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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.

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.

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Add a Bend modifier to give the poly planes a slight curve. This will spread the emitted fluid in an arc rather than producing a dull, cubic shape.

Change the Bend Axis for both planes to X.

If you'd like your setup to be identical to the provided scene files, here are the Bend Angle values for the planes:

Right Left Plane: [Bend Angle: -25 34]

Left Right Plane: [Bend Angle: -34 25]

You may want to experiment with the Bend Angle parameter - different values will produce slightly different results.

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Add a Shell modifier to both planes and leave its settings at their default values.

The purpose of this step is to give the planes volume by turning them into an actual 3D shape with thickness. Phoenix (and fluid simulators in general) don't work well with open geometry.

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Create 2 two Phoenix Liquid Sources. Position them next to the planes and give them meaningful names so you can keep organized and focused.

Select the Phoenix Source on the right and use the Add button to add the poly plane on the right as its source geometry.

Repeat this step for the Phoenix Source on the left.

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Create a Phoenix Liquid Simulator that envelops both source geometry objects.

Under the Grid rollout, set the Scene Scale to 2. The Scene Scale parameter will affect most of the dynamics in the simulatorSimulator. A smaller value will produce splashier, more turbulent movement while a larger Scene Scale will give the impression of a heavier, more inert fluid. You may experiment with this parameter but keep in mind that you will also have to adjust the Outgoing Velocity of the Phoenix Sources as well.

Enable Adaptive Grid and Maximum Expansion. The red box in the screenshot to the right is a preview of the Maximum Bounds for the simulation. Adaptive Grid is a huge time saver - the initial grid is dynamically expanded to accommodate the movement of the fluids, cutting down on both processing time and memory. If you notice any clipping, increase the Extra Margin to a value of 5 - 10. This will add a few extra voxels at the borders during adaptation.

If you'd like your setup to be identical to the provided scene file, here are the exact values for the Grid parameters:

Cell Size is set to 0.64cm for the final simulation.

The X/Y/Z dimensions of the grid Grid are 90/650/70 respectively.

The Adaptive Grid Maximum Expansion settings are [200, 200] for X, [0, 0] for Y and [50, 200] for Z.

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Now that we have a Simulator, Sources and source geometry, we are ready to start the R&D process.

Reduce the resolution of the grid by hitting the Decrease Resolution button once. This will reduce the number of voxels in the grid by half, thus reducing the simulation time.

You can easily go back to the final resolution by pressing the Increase Resolution button.

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Hit the Start button and wait for 50 or so frames.

To get a preview of the liquid surface, you can enable Preview → Show Mesh.

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Here's how the simulation looks so far:

You can see from the Preview Animation above that the default behavior of the Phoenix Liquid Source is to emit liquid from the surface of the source geometry. Phoenix provides you with 2 ways to direct the discharge:

1. Using Polygon/Face IDs to limit the emission to manually specified faces.
2. Using Discharge Modifiers on the Liquid Source to limit the emission based on the position, orientation or animation speed of the faces.

In this tutorial, we use the simpler option 1.

Add an Edit Poly modifier and go into Face selection mode. Select the faces pointing towards the Origin and give them a unique material Material ID. By default, all faces have an ID of 1, so switching the ID to 2 for the selected faces should do the job.

The Phoenix Liquid Source in 3ds Max can use the polygon face IDs as a 'mask' - only the faces with a given ID will be used for emission.

Repeat this step for the other poly plane.

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Select each Phoenix Liquid Source and set the Polygon ID parameter to the same value that you gave the source geometry faces in the previous step.

This will force the liquid source to only emit from the polygons with the specified ID value.

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No Polygon IDs for the Liquid Source

Currently, the emission is too weak. Instead of colliding with each other and producing an interesting splash, the liquids emitted from the 2 two sources simply fall down.

Increase the Outgoing Velocity parameter of the Liquid Sources. If you'd like your setup to be identical to the provided scene files, here are the exact values:

Right Left Phoenix Source: [Outgoing Velocity: 450cm 400cm].

Left Right Phoenix Source: [Outgoing Velocity: 400cm 450cm].

