Bezier line animation in Processing

welcome.pde

float x1, y1;
float x2, y2;
float x3, y3;
float x4, y4;

int frame = 0;

PImage title;

float a = 0;

void setup() {
  size(500, 500);
  smooth();
  ellipseMode(CENTER);
  
  title = loadImage("title.png");

  x1 = 250;
  y1 = 25;
  x2 = 380; // CP 1
  y2 = 25; //250;
  x3 = 25; // CP 2
  y3 = 380; //250;
  x4 = 250;
  y4 = 380;

  background(0);
  strokeWeight(3);
}

void draw() {

  noStroke();
  fill(0, 10);
  rect(0, 0, width, height);
  imageMode(CENTER);
  image(title, width * 0.5, 450);

  fill(255, 50);

  x2 = 250 + (sin(radians(a)) * 355);
  x3 = 250 - (cos(radians(a)) * 355);

  noFill();
  
  stroke(255, 155 + sin(radians(a * 2)) * 100, 155 + cos(radians(a * 2)) * 100);
  bezier(x1, y1, x2, y2, x3, y3, x4, y4);

  x2 = 250 - (sin(radians(a)) * 355);
  x3 = 250 + (cos(radians(a)) * 355);
  
  stroke(255, 155 + sin(radians(a * 2)) * 100, 155 + cos(radians(a * 2)) * 100);
  bezier(x1, y1, x2, y2, x3, y3, x4, y4);

  a += 3;
  
  if(frame > 59 && frame < 160){
    saveFrame("welcome###.png");
  }
  frame ++;
}