Connecting particles with spline

In this recipe we are going to learn how to connect particles with splines in 3D.

Getting started

In this recipe we are going to use the particle's code base from the recipe Creating a particle system, from Chapter 5, Building Particle Systems. We are going to use the 3D version.

How to do it…

We will create splines connecting particles.

  1. Include the necessary header file inside ParticleSystem.h:
    #include "cinder/BSpline.h"
  2. Add a new property to the ParticleSystem class:
    ci::BSpline3f spline;
  3. Implement the computeBSpline method for the ParticleSystem class:
    void ParticleSystem::computeBspline(){ 
 std::vector<ci::Vec3f> splinePoints;
 std::vector<Particle*>::iterator it;
 for(it = particles.begin(); it != particles.end(); ++it ){
  ++it;
  splinePoints.push_back( ci::Vec3f( (*it)->position ) );
 }
 spline = ci::BSpline3f( splinePoints, 3, false, false );
}
  4. At the end of the ParticleSystem update method, invoke the following code:
    computeBSpline();
  5. Replace the draw method of ParticleSystem with the following:
    void ParticleSystem::draw(){
 ci::gl::color(ci::Color::black());
 if(spline.isUniform()) {
  glBegin(GL_LINES);
  float step = 0.001f;
  for( float t = step; t <1.0f; t += step ) {
   ci::gl::vertex( spline.getPosition( t-step ) );
   ci::gl::vertex( spline.getPosition( t ) );
  } 
  glEnd();
 }
 ci::gl::color(ci::Color(0.0f,0.0f,1.0f));
 std::vector<Particle*>::iterator it;
 for(it = particles.begin(); it != particles.end(); ++it ){
  (*it)->draw();
 }
}
  6. Add headers to your main Cinder application class files:
    #include "cinder/app/AppBasic.h"
#include "cinder/gl/Texture.h"
#include "cinder/Rand.h"
#include "cinder/Surface.h"
#include "cinder/MayaCamUI.h"
#include "cinder/BSpline.h"

#include "ParticleSystem.h"
  7. Add members for your main class:
    ParticleSystem mParticleSystem;

float repulsionFactor;
float maxAlignSpeed;

CameraPersp        mCam;
MayaCamUI mMayaCam;

Vec3f mRepPosition;

BSpline3f   spline;
  8. Implement the setup method as follows:
    void MainApp::setup()
{
repulsionFactor = -1.0f;
maxAlignSpeed = 10.f;
mRepPosition = Vec3f::zero();

mCam.setPerspective(45.0f, getWindowAspectRatio(), 0.1, 10000);
mCam.setEyePoint(Vec3f(7.f,7.f,7.f));
mCam.setCenterOfInterestPoint(Vec3f::zero());
mMayaCam.setCurrentCam(mCam);
vector<Vec3f> splinePoints;
float step = 0.5f;
float width = 20.f;
for (float t = 0.f; t < width; t += step) {
 float mass = Rand::randFloat(20.f, 25.f);
 float drag = 0.95f;
 splinePoints.push_back( Vec3f(math<float>::cos(t),
 math<float>::sin(t),
 t - width*0.5f) );
 Particle *particle;
 particle = new Particle( 
  Vec3f( math<float>::cos(t)+Rand::randFloat(-0.8f,0.8f),
   math<float>::sin(t)+Rand::randFloat(-0.8f,0.8f),
   t - width*0.5f), 
  1.f, mass, drag );
 mParticleSystem.addParticle( particle );
}
spline = BSpline3f( splinePoints, 3, false, false );
}
  9. Add members for camera navigation:
    void MainApp::resize( ResizeEvent event ){
  mCam = mMayaCam.getCamera();
  mCam.setAspectRatio(getWindowAspectRatio());
  mMayaCam.setCurrentCam(mCam);
}

void MainApp::mouseDown(MouseEvent event){
  mMayaCam.mouseDown( event.getPos() );
}

void MainApp::mouseDrag( MouseEvent event ){
  mMayaCam.mouseDrag( event.getPos(), event.isLeftDown(), event.isMiddleDown(), event.isRightDown() );
}
  10. Implement the update method as follows:
    void MainApp::update() {
 float pos=math<float>::abs(sin(getElapsedSeconds()*0.5f));
 mRepPosition = spline.getPosition( pos );
 std::vector<Particle*>::iterator it;
 it = mParticleSystem.particles.begin();
 for(; it != mParticleSystem.particles.end(); ++it ) {
  Vec3f repulsionForce = (*it)->position - mRepPosition;
  repulsionForce = repulsionForce.normalized() *
   math<float>::max(0.f, 3.f-repulsionForce.length());
  (*it)->forces += repulsionForce;
  Vec3f alignForce = (*it)->anchor - (*it)->position;
  alignForce.limit(maxAlignSpeed);
  (*it)->forces += alignForce;
 }
 mParticleSystem.update();
}
  11. Implement the draw method as follows:
    void MainApp::draw() {
 gl::enableDepthRead();
 gl::enableDepthWrite();
 gl::clear( Color::white() );
 gl::setViewport(getWindowBounds());
 gl::setMatrices(mMayaCam.getCamera());
 gl::color(Color(1.f,0.f,0.f));
 gl::drawSphere(mRepPosition, 0.25f);
 mParticleSystem.draw();
}

How it works…

B-spline lets us draw a very smooth curved line through some given points, in our case, particle positions. We can still apply some attraction and repulsion forces so that the line behaves quite like a spring. In Cinder, you can use B-splines in 2D and 3D space and calculate them with the BSpline class.

How it works…

See also

More details about B-spline are available at http://en.wikipedia.org/wiki/B-spline.

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