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Here is the simulation with a higher Outgoing Velocity rate:

Note the stepping in the emission. This is caused by a combination of factors - the high Outgoing Velocity of the source and the low Steps per Per Frame of the simulatorSimulator.

The Steps per Per Frame (SPF) parameter controls how many calculations are made between 2 two consecutive frames. This includes the emitters as well. If you see stepping in the emission, increasing the SPF is most likely your best bet.

Select the Phoenix Liquid Simulator and set the Steps per Per Frame parameter to 2. This value should suffice for the current setup.

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Because the liquid is moving too fast, we will reduce the Gravity and Time Scale from the Dynamics rollout of the Simulator.

Here are the exact values:

Gravity: Set the Gravity to 0.5
Time Scale: 0. 5 The Gravity parameter is self-explanatory - the liquid will fall down slower when this value is lower.

The  Set the Time Scale, however, warrants an explanation. to 0.5. The Time Scale is very similar to the Steps per Per Frame parameter in a sense that it also scales down the velocities in your simulation, thus removing detail from the simsimulation. You should be mindful of this when tweaking your setup.

If you'd like to significantly slow down a liquid simulation, a better approach would be to use the Time Bend Controls section of the Input rollout. We don't make use of these as the RGB channel blending (for slow-down) is not supported at the time of writing this tutorial.

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Gravity: 1 and Time Scale: 1

Set the Default Viscosity to 0.01. Viscosity emulates thickness - the higher this value is, the more the liquid will resemble thick mud, honey or tar.

Set the Surface Tension parameter values as followsSet the Surface Tension parameter values as follows:

Strength: 0.5;
Droplet Breakup: 1.0;
Droplet Radius: 2.5.

Strength controls the force produced along the curvature of the liquid surface - this keeps the liquid from breaking into individual particles when a force is applied to it.

Droplet Breakup controls balances between the liquid forming tendrils or droplets. Larger values will produce more droplets.

Droplet Radius controls the radius of the droplets formed by the Droplet Breakup parameter (in voxels). Therefore, if you increase the resolution of the grid, you should also increase the Droplet Radius.

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No Viscosity and Surface Tension tweaks

Enable RGB on both Phoenix Sources.

Set the RGB color for the LEFT source to pure white (255, 255, 255).

Set the RGB color for the RIGHT source to black [ (1, 1, 1]). The value is set to (1, 1, 1) instead of (0, 0, 0) so we can preview the channel in the Preview rollout of the simulator.Set the RGB color for the LEFT source to pure white [ 255, 255, 255 ].

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Enable RGB channel export from the Output rollout of the Phoenix Simulator.

If disabled, the RGB channel will not be cached to disk and cannot be queried by the Phoenix Grid Texture later on.

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To get a viewport preview of the RGB channel, disable Mesh Preview (if enabled) and enable RGB at the bottom of the Voxel Preview section. You may also disable all other Voxel Preview channels so they don't interfere.

Disable the Auto Range checkbox (this tries to automatically find the min/max range of each channel and adjust the preview range accordingly). Set the RGB max value to 1.0, and the RGB min value to 0.001.

If you now Start the sim, you should get a preview of the RGB channel in the viewport.

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Here is how the RGB channel looks:

Here are the values for the Stucco Parameters rollout:

Size: 10;
Thickness: 0.14;
Threshold: 0.43.

You may experiment with those - the emission pattern will be heavily affected.

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Select one of the Phoenix Sources and Left-Mouse-Button drag from the Stucco output to the Outgoing Velocity Mask parameter.

This will set up the Stucco to work as an emission modulator - black values will emit no liquid, white values will emit with full Outgoing Velocity.

Make sure to choose the Instance setting (as the Copy option will create a copy of this map - you want to use the same texture for both sources instead of being forced to deal with 2 separate Stuccos).

Repeat this step for the other Phoenix Source.

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Here's how the emission looks when modulated by the Stucco.

Note that the break-up is static. We will animated the Offset of the Stucco texture to resolve this.

To animate the Offset parameter from the Coordinates rollout of the Stucco texture, simply enable Auto Key from the bottom right of the 3ds Max window, go to frame 100 and set the Offset X value to 100. This should set the Offset X such that it is equal to 0 at frame 0 and 100 at frame 100.

