source: other-projects/nz-flag-design/trunk/render-3d/Flag_files/Flag.js@ 29693

/*
 * Aug 9 2012
 * Its Singapore's National Day, so
 * Making a quick tweaks to simulate the Singapore flag in the wind 
 *
 */
/*
 * Aug 3 2012
 *
 * Since I started working for a new startup not too long ago,
 * I commute between home and work for over 2 hours a day.
 * Although this means less time on three.js,
 * I try getting a little coding on the train.
 *
 * This set of experiments started from a simple hook's law doodle,
 * to spring simulation, string simulation, and I realized
 * I once again stepped onto physics and particle simulation, 
 * this time, more specifically soft body physics.
 *
 * Based on the "Advanced Character Physics" article,
 * this experiment attempts to use a "massless"
 * cloth simulation model. It's somewhat similiar 
 * but simplier to most cloth simulations I found.
 *
 * This was coded out fairly quickly, so expect more to come
 * meanwhile feel free to experiment yourself and share
 *
 * Cheers,
 * Graphics Noob (aka @Blurspline, zz85)
 */

// Suggested Readings

// Advanced Character Physics by Thomas Jakobsen Character - http://web.archive.org/web/20070610223835/http:/www.teknikus.dk/tj/gdc2001.htm
// http://freespace.virgin.net/hugo.elias/models/m_cloth.htm
// http://en.wikipedia.org/wiki/Cloth_modeling
// http://cg.alexandra.dk/tag/spring-mass-system/
// Real-time Cloth Animation http://www.darwin3d.com/gamedev/articles/col0599.pdf

var DAMPING = 0.02;
var DRAG = 1 - DAMPING;
var MASS = .2;
var restDistance = 20;


var xSegs = 15; // ratio is 2:3
var ySegs = 10; //

var clothFunction = plane(restDistance * xSegs, restDistance * ySegs);

var cloth = new Cloth(xSegs, ySegs);

var GRAVITY = 981 * 1.4; // 
var gravity = new THREE.Vector3( 0, -GRAVITY, 0 ).multiplyScalar(MASS);


var TIMESTEP = 18 / 1000;
var TIMESTEP_SQ = TIMESTEP * TIMESTEP;

var pins = [];
var pinning = true;

var wind = true;

var windForce = new THREE.Vector3(0,0,0);

var ballPosition = new THREE.Vector3(0, -45, 0);
var ballSize = 60; //40

var tmpForce = new THREE.Vector3();

var lastTime;




function plane(width, height) {

    return function(u, v) {
        var x = u * width; //(u-0.5)
        var y = v * height;
        var z = 0;

        return new THREE.Vector3(x, y, z);
    };
}

function Particle(x, y, z, mass) {
    this.position = clothFunction(x, y); // position
    this.previous = clothFunction(x, y); // previous
    this.original = clothFunction(x, y); 
    this.a = new THREE.Vector3(0, 0, 0); // acceleration
    this.mass = mass;
    this.invMass = 1 / mass;
    this.tmp = new THREE.Vector3();
    this.tmp2 = new THREE.Vector3();
}

// Force -> Acceleration
Particle.prototype.addForce = function(force) {
    this.a.add(
        this.tmp2.copy(force).multiplyScalar(this.invMass)
    );
};


// Performs verlet integration
Particle.prototype.integrate = function(timesq) {

    var newPos = this.tmp.subVectors(this.position, this.previous);
    newPos.multiplyScalar(DRAG).add(this.position);
    newPos.add(this.a.multiplyScalar(timesq));
    
    this.tmp = this.previous;
    this.previous = this.position;
    this.position = newPos;

    this.a.set(0, 0, 0);
    
}


var diff = new THREE.Vector3();

function satisifyConstrains(p1, p2, distance) {
    
    diff.subVectors(p2.position, p1.position);
    var currentDist = diff.length();
    if (currentDist==0) return; // prevents division by 0
    var correction = diff.multiplyScalar(1 - distance/currentDist);
    var correctionHalf = correction.multiplyScalar(0.5);
    p1.position.add(correctionHalf);
    p2.position.sub(correctionHalf);
    
