Curtain

// The Link class is used for handling constraints between particles.

class Link {

  float restingDistance;

  float stiffness;

  Particle p1;

  Particle p2;

  // the scalars are how much "tug" the particles have on each other

  // this takes into account masses and stiffness, and are set in the Link constructor

  float scalarP1;

  float scalarP2;

  // if you want this link to be invisible, set this to false

  boolean drawThis = true;

  Link (Particle which1, Particle which2, float restingDist, float stiff) {

    p1 = which1; // when you set one object to another, it's pretty much a reference. 

    p2 = which2; // Anything that'll happen to p1 or p2 in here will happen to the paticles in our array

    restingDistance = restingDist;

    stiffness = stiff;

    // although there are no differences in masses for the curtain, 

    // this opens up possibilities in the future for if we were to have a fabric with particles of different weights

    float im1 = 1 / p1.mass; // inverse mass quantities

    float im2 = 1 / p2.mass;

    scalarP1 = (im1 / (im1 + im2)) * stiffness;

    scalarP2 = (im2 / (im1 + im2)) * stiffness;

  }

  void constraintSolve () {

    // calculate the distance between the two particles

    PVector delta = PVector.sub(p1.position, p2.position);  

    float d = sqrt(delta.x * delta.x + delta.y * delta.y);

    float difference = (restingDistance - d) / d;

    // if the distance is more than curtainTearSensitivity, the cloth tears

    // it would probably be better if force was calculated, but this works

    if (d > curtainTearSensitivity) 

      p1.removeLink(this);

    // P1.position += delta * scalarP1 * difference

    // P2.position -= delta * scalarP2 * difference

    p1.position.add(PVector.mult(delta, scalarP1 * difference));

    p2.position.sub(PVector.mult(delta, scalarP2 * difference));

  }

​

  void draw () {

    if (drawThis)

      line(p1.position.x, p1.position.y, p2.position.x, p2.position.y);

  }

}

​