public void ComputeC(Particle particle){
		//density
		float density = 0;
		List<Particle> neighbours = particle.getNeighbours();
		for(Particle neigh : neighbours){
			density += Kernel(particle.getPosStar(), neigh.getPosStar());
		}
		particle.setDensity(density);
		particle.setConstraint(density / REST_DENSITY -1);
	//	System.out.println("density = " + density);
	}