.   translate(mouseX, mouseY);
13.   noFill();
14.   stroke(0);
15.   rect(-l/2, -l/2, l, l);
16.   popMatrix();
17. }

The problem that I have with this code is that the rectangles never completely fade out of view. Even stranger, commenting out the background() function in setup() allows them to fade out all of the way, but the background is gray  instead of white. I'm up for any solution that has the same functionality as this code minus the background() line, but leaves a white background. Thanks in advance, guys.