Note that the Offset parameter value does not update on Timeline changes. The best way to make sure that the keyframes are properly set up is to drag the Time slider at the bottom and look for changes in the Viewport. The texture should be 'swimming' over the surface of the source geometry.

Consider setting the tangents for the Offset animation to linearLinear.

If you can't see the animation curve for the Offset in the Curve Editor, go to View → Filters and disable all options under the Show Only dialog. You should now be able to find it under the VRayMaterial → Diffuse map parameters.

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Here's the emission when modulated by the Stucco with an animated Offset parameter.

Enable Liquid Channel Smoothing from the Input rollout. This option applies a smoothing algorithm over the cache as its read back into the simulator Simulator for preview and rendering.

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Open the Rendering rollout of the Phoenix Simulator and set the Smoothness parameter to 100. This is yet another smoothing option dedicated especially to meshes, available to you to combat grid artifacts and produce a clean liquid surface.

Enable Use Liquid Particles and set the Particle Size to 0.8. The Liquid Particles option allows the meshing algorithm to produce a surface with varying thickness. You can use the Particle Size parameter to control the thickness of the mesh.

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No Smoothing

We are now ready to Start the simulation at the final Cell Size of 0.64cm.

You may want to set the Preview rollout → Read Cache for Preview option to Disable During Sim - this will save some time as the CPU and Graphics card won't be bogged down by the smoothing operations over the cache files read by the simulator Simulator on each frame.

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Create a VRayBlendMtl and assign it to the Phoenix Simulator.

Then, create 2 two more materials - a VRayFastSSS2 material and a VrayMtl.

Plug the VRayFastSSS2 into the Base slot of the Blend material.

Plug the VRayMtl into the Coat1 slot of the Blend material.

Here's how this setup is going to work: we use a Phoenix Grid Texture to read the RGB channel of the Simulator and use that as the blending factor between the 2 two materials. The VrayFastSSS2 material will be revealed where the RGB channel is black, the VRayMtl will show where the RGB is white, and the RGB values in-between will produce a mix between the two.

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The camera is placed right in front of the Phoenix Simulator. You can experiment with different angles.

Exposure Gain is set to Manual 100 ISO and the Shutter speed to 1/600 - this should give you a balanced exposition that you can easily tweak further in a compositing package or the V-Ray Frame Buffer.

Aperture is reduced to 1.4 and Depth of Field is enabled. A small aperture in conjunction with Depth of Field produces a beautiful blooming effect of the highlights that are out of focus.

To further tweak this, the Aperture Shape is set to Bladed, with the Blades set to 7 and the Rotation to 15 degrees.

Feel free to experiment with these settings and find something that suits your taste.

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The lighting in the scene is crucial for a good looking render. 

The key light in this scene is the V-Ray Plane light, placed above of the simulation grid.

The Half-Length/Width values are: [230cm, 350cm].

The Intensity Multiplier is set to 10 5 and the Invisible option is enabled under the Options rollout.

A VRaySoftbox texture is applied to modulate the light intensity. The parameters for it are provided in the next step of this tutorial.

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Finally, a VRay Sphere Light is used for backlighting. Recall that the milk is using a VRay FastSSS2 shader - the backlight will bring out the translucency and help produce more believable shading.

The Multiplier value is set to 30 and the Mode is set to Color RGB (144, 105, 98).

The Invisible option is enabled under the Options rollout.

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A regular poly plane is used as a backdrop for rendering. A default VRayMtl is assigned to it with the only change being the value of the Diffuse color - [ RGB (10, 10, 10 ]).

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Now that we are happy with the render result, we can make small adjustments to the image directly from the V-Ray frame buffer.

Here we have tweaked the Exposure, the White Balance and the Hue/Saturation Saturation to make the image more livelyinteresting.

Also, a V-Ray Denoiser Render Element is added to the final image. The Denoiser takes an existing render and applies a denoising operation to it after the image is completely rendered in order to remove the noise in the image.

Experiment with these parameters to find the best photorealistic look for your beautiful Chocolate Milk fusion. 

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