> simulations

FLIP

simulations/flip·v1.5.1

IN:1 OUT:4

SUMMARY

The FLIP operator turns the points of any POP into liquid particles that transfer their velocity to a grid, where gravity, viscosity, vorticity, surface tension and an incompressible pressure solve act, then back to the particles with FLIP or APIC transfer. It outputs the particles, white water particles, a meshed surface and the solved grid as a POPX field.

Simple mode applies one solver step to the input every frame, while Advect runs a feedback loop (Initialize, Start, Play) with a Target Feedback Loop POP for forces downstream. Viscosity can vary per particle for water and honey in one simulation, the ground and a mesh or field collider act as solid walls, and the surface renders with a refractive water shader or a PBR material.

PARAMETERS

PAGEFLIP

[+]Solver Mode	Solvermode	Simple is stateless and recomputed from the input each frame. Advect runs a feedback loop, started with Initialize then Start.
Target Feedback Loop POP	Targetpop	A reference to a POP node downstream in the network whose data is retrieved one frame later.
[+]Velocity Transfer	Velocitytransfer	Selects the velocity transfer. FLIP is splashy and energetic and uses PIC/FLIP Ratio. APIC is swirly with a clean surface and keeps rotation.
PIC/FLIP Ratio	Picflipratio	In FLIP mode, the blend of FLIP to PIC velocity. 1 is pure FLIP and lively, lower values are calmer and more damped.
Max Axis Resolution	Maxaxisres	Sets the number of voxels along the longest axis. The other axes follow the box proportions.
[+]Simulation Resolution	Simres	Shows the resulting grid resolution (read-only).
[+]Simulation Size	Simsize	Size of the simulation box, in world units.
[+]Simulation Center	Simcenter	Sets the center of the simulation box.
Display Bounds	Displaybounds	Display the simulation box.
Time Scale	Timescale	Speed multiplier on the time step. 0 freezes and 2 doubles.
Time Step	Timestep	The time step per frame in seconds, 1 divided by the cook rate (read-only). Time Scale multiplies it.
Substeps	Substeps	Number of solver substeps per frame. More substeps are more stable.
Initialize	Initializepulse	Initializes the POP from its input and parameters. The initial particles are spawned.
Start	Startpulse	Start the simulation from the initialized state.
Play	Play	Pause or play the feedback loop.
Step	Steppulse	Advance the simulation by one frame while paused.

PAGEProperties

Particle Separation	Particleseparation	Sets the rest distance between the particles in world units. It sets the rest density and the particle size.
Particle Radius Scale	Particleradiusscale	Scales the particle spacing used for push-apart, over-packing correction and the particle radius (PointScale).
Separation Iterations	Separationiters	Sets the number of push-apart passes per frame that keep the particles at their separation below grid scale. 0 turns it off.
Pressure Iterations	Pressureiters	Sets the number of pressure solve iterations. More iterations make the fluid less compressible but are slower.
Over Relaxation	Overrelaxation	Sets the over-relaxation of the pressure solve, from 1 to 1.95. Higher values converge faster, 1 is plain Gauss-Seidel.
Viscosity	Viscosity	Kinematic viscosity in units squared per second. 0 is inviscid.
Viscosity Attribute	Viscosityattrib	Sets an optional float point attribute that multiplies Viscosity per particle. When empty, Viscosity is used everywhere.
Viscosity Iterations	Viscosityiters	Sets the number of iterations of the implicit viscosity solve per frame, used only while Viscosity is above 0.
Slip Length	Sliplength	Sets the wall friction of viscous liquids in world units. 0 is no-slip, larger values slip more freely along walls and colliders.
Vorticity	Vorticity	Sets the amount of vorticity confinement, which restores small swirls. 0 is none.
Surface Tension	Surfacetension	Strength of the surface tension, which flattens the free surface and pulls small fluid into droplets. 0 is off.
Enable Surface Tension Blur	Surfaceblur	Blur the surface curvature before applying tension, so it follows the overall shape instead of cell noise.
Blur Radius	Blurradius	Sets the blur radius of the surface tension in voxels. Useful values are 1 to 3, up to 4.

