vertex

int frames=300, totalSegments=6;
animatedPolygon ap1;
float t, new_t, partialSegment, fullSegments;

void setup() {
  size(500, 500);
  ap1 = new animatedPolygon(totalSegments);
  strokeWeight(40);
}


void draw() {
  t = (frameCount%frames)/float(frames);
  background(#35465c);
  ap1.run();
  //if (frameCount<=frames) saveFrame("image-###.gif");
}

class animatedPolygon {

  PVector[] vertices;
  PVector currPos;
  int counter, v, v1, v2;
  float currentPosBetweenVertices;
  float angle, cx, cy, r;

  animatedPolygon(int v) {
    vertices = new PVector[v];
    r = 200;
    cx = width/2;
    cy = height/2;
    for (int i=0; i<vertices.length; i++) {
      float x = cx + cos(angle)*r;
      float y = cy + sin(angle)*r;
      vertices[i] = new PVector(x, y);
      angle += TWO_PI/v;
    }
  }

  void run() {
    update();
    display();
  }

  void update() {

    if (t<0.5) {
      new_t = map(t, 0, 0.5, 0, 1);
    } else {
      new_t = map(t, 0.5, 1, 0, 1);
    }

    fullSegments = floor(new_t * totalSegments);
    partialSegment = (new_t * totalSegments) % totalSegments;
    currentPosBetweenVertices = partialSegment-fullSegments;

    v1 = int(fullSegments);
    v2 = int((fullSegments+1)%totalSegments);

    currPos = PVector.lerp(vertices[v1], vertices[v2], currentPosBetweenVertices);
  }

  void display() {
    stroke(255);
    if (t<0.5) {
      for (int i=0; i<fullSegments; i++) {
        line(vertices[i].x, vertices[i].y, vertices[(i+1)%totalSegments].x, vertices[(i+1)%totalSegments].y);
      }
      line(vertices[v1].x, vertices[v1].y, currPos.x, currPos.y);
    } else {
      for (int i= (v1+1); i<totalSegments; i++) {
        line(vertices[i].x, vertices[i].y, vertices[(i+1)%totalSegments].x, vertices[(i+1)%totalSegments].y);
      }
      line(currPos.x, currPos.y, vertices[v2].x, vertices[v2].y);
    }
  }
}