anchor.diff:  You have learned that many different effects can be created by changing the parameters and textures of one general emitter object.
   
   Now you move on to another exciting chapter – the simulation of <<jme3/beginner/hello_physics#,physical objects>>. Let's shoot some cannon balls at a brick wall!
  -'''
  
positive.code: a| How many new particles are emitted per second. 
  
  <a| color               
  a| `setStartColor()` +`setEndColor()` 
  a| gray 
  a| Set to the same colors, or to two different colors for a gradient effect. 
  
  <a| size                
  a| `setStartSize()` +`setEndSize()` 
  a| 0.2f +2f 
  a| Set to two different values for shrink/grow effect, or to same size for constant effect. 
  
  <a| gravity             
  
negative.code: a| Specify a vector how fast particles move and in which start direction. 
  
  <a| direction           
  a| `getParticleInfluencer(). setVelocityVariation()` +
  `setFacingVelocity()` +
  `setRandomAngle()` +
  `setFaceNormal()` +
  `setRotateSpeed()` 
  a| 0.2f +
  false +
  false +
  Vector3f.NAN +0.0f 
  a| Optional accessors that control in which direction particles face while flying. 
  
  <a| lifetime            
  a| `setLowLife()` +
  `setHighLife()` 
  <a| 3f +
  7f  
  a| Minimum and maximum time period before particles fade. 
  
  <a| emission rate       
  