Segundo Processing...

Código:
void setup(){
size(400,400);
stroke(255);
smooth();
noFill();
}
void draw(){
for (int X=0; X<400; X=X+50) {
for (int Y=0; Y<400; Y=Y+50) {
fill(mouseX,100,mouseY,50);
rect(X,Y,40,40);
}
}
}