OSG粒子系统

ParticleSystem : 是一个drawable,有很多属性可以设置。A particle system can only use one texture。

ModularEmitter: ModularEmitter->Emitter->ParticleProcessor->node :每帧都会产生新的粒子，最好使用 "conter"、"placer"、"shotter" achieve a wide variety of emitting styles

conter ：tell the ModularEmitter how many particles it has to create for the current frame。然后会指示placer and shooter去设置粒子的位置和velocity vector。

Particle: 通過設置不同屬性来给粒子系统设置不同的粒子模板，可以给一个发射器设置不同的模板。

Program (ModularProgram) : create a Program. It is a particle processor just like the Emitter
class, but it allows to modify particle properties after they have been created. 创建多个Operator对象,然后将这些Operator对象加入到ModularProgram中。

Particle
counter
placer
shooter
program

emitter->setParticleTemplate(Particle)
emitter->setCounter(counter);
emitter->setParticleSystem(ps);
emitter->setPlacer(placer);
emitter->setShooter(shooter);

program->setParticleSystem(ps)

root->addchild(ps)
root->addchild(program)
root->addchild(emitter)