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/**
* @classdesc Class that handles the water dynamics.
*/
class WaterDynamics {
constructor(gravity, drag_mod=0.25, lift_mod=0.25, push_mod=0.05,
max_drag=2000, max_lift=500, max_push=13){
this.gravity = gravity;
this.drag_mod = drag_mod;
this.lift_mod = lift_mod;
this.max_drag = max_drag;
this.max_lift = max_lift;
this.push_mod = push_mod;
this.max_push = max_push;
}
compute_centroids(vectors){
let count = vectors.length;
console.assert(count >= 3);
let c = new b2.Vec2(0, 0);
let area = 0;
let ref_point = new b2.Vec2(0, 0);
let inv3 = 1/3;
for(let i = 0; i < count; i++){
// Triangle vertices
let p1 = ref_point;
let p2 = vectors[i];
let p3 = i + 1 < count ? vectors[i + 1] : vectors[0];
let e1 = b2.Vec2.Subtract(p2, p1);
let e2 = b2.Vec2.Subtract(p3, p1);
let d = b2.Cross_v2_v2(e1, e2);
let triangle_area = 0.5 * d;
area += triangle_area;
// Area weighted centroid
c.Add(b2.Vec2.Multiply(triangle_area * inv3, b2.Vec2.Add(p1, b2.Vec2.Add(p2, p3))));
}
if(area > b2.epsilon){
c.Multiply(1/area);
}
else{
area = 0;
}
return [c, area];
}
inside(cp1, cp2, p){
return (cp2.x - cp1.x) * (p.y - cp1.y) > (cp2.y - cp1.y) * (p.x - cp1.x);
}
intersection(cp1, cp2, s, e){
let dc = new b2.Vec2(cp1.x - cp2.x, cp1.y - cp2.y);
let dp = new b2.Vec2(s.x - e.x, s.y - e.y);
let n1 = cp1.x * cp2.y - cp1.y * cp2.x;
let n2 = s.x * e.y - s.y * e.x;
let n3 = 1.0 / (dc.x * dp.y - dc.y * dp.x);
return new b2.Vec2((n1 * dp.x - n2 * dc.x) * n3, (n1 * dp.y - n2 * dc.y) * n3);
}
find_intersection(fixture_A, fixture_B){
// TODO : assert polygons
let output_vertices = [];
let polygon_A = fixture_A.GetShape();
let polygon_B = fixture_B.GetShape();
// fill 'subject polygon' from fixture_A polygon
for(let i = 0; i < polygon_A.m_count; i++){
output_vertices.push(fixture_A.GetBody().GetWorldPoint(polygon_A.m_vertices[i]));
}
// fill 'clip polygon' from fixture_B polygon
let clip_polygon = [];
for(let i = 0; i < polygon_B.m_count; i++){
clip_polygon.push(fixture_B.GetBody().GetWorldPoint(polygon_B.m_vertices[i]));
}
let cp1 = clip_polygon[clip_polygon.length - 1];
for(let j = 0; j < clip_polygon.length; j++){
let cp2 = clip_polygon[j];
if(output_vertices.length == 0){
break;
}
let input_list = output_vertices.slice();
output_vertices = [];
let s = input_list[input_list.length - 1];
for(let i = 0; i < input_list.length; i++){
let e = input_list[i];
if(this.inside(cp1, cp2, e)){
if(!this.inside(cp1, cp2, s)){
output_vertices.push(this.intersection(cp1, cp2, s, e));
}
output_vertices.push(e);
}
else if(this.inside(cp1, cp2, s)){
output_vertices.push(this.intersection(cp1, cp2, s, e));
}
s = e;
}
cp1 = cp2
}
return [(output_vertices.length != 0), output_vertices];
}
calculate_forces(fixture_pairs){
for(let pair of fixture_pairs){
let density = pair[0].GetDensity();
let [has_intersection, intersection_points] = this.find_intersection(pair[0], pair[1]);
if(has_intersection){
let [centroid, area] = this.compute_centroids(intersection_points);
// apply buoyancy force
let displaced_mass = pair[0].GetDensity() * area;
let buoyancy_force = b2.Vec2.Multiply(displaced_mass, b2.Vec2.Negate(this.gravity));
pair[1].GetBody().ApplyForce(buoyancy_force, centroid, true);
// apply complex drag
for(let i = 0; i < intersection_points.length; i++) {
let v0 = intersection_points[i];
let v1 = intersection_points[(i + 1) % intersection_points.length];
let mid_point = b2.Vec2.Multiply(0.5, b2.Vec2.Add(v0, v1));
// DRAG
// find relative velocity between object and fluid at edge midpoint
let vel_dir = b2.Vec2.Subtract(pair[1].GetBody().GetLinearVelocityFromWorldPoint(mid_point),
pair[0].GetBody().GetLinearVelocityFromWorldPoint(mid_point));
let vel = vel_dir.Normalize();
let edge = b2.Vec2.Subtract(v1, v0);
let edge_length = edge.Normalize();
let normal = b2.Cross_f_v2(-1, edge);
let drag_dot = b2.Dot_v2_v2(normal, vel_dir);
if(drag_dot >= 0){ // normal points backwards - this is not a leading edge
// apply drag