PAGECollisions

Enable Ground Collision	Enablegroundcollision	Collide with an infinite ground plane.
Display Ground	Displayground	Display the ground plane.
[+]Ground Position	Groundposition	A point on the ground plane.
[+]Ground Direction	Grounddirection	Normal of the ground plane. 0 1 0 is a level floor.
Ground Collision Margin	Groundmargin	Distance the particles keep from the ground.
[+]Collision Type	Collisiontype	Selects the collider: none, a POP mesh (ray traced, Windows only) or a field used as an SDF volume.
Display Geometry	Displaygeo	Enable drawing the collider, as the mesh or the field's zero surface.
Collision Passes	Geocollisionpasses	Number of collider solves per substep. More passes hold fast moving geometry.
Collision POP	Collisionpop	The collider, either a mesh POP or an SDF field volume.
[+]Dist Attribute (SDF)	Distattr	For a field collider, the signed distance attribute of the Collision POP, negative inside.
Solid	Solid	When on, particles inside the collider are pushed out. When off, the collider is hollow.
Keep Inside	Keepinside	When on, the collider is hollow and every outside particle is pulled back in. When off, only escaping particles are caught.
Project	Project	Place every particle on the collider surface instead of colliding.
Collision Offset	Collisionoffset	Distance the particles keep from the collider surface.
Collision Damping	Collisiondamping	Velocity lost on contact. 0 bounces and 1 stops.
[+]Transform Order	Xord	Sets the transform order of the collider.
[+]Rotate Order	Rord	Sets the order of the rotations of the collider.
[+]Translate	T	Translate the collider in the three axes.
[+]Rotate	R	Rotate the collider around the X, Y and Z axes. Angles are given in degrees.
[+]Scale	S	Scale factor for each axis of the collider.
[+]Pivot	P	The pivot point for the rotate and scale of the collider.
Uniform Scale	Scale	Specifies a uniform scale factor for the collider.
[+]Display Color	Displaycolor	Sets the color of the displayed guides.

PAGEForces

[+]Gravity	Gravity	The gravity vector in units per second squared.
Gravity Multiplier	Gravitymultiplier	Multiplies the Gravity vector.
[+]Wind Direction	Winddirection	The direction the wind blows in. Its length does not matter.
Wind Speed	Windspeed	The wind acceleration along Wind Direction, in units per second squared.
Velocity Damping	Velocitydamping	Amount of damping applied to the velocity. 0 applies no damping.
Static Threshold	Staticthreshold	Sets the static friction coefficient. Contacts below it grip.
Dynamic Scale	Dynamicscale	Sets the kinetic friction coefficient that slows sliding contacts.
Limit Acceleration	Limitacc	Enable the particle speed cap set by Max Acceleration, to prevent instability.
Max Acceleration	Maxacc	Sets the particle speed cap, the same at any frame rate. 100 gives 16.7 units per second.

