Particle system properties' unit

cronvel · May 20, 2021, 9:38am

Hi,

I fail to understand which unit the properties of the particle system are expressed in.
For example lifeTime: it doesn’t seems to be expressed in seconds, neither in millisecond.

Also everything seems to depend on updateSpeed, but again, I fail to see in which unit it is expressed, and even slow update speed still produce fluid particle movements.

Even emitRate which is said to be expressed in “particle per second” experience slowdown/speedup when lowering/raising the updateSpeed.

cronvel · May 20, 2021, 10:19am

Ok it seems that the life time in seconds is : lifeTime / ( 100 * updateSpeed ).

But I’m still unsure what is updateSpeed (number of cycle per ms?).

mawa · May 20, 2021, 12:52pm

Obviously the update speed is the time before each call for a particle frame update. It can be tuned independently because particles can heavily impact performance. So you can adjust the update speed of your particles to your best ratio of performance vs quality/smoothness for a particlesystem effect. You can do it separately for each PS.

cronvel · May 20, 2021, 2:42pm

Raising updateSpeed make things faster, so it is not the time (period, or delay) between update. It’s more a kind of frequency, but in which unit?

mawa · May 20, 2021, 2:46pm

Yes, it’s in the text updateSPEED. If you increase speed you will increase the number of times the particles frame is updated (and therefor your animation goes faster). And I’m not sure, but I believe it must be ms. However note that it will also depend on your framerate (as far as I could see from my tests).

Evgeni_Popov · May 21, 2021, 12:03pm

For GPUParticleSystem the lifetime would be in seconds, but for the (CPU) ParticleSystem it seems more complicated…

You can have a look here:

github.com

BabylonJS/Babylon.js/blob/master/src/Particles/particleSystem.ts#L1789-L1834

    
      
          this._scaledUpdateSpeed = this.updateSpeed * (preWarmOnly ? this.preWarmStepOffset : this._scene?.getAnimationRatio() || 1);
          
          
// Determine the number of particles we need to create
          var newParticles;
          
          
if (this.manualEmitCount > -1) {
              newParticles = this.manualEmitCount;
              this._newPartsExcess = 0;
              this.manualEmitCount = 0;
          } else {
              let rate = this.emitRate;
          
          
    if (this._emitRateGradients && this._emitRateGradients.length > 0 && this.targetStopDuration) {
                  const ratio = this._actualFrame / this.targetStopDuration;
                  GradientHelper.GetCurrentGradient(ratio, this._emitRateGradients, (currentGradient, nextGradient, scale) => {
                      if (currentGradient !== this._currentEmitRateGradient) {
                          this._currentEmitRate1 = this._currentEmitRate2;
                          this._currentEmitRate2 = (<FactorGradient>nextGradient).getFactor();
                          this._currentEmitRateGradient = (<FactorGradient>currentGradient);
                      }

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and:

github.com

BabylonJS/Babylon.js/blob/master/src/Particles/particleSystem.ts#L333-L516

    
      
          for (var index = 0; index < particles.length; index++) {
              var particle = particles[index];
          
          
    let scaledUpdateSpeed = this._scaledUpdateSpeed;
              let previousAge = particle.age;
              particle.age += scaledUpdateSpeed;
          
          
    // Evaluate step to death
              if (particle.age > particle.lifeTime) {
                  let diff = particle.age - previousAge;
                  let oldDiff = particle.lifeTime - previousAge;
          
          
        scaledUpdateSpeed = (oldDiff * scaledUpdateSpeed) / diff;
          
          
        particle.age = particle.lifeTime;
              }
          
          
    let ratio = particle.age / particle.lifeTime;
          
          
    // Color

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Particle system properties' unit

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