Using TexUVW for Creating Thin Smoke

Phoenix 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 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 4.10 Nightly Build from 11 March 2020 and V-Ray Next Official Release for 3ds Max 2016 at least. You can download nightlies from https://nightlies.chaos.com or get the latest official Phoenix and V-Ray from https://download.chaos.com. If you notice a major difference between the results shown here and the behavior of your setup, please reach us using the Support Form.

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

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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 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 Fire/Smoke Simulator.
3. Phoenix 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.

Set the Time Configuration → Animation Length to 200 so that the Timeslider goes from 0 to 200.

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).

Assign an Edit Poly modifier to the Box geometry and go into Face Selection mode.

Select all faces and give them an ID of 1.

Then, select only the top face which is used for emission of liquid and assign its ID to 2. This is the surface we are going to emit fluid from in the following steps.

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 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 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 Simulator → Output rollout and enable the Texture UVW.

Go to the Phoenix 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 Simulator → Preview rollout → GPU Preview → Enable in Viewport.

Start the simulation. To the right is a Viewport Preview showing the result of the simulation so far.

As you can see in the preview animation, we have unwanted fire in the smoke.

Keep the simulation unchanged. Go to Simulator → Rendering rollout, click on the Volumetric Options, disable the Fire.

We still need to simulate the temperature so it would make the smoke rise up, so we don't disable it from the simulation via the Output rollout, but instead just turn it off for rendering.

Set the Smoke Color to light gray color (RGB = 245, 245, 245). Also, disable the Scattering.

Now the smoke appears too thin, so let's increase the Simple Smoke Opacity to 0.9 (found in the Smoke Opacity rollout).

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

Our final goal for the simulation is a smooth, natural-swirling, cigarette smoke look. So all of the following steps aim at this goal. Now the smoke appears to be running too fast.

To slow it down, change the Time Scale from 1.0 to 0.7.

Here is how the simulation looks so far.

Create Helpers → Phoenix FD → PHXTurbulence.

Position to (X, Y, Z): (0, 0, 0).

Set the Strength to 40.0 and the Size to 2600.0cm.

Reduce the Fractal Depth to 2.

The Turbulence is used to bring more swirls in the smoke. You can use the Preview rollout's Force preview in order to see what velocities the turbulence adds to the simulation. Note that you have to turn off the GPU preview in order to see the forces.

Then go to the Fire/Smoke Simulator and run the simulation again.

Here is a 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'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).

During the scene setup we use Scanline Renderer as it's good enough for this purpose.

For the final render we will use V-Ray Next Official Release.

Set the Time Output Range from 0 to 120. Press the Render button to render the animation.

Here's a rendered animation so far.

Now, let's modulate the Smoke Opacity.

Set the Smoke Opacity to be Based on Simple Smoke.

Enable the Modulate option.

Keep the Noise texture that we used in the previous step and change the following settings:

Set the Size to 0.2;

Noise Threshold High to 0.5, Low to 0.45;

Levels to 2.0.

The images illustrate what the Modulation does when you modulate smoke with a Noise texture. On the left, we have smoke's opacity Based on Simple Smoke channel data. When Modulate option is enabled, and there is a Noise texture in the texture slot, it multiplies the Noise texture to create the final image on the right. You can think of it as if we are using a mask over the Noise texture in order to keep only the "islands" inside the smoke volume and remove the ones which are outside the simulated smoke. This is how you get this wispy smoke result. The key is having most of the Noise texture black and keeping only these high contrast spots in it, which are dragged along and stretched by the fluid.

Here is a Viewport Preview showing the result of the simulation so far. You can see that currently there is an issue in the video. Some smoke areas are appearing out of nothing during the animation. This is because as the fluid travels through space, it meets new "islands" of opacity and starts dragging them along. Which makes parts of the fluid suddenly appear.

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's grid Texture UVW.

Here is a rendered animation of the simulation so far.

The smoke looks quite choppy, but we are going for a smoother look. The noise in the smoke is caused by the Vorticity parameters in the Dynamics rollout of the Simulation. We can resolve this by reducing the Smoke Surface Vorticity to 0.

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

Now we have smoother simulation results.

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.

Here is an animation showing what we've achieved so far. You can see the better swirling and rolling of the smoke. The behavior is now more like that of a thin smoke.

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 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 Simulator → Simulation rollout and enable the Stop Frame Timeline. Press the Start to run the simulation.

Now that our simulation is ready, switch the Renderer to V-Ray Next Official Release.

Let's render the full sequence from frame 0 to 200.

And this is our final animation.

We have already finished this tutorial. But since Phoenix 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.

Here we summarized the pros and cons of the two methods.

The technique we demonstrate in this tutorial creates softer and smoother results. The simulation and rendering are very fast compared to the particle-based ones. However, you can get more accurate results when using the cigarette smoke quick preset.

Feel free to use whichever method suits you best for a project.