PAGEWhite Water

Generate White Water	Generatewhitewater	Generate spray, foam and bubbles from the fluid.
Display White Water	Displaywhitewater	Show the white water particles in the viewer.
Render White Water	Renderwhitewater	Render the white water in your Render TOP. Needs Generate Surface and Render Surface on.
Max White Particles to Allocate	Maxwhiteparticles	Sets the size of the white water particle pool, the most white water that can exist at once.
Precise Classification	Preciseclassification	Classify white water from the neighbors of the fluid particles instead of the grid. Gives finer detail at a higher GPU cost.
Trapped Air Rate	Trappedairrate	Sets the emission rate of white water from trapped air, in converging turbulent flow.
Trapped Air Range	Trappedairrange	Sets the low and high trapped-air potential, per second. Emission starts at the low value and is full at the high value.
Wave Crest Rate	Wavecrestrate	Sets the emission rate from wave crests, where the surface is convex and fast-moving.
Wave Crest Range	Wavecrestrange	Sets the low and high wave crest potential. Emission starts at the low value and is full at the high value.
Kinetic Energy Range	Kineticenergyrange	Sets the low and high kinetic energy that scale the emission. Slow fluid emits little, fast fluid the most.
Lifespan Range	Lifespanrange	Sets the minimum and maximum foam lifespan in seconds before it dissolves.
Spray Threshold	Spraythreshold	Sets the fraction of rest density below which a white water particle is ballistic spray.
Bubble Threshold	Bubblethreshold	Sets the fraction of rest density above which a white water particle is a bubble. Between the two thresholds it is foam.
Spray Drag	Spraydrag	Sets the quadratic air drag on the spray. Higher values slow the plumes and feather them out sooner, 0 is ballistic.
Bubble Buoyancy	Bubblebuoyancy	Sets the upward push on bubbles, as a multiple of gravity.
Bubble Drag	Bubbledrag	Sets how fast bubbles take on the surrounding fluid velocity, per second.
[+]Foam Color	Foamcolor	Sets the color of the foam and spray.
Foam Density	Foamdensity	Sets the optical depth of a foam puff. Higher values turn foam opaque white sooner.
Foam Scale	Foamscale	Sets the size of the foam and spray puffs as a multiple of the particle radius.
Foam Brightness	Foambrightness	Sets the exposure trim of the foam. 1 is physically based.
Foam Blur	Foamblur	Sets the blur of the foam medium in pixels, merging puffs into a continuous sheet.
[+]Bubble Color	Bubblecolor	Sets the color of the bubble shells.
Bubble Opacity	Bubbleopacity	Scales the bubble reflection and coverage from 0 to 1. At 1 each bubble is a mirror ring with a nearly clear center.
Bubble Scale	Bubblescale	Sets the bubble size relative to the particle radius, with 30 percent random variation. 0.5 gives small champagne bubbles, 1.5 large ones.
Bubble Brightness	Bubblebrightness	Sets the exposure trim of the bubbles. 1 is physically based.
Underwater Softness	Underwatersoftness	Sets how much the bubbles and foam soften with depth under the water. 0 keeps them as sharp deep as shallow.
Scale Speed	Scalespeed	Sets how fast the white water particle size animates on birth and on changing type, per second.
Fade Time	Fadetime	Sets the time in seconds before death over which a white water particle shrinks to zero, so it does not pop out.

PAGESurface

Generate Surface	Generatesurface	Enable building a liquid surface mesh from the particles.
Display Surface	Displaysurface	Show the surface in the viewer.
Render Surface	Rendersurface	Whether to render the surface in your Render TOP.
Render TOP	Rendertop	The Render TOP whose camera and lights draw the surface.
Max Axis Resolution	Surfacemaxaxisres	Sets the surface grid resolution along the longest axis of the simulation box.
[+]Field Resolution	Fieldres	Shows the resulting field resolution (read-only).
Kernel Radius	Kernelradius	Sets the radius of the particle averaging, in particle spacings. Larger values are smoother.
Particle Radius	Particleradius	Sets the radius of each particle in the surface, in particle spacings. Values below about 0.5 leave holes.
Adaptive Radius	Adaptiveradius	Grows the radius up to this many times where particles are sparse, so that drops and sheets hold. 1 disables it.
Concavity Range	Concavity	Sets the low and high concavity range, which pulls the surface in at concave gaps to remove spurious blobs.
Field Blur Iterations	Fieldbluriters	Sets the number of Gaussian blur passes on the surface field. 0 disables the blur.
Field Blur Radius	Fieldblurradius	Sets the blur radius of the surface field, in voxels.
One-Sided Blur	Onesidedblur	When on, the blur only fills dimples between particles and never erodes droplets or crests.
Curvature Flow Iterations	Curvatureiters	Sets the number of curvature flow steps near the surface. Flattens particle bumps while keeping broad waves.
Protect Radius	Protectradius	Curvature flow fades where the surface bends tighter than this, in particle spacings. 0 turns it off.
Density Mask	Densitymask	Sets the particle densities, from 0 to 1, where curvature flow and mesh smoothing start and reach full strength.
Erode / Dilate	Erodedilate	Offsets the surface in voxels. Positive values grow the liquid, negative values shrink it.
Open / Close	Openclose	Morphology in voxels. Positive values close small holes and gaps, negative values remove thin