int oldX, oldY;
int olderX, olderY;

void setup() {
  frameRate(15);
  size(600, 600);
  smooth();
  stroke(0,255,0,127);
  background(0);
}

void draw() {
  fill(0, 16);
  noStroke();
  rect(0, 0, width, height);
  fill(0,255,0,127);
  stroke(0,255,0,127);
  triangle(oldX, oldY, mouseX, mouseY, olderX, olderY);
  olderX = oldX;
  olderY = oldY;
  oldX = mouseX;
  oldY = mouseY;
}