When you play the simulation, notice that the nParticles are not dissipating into the air like smoke does. This is because the nParticles lifespan is not yet defined.

In this lesson, you make the nParticles disappear from the scene after they reach a specified age by using the nParticle Lifespan attributes. You can set the nParticle object’s size (Radius) according to its lifespan so that nParticles decrease or increase in size as they age.

To set nParticle Lifespan

1. In the Outliner, select nParticle_Smoke.
2. In the Attribute Editor, click the nParticle_ShapeSmoke tab.
3. In the Lifespan section, select Random range from the Lifespan Mode list.
4. Play the simulation.

   The nParticles now die (disappear from the scene) shortly after they are emitted. nParticle Lifespan is measured in seconds, meaning that at a Lifespan of 1.0, the nParticles die after one second.

   

5. Set Lifespan to 5.
6. To add some randomization to the nParticle’s Lifespan, set Lifespan Random to 5.
7. Rewind the simulation and play it back.

   The nParticles die off more randomly, but the nParticle object is still not dense enough to look like smoke. You can make your nParticle object denser by increasing the nParticle Radius.

   

For a realistic smoke simulation, the nParticles must gradually disperse as they rise in the air. You can use the Radius Scale ramp to increase the radius of nParticles as they age, which creates the effect of smoke dispersing until it gradually disappears.

To set nParticle size

1. In the Particle Size section, set Radius to 5.0.
2. In the Radius Scale section, select the first marker in the ramp, set Selected Position to 0.016, and set Selected Value to 0.04.

   Creating a new marker allows you to set two Selected Value and Selected Position values. Each Selected Value specifies a scaling value for the nParticle object's Radius attribute. By default, ramps define a scaling value of 1, meaning that only one mark exists on the ramp and that marker represents an attribute scaling factor of 1. Setting a selected position to 0.5 sets per-particle radius to half the Radius value.

3. Click in the ramp to create additional markers and set the Selected Position and Selected Value for the new markers to the following values:

   Marker	Selected Position	Selected Value
   2	0.732	0.180
   3	0.882	0.480
   4	1.0	1.0

   

   For each point in the ramp, leave Interpolation at its default setting of Linear.

4. To map nParticle Radius to the nParticles’ age, set Radius Scale Input to Normalized Age.

   When Normalized Age is used, per-particle radius is mapped within the range of the nParticle object’s lifespan.

5. To add randomization to the per-particle radius, set Radius Scale Randomize to 0.25.
6. Play the simulation.

   You can continue to adjust the per-particle radius by adding more markers to the ramp, or optimize the ramp values as the simulation plays back.