Environmental Animation - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 373

Smoking Is Good for You
Although it also won't appear in the “final” version of the scene, let's add some smoke to the room.
Blender's smoke simulator is fairly easy to use, and produces cool results even at lower resolutions. It is
based around particle systems, which you were just introduced to, and another material type: volume.

Smoke is emitted from particles, so to begin the process of creating smoke, we need a particle system.
Smoke will continue to pour from any particle that is still alive, so it is important to decide what you
would like your smoke to do. Should there be a burst of smoke that rolls into the sky, like an explosion?
Make a particle system that emits all of its particles within a few frames, and of which the particle life is
very short. Should it be a constant emission, like a smoldering fire? Create a particle system that emits
over the length of your animation, or one that emits all at once with long particle lifetimes. So before
you even add a smoke simulation, get your particle system squared away, which is a job in its own right.
So we don't have to create and tweak an all new particle system, let's use the one from the previous
example. If we were to leave it just as it is, there would be a ton of smoke flying around, so reduce the
total particle amount from 1000 to 100, and reduce the particle life to around 3 or 4. Bake the particle
system so that you don't get any shenanigans involving particles not updating properly. Of course, you
could leave these values as they are and use your original bake but we're going for speed since we're
learning the ropes.

To enable the first part of the smoke simulation, make sure that the cube toy is still selected and LMB
click the Add button in the Smoke panel of the Physics properties. This brings up a set of buttons with
the labels: None, Domain, Flow, and Collision . The cube and particle system will be the Flow object,
which means that the smoke system uses the particles to designate the flow of the smoke. Setting the cube
to Flow brings up the Flow controls, shown in Figure 13.17 .

Because your object might have more than one particle
system, a selector is available at the bottom of the panel.
Choose the particle system that we've just tweaked. Temp
Diff affects the speed with which the smoke moves. At 1.0,
the default, the smoke and surrounding “air” are balanced,
so the smoke spreads, dissipates, and generally mixes with
the air at a fixed rate. If you raise this difference (10.0 is
the max) it puts more energy into the system, causing things
to mix and spread faster. That's the flow control.

Figure 13.17 The Flow object's smoke controls.

It would be nice if Blender could just determine a smoke simulation based on that information, but such
calculations are labor and memory intensive. To make it work, we have to confine the smoke calculations
to a finite area. This is done by adding a cube to your scene, and sizing it to it the area in which the
smoke will live. Particles that are emitted outside of this cube will generate no smoke. Smoke that rises
(or sinks) outside of the cube vanishes. The cube can be scaled and rotated to best fit the scenario, but
only a rectangular object will work. It you add a sphere, or some kind of strange extruded shape, Blender
will simply use that shape's bounding box. To keep it simple, always use a cube. This cube within which
the smoke simulation takes place is called the Domain .