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Phoenix FD 4.10 offers a new feature called TexUVW that allows fluids to transport UVW information along with the moving fluid. Before that, mapping the fluid's color or opacity for rendering had the texture maps fixed in one place as the fluid flows through them. In this tutorial, we are going to use this new grid data for controlling the rendering opacity of smoke.

Although Phoenix FD already provides a Cigarette Smoke Quick Preset, it uses a particle-based method that requires a large number of particles small enough to produce smooth thin smoke. This could require a lot of memory and could be quite slow to simulate if you need to render it in high resolution. In this tutorial, we show you an alternative method. First, we simulate regular smoke (non-particle based) and then use a Noise texture to modulate the opacity of the smoke. In the end, we can have convincing thin, wispy, cigarette-type smoke results.

This simulation requires Phoenix FD 4.10 Nightly Build from 11 March 2020 and V-Ray Next Official Release at the least. If you notice a major difference between the results shown here and the behavior of your setup, please send an email to support@chaosgroup.com.

The Download button below provides you with an archive containing the start and end scenes.

Scale is crucial for the behavior of any simulation. The real-world size of the Simulator in units is important for the simulation dynamics. Large-scale simulations appear to move more slowly, while mid-to-small scale simulations have lots of vigorous movement. When you create your Simulator, you must check the Grid rollout where the real-world extents of the Simulator are shown. If the size of the Simulator in the scene cannot be changed, you can cheat the solver into working as if the scale is larger or smaller by changing the Scene Scale option in the Grid rollout.

The Phoenix FD solver is not affected by how you choose to view the Display Unit Scale - it is just a matter of convenience.

The thin smoke effect we're creating is about 1 meter in height so we choose to view the units as Centimeters.

Go to Customize → Units Setup and set Display Unit Scale to Metric Centimeters.

Also, set the System Units such that 1 Unit equals 1 Centimeter.

The final scene consists of the following elements:

1. Box geometry used for the smoke source.
2. Phoenix FD Fire/Smoke Simulator.
3. Phoenix FD Fire/Smoke Source emitting Smoke from the Box geometry.
4. Target Directional Light for lighting.
5. V-Ray Physical Camera for rendering.
6. PHXTurbulence for disturbing the smoke motion.

Create a Standard Primitives → Box. The box is used as an emission object for the smoke.

Set its Length / Width / Height to 3.0 cm / 32.0 cm / 3.0 cm.

Set its Length / Width / Height Segs to 1.

Position it to (X, Y, Z): (0, 0, 0). 

Go to Create Panel → Lights → Standard and add a Target Directional Light in the scene. Enable the Atmosphere Shadows option in the Shadow Parameters rollout.

Exact position of the light is (X, Y, Z): (0, -41, 300).

Exact position of the light target is (X, Y, Z): (0, 0, 0) (targeted at the box).

Add a Helpers → Phoenix FD → Fire/Smoke Source. The Fire/Smoke Source is a Phoenix FD helper node used to tell the Simulator which objects in the scene emit, how strong the emission is, etc.

Add the Box geometry to the Emitter Nodes list.

Set the Polygon ID to 2. This way smoke is emitted only from the faces with this polygon ID.

Go to Create → Geometry → PhoenixFD → FireSmokeSim.

The exact position of the Phoenix FD Simulator in the scene is (X, Y, Z): (0, 0, 0).

Open the Grid rollout and set the following values:

• Cell Size: 1.0 cm;
• Size (X, Y, Z): (46, 24, 11);
• Container Walls → all Open;
• Adaptive Grid: Smoke; 
  The Adaptive Grid algorithm allows the bounding box of the simulation to dynamically expand when needed. With a Threshold of 0.02, the Simulator expands when the cells near the walls of the simulation bounding box reach a Smoke value of 0.02 or greater.
• Extra Margin: 5;
  The Extra Margin option is useful when the Adaptive Grid algorithm is unable to expand fast enough to accommodate quick movement in the simulation (e.g. an explosion). The Extra Margin attempts to remedy this by expanding the grid preemptively with the specified number of voxels on each side.
• Enable Expand and Don't Shrink - this way the Adaptive grid will not contract back when there is very thin smoke at the borders of the grid.
• Enable Max Expansion: X: (193, 151), Y: (98, 105), Z: (0, 189) to save memory and simulation time by limiting the Maximum Size of the simulation grid.

