Paint 0

final int sfondo=255;
final int matita=0;

void setup()
{
   size(500, 500);
   frameRate(30);
   smooth();
   stroke(matita);
   strokeWeight(2);
   cornice();
   
   noLoop();
}

void cornice()
{
   background(sfondo);
   rect(0,0,width-1, height-1);
   redraw();
}

void keyPressed()
{
   cornice();
}

void mouseDragged()
{
   line(mouseX, mouseY, pmouseX, pmouseY);
   redraw();
}

void draw()
{
   // niente...
}