ion && firstActive != firstFree) {
        firstActive++;
        if (firstActive == particles.length) firstActive = 0;
      }
    };
    ParticlePool.prototype.draw = function(context, image) {
// draw active particles
      if (firstActive < firstFree) {
        for (i = firstActive; i < firstFree; i++)
          particles[i].draw(context, image);
      }
      if (firstFree < firstActive) {
        for (i = firstActive; i < particles.length; i++)
          particles[i].draw(context, image);