We want to modulate the smoke opacity with Grid Texture UVW, so let's go to the Phoenix FD Simulator → Output rollout and enable the Texture UVW.

Go to the Phoenix FD Simulator → Simulation rollout and disable the Stop Frame Timeline. Set the Stop Frame to 120. Press the Start button to simulate.

To enable the GPU Preview as seen in the video below, select the Phoenix FD Simulator → Preview rollout → GPU Preview → Enable in Viewport.

Without re-running the simulation, here is the Viewport Preview showing the result of the simulation so far.

To preview exactly how the TexUVW mapping looks like, go to the Simulator / Rendering rollout and click on the Volumetric Options. In the Smoke Opacity rollout change the Based on to Texture. Plug a Noise texture in the Texture slot.

As shown, set the Noise Type to Turbulence, Size to 0.8.

Noise Threshold High to 0.7, Low to 0.45.

Levels to 2.0.

For the Coordinates, set Source to Explicit Map Channel. By doing so, we can take advantage of Phoenix FD's grid Texture UVW. Also, set the UVW tiling to 2.0.

All of these Noise texture settings are for demonstration purposes, so we can clearly see how the growing smoke distorts the texture UVW and affects the applied Noise texture.

To alleviate grid artifacts during render time, set the Sampler Type to Spherical (found in the Rendering rollout).

Here's a rendered animation so far.

Now you understand how this technique works and its possible side effects.

Let's adjust the final Noise texture for the texture slot of the Smoke Opacity. The settings for the texture are now optimized in order to minimize the unnatural look of suddenly appearing/disappearing islands of smoke.

Plug in a new Noise texture in the Texture slot.

As shown, set the Noise Type to Turbulence.

Size to 0.1.

Noise Threshold High to 0.8, Low to 0.32.

Levels to 5.0.

Keep the UVW Tiling to 1.0.

For the Coordinates, set Source to Explicit Map Channel. By doing so we can take advantage of Phoenix FD's grid Texture UVW.

To achieve a better swirling motion in the simulation, we can change the conservation mode and increase the Conservation Quality parameter under the Dynamics rollout. Set the Conservation Mode to PCG Symmetric, Quality to 50.

The PCG Symmetric conservation mode is ideal for smoke simulations or explosions because it tends to produce strong conservation which needs less conservation quality in order to look realistic. This method is also used for the Cigarette Smoke quick preset. So we choose this mode.

Go to the Fire/Smoke Simulator, and run the simulation again.

Now, in order to make our animation more interesting, we can animate the Time Scale. This section of the tutorial is optional – you may skip to the Rendering section below.

To create the bullet-time effect, open the Dynamics rollout of the Phoenix FD Simulator and animate the Time Scale parameter as follows:

Frame 0: 0.7
Frame 100: 0.7
Frame 130: 0.3
Frame 145: 0.3
Frame 164: 0.7

The Time Scale is a global multiplier for the dynamics of the Simulator. Reducing it uniformly scales down all sources of velocity in the simulation.

Now we are ready for the final simulation. Go to the Phoenix FD Simulator → Simulation rollout and enable the Stop Frame Timeline. Press the Start to run the simulation.

We have already finished this tutorial. But since Phoenix FD comes with a Cigarette Smoke Quick Preset, let's see the same settings as shown in this tutorial but using the quick preset.

The quick preset is a particle-based method, which emits a large amount of particles from Fire/Smoke source and renders with the help of the Particle Shader. Because of the nature of this method, you need lots of particles in the scene. Furthermore, the particle size has to be small enough to alleviate grainy look. See the rendered clip of the cigarette smoke quick preset simulation.