}

function Cloth(w, h) {
    w = w || 10;
    h = h || 10;
    this.w = w;
    this.h = h;

    var particles = [];
    var constrains = [];

    var u, v;

    // Create particles
    for (v=0;v<=h;v++) {
        for (u=0;u<=w;u++) {
            particles.push(
                new Particle(u/w, v/h, 0, MASS)
            );
        }
    }

    // Structural
    
    for (v=0;v<h;v++) {
        for (u=0;u<w;u++) {

            constrains.push([
                particles[index(u, v)],
                particles[index(u, v+1)],
                restDistance
            ]);

            constrains.push([
                particles[index(u, v)],
                particles[index(u+1, v)],
                restDistance
            ]);

        }
    }

    for (u=w, v=0;v<h;v++) {
        constrains.push([
            particles[index(u, v)],
            particles[index(u, v+1)],
            restDistance

        ]);
    }

    for (v=h, u=0;u<w;u++) {
        constrains.push([
            particles[index(u, v)],
            particles[index(u+1, v)],
            restDistance
        ]);
    }
    

    // While many system uses shear and bend springs,
    // the relax constrains model seem to be just fine
    // using structural springs.
    // Shear
    var diagonalDist = Math.sqrt(restDistance * restDistance * 2);


    for (v=0;v<h;v++) {
        for (u=0;u<w;u++) {

            constrains.push([
                particles[index(u, v)],
                particles[index(u+1, v+1)],
                diagonalDist
            ]);

            constrains.push([
                particles[index(u+1, v)],
                particles[index(u, v+1)],
                diagonalDist
            ]);

        }
    }
    
    this.particles = particles;
    this.constrains = constrains;

    function index(u, v) {
        return u + v * (w + 1);
    }

    this.index = index;

}

function setPinning(bool){
    pinning = bool;
}

function simulate(time) {
    if (!lastTime) {
        lastTime = time;
        return;
    }
    
    // TIMESTEP = (time - lastTime);
    // TIMESTEP = (TIMESTEP > 30) ? TIMESTEP / 1000 : 30 / 1000;
    // TIMESTEP_SQ = TIMESTEP * TIMESTEP;
    // lastTime = time;
    // console.log(TIMESTEP);
    
    var i, il, particles, particle, pt, constrains, constrain;

    // Aerodynamics forces
    if (wind) {
        var face, faces = clothGeometry.faces, normal;

        particles = cloth.particles;

        for (i=0,il=faces.length;i<il;i++) {
            face = faces[i];
            normal = face.normal;

            tmpForce.copy(normal).normalize().multiplyScalar(normal.dot(windForce));
            particles[face.a].addForce(tmpForce);
            particles[face.b].addForce(tmpForce);
            particles[face.c].addForce(tmpForce);
        }
    }
    
    for (particles = cloth.particles, i=0, il = particles.length
            ;i<il;i++) {
        particle = particles[i];
        particle.addForce(gravity);
        //
        var x = particle.position.x, y = particle.position.y, z = particle.position.z, t=Date.now() / 1000;
        //windForce.set(Math.sin(x*y*t), Math.cos(z*t), Math.sin(Math.cos(5*x*y*z))).multiplyScalar(0);
        particle.addForce(windForce);
        particle.integrate(TIMESTEP_SQ);
    }

    // Start Constrains

    constrains = cloth.constrains,
    il = constrains.length;
    for (i=0;i<il;i++) {
        constrain = constrains[i];
        satisifyConstrains(constrain[0], constrain[1], constrain[2]);
    }

    // Ball Constrains


    ballPosition.z = -Math.sin(Date.now()/300) * 90 ; //+ 40;
    ballPosition.x = Math.cos(Date.now()/200) * 70

    // Pin Constrains
    if(Boolean(pinning)){
        for (i=0, il=pins.length;i<il;i++) {
            var xy = pins[i];
            var p = particles[xy];
            p.position.copy(p.original);
            p.previous.copy(p.original);
        }
    }

}