let drag_mag = drag_dot * this.drag_mod * edge_length * density * vel * vel;
drag_mag = Math.min(drag_mag, this.max_drag);
let drag_force = b2.Vec2.Multiply(drag_mag, b2.Vec2.Negate(vel_dir));
pair[1].GetBody().ApplyForce(drag_force, mid_point, true);
// apply lift
let lift_dot = b2.Dot_v2_v2(edge, vel_dir);
let lift_mag = drag_dot * lift_dot * this.lift_mod * edge_length * density * vel * vel;
lift_mag = Math.min(lift_mag, this.max_lift);
let lift_dir = b2.Cross_f_v2(1, vel_dir);
let lift_force = b2.Vec2.Multiply(lift_mag, lift_dir);
pair[1].GetBody().ApplyForce(lift_force, mid_point, true);
}
// PUSH
let body_to_check = pair[1].GetBody();
// Simplification /!\
let joints_to_check = [];
let joint_edge = body_to_check.GetJointList();
while(joint_edge != null){
if(joint_edge.joint.GetBodyB() == body_to_check){
joints_to_check.push(joint_edge.joint);
}
joint_edge = joint_edge.next;
}
for(let joint of joints_to_check){
if(joint.GetLowerLimit() < joint.GetJointAngle() && joint.GetJointAngle() < joint.GetUpperLimit()){
let torque = joint.GetMotorTorque(60);
// Calculate angular inertia of the object
let moment_of_inertia = body_to_check.GetInertia();
let angular_velocity = body_to_check.GetAngularVelocity();
let angular_inertia = moment_of_inertia * angular_velocity;
// Calculate the force applied to the object
let world_center = body_to_check.GetWorldCenter();
let anchor = joint.GetAnchorB();
let lever_vector = b2.Vec2.Subtract(world_center, anchor); // vector from pivot to point of application of the force
let force_applied_at_center = b2.Cross_v2_f(lever_vector, -torque);
let push_dot = b2.Dot_v2_v2(normal, force_applied_at_center);
if(push_dot > 0){
vel = torque + angular_inertia;
// Wrong approximation /!\
let push_mag = push_dot * this.push_mod * edge_length * density * vel * vel;
let force = b2.Vec2.Multiply(push_mag, b2.Vec2.Negate(force_applied_at_center));
let clip_force_x = Math.max(-this.max_push, Math.min(force.x, this.max_push));
let clip_force_y = Math.max(-this.max_push, Math.min(force.y, this.max_push))
let push_force = new b2.Vec2(clip_force_x, clip_force_y);
body_to_check.ApplyForce(push_force, joint.GetAnchorB(), true);
}
}
}
}
}
}
}
}
/**
* @classdesc Stores fixtures of objects in contact with water.
* @constructor
*/
function WaterContactDetector() {
b2.ContactListener.call(this);
this.fixture_pairs = [];
}
WaterContactDetector.prototype = Object.create(b2.ContactListener.prototype);
WaterContactDetector.prototype.constructor = WaterContactDetector;
WaterContactDetector.prototype.BeginContact = function (contact){
if(contact.GetFixtureA().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.WATER
&& contact.GetFixtureB().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.BODY_OBJECT){
this.fixture_pairs.push([contact.GetFixtureA(), contact.GetFixtureB()]);
}
else if(contact.GetFixtureB().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.WATER
&& contact.GetFixtureA().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.BODY_OBJECT){
this.fixture_pairs.push([contact.GetFixtureB(), contact.GetFixtureA()]);
}
};
WaterContactDetector.prototype.EndContact = function (contact) {
if(contact.GetFixtureA().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.WATER
&& contact.GetFixtureB().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.BODY_OBJECT){
let index = this.fixture_pairs.indexOf([contact.GetFixtureA(), contact.GetFixtureB()]);
if (index !== -1) {
this.fixture_pairs.splice(index, 1);
}
}
else if(contact.GetFixtureB().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.WATER
&& contact.GetFixtureA().GetBody().GetUserData().object_type == CustomUserDataObjectTypes.BODY_OBJECT){
let index = this.fixture_pairs.indexOf([contact.GetFixtureB(), contact.GetFixtureA()]);
if (index !== -1) {
this.fixture_pairs.splice(index, 1);
}
}
};
WaterContactDetector.prototype.Reset = function (){
this.fixture_pairs = [];
};
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