My Sketch

int t=0;

float w1=0.01;  // TRY different value: 0.1, 0.01, 0.001

float amp1=height/4;

int rad=15;

PVector pos1=new PVector();

PVector pos2=new PVector();

//===========================================================================

// PROCESSING DEFAULT FUNCTIONS:

void settings() {

  size(400, 600);

}

void setup() {

  textAlign(CENTER, CENTER);

  rectMode(CENTER);

  fill(255);

  strokeWeight(2);

  background(0);

}

void draw() {

  //background(0);  //DISABLE this line to see the accumulated effect

  float calcY;

  //TOP animation: Continuos sinusoidal

  pushMatrix();

  translate(0, height/4);

  calcY=amp1*sin(w1*millis());

  pos1.set(pos1.x+0.5, calcY);

  pos2.set(pos1.x, -pos1.y);

  fill(255, 0, 0);

  ellipse(pos1.x, pos1.y, rad, rad);

  ellipse(pos2.x, pos2.y, rad, rad);

  popMatrix();

  //BOTTOM animation: Steppping in and out aka. center-out

  pushMatrix();

  translate(0, 3*height/4);  

  fill(255, 0, 0);

  calcY=amp1*sin(w1*millis());

  pos1.set(pos1.x+0.5, abs(calcY)>amp1*0.95?amp1:0.0);

  pos2.set(pos1.x, -pos1.y);

  fill(250, 250, 0);

  ellipse(pos1.x, pos1.y, rad, rad);

  ellipse(pos2.x, pos2.y, rad, rad);

  popMatrix();

  if (pos1.x>width) {

    pos1.x=0;

    background(0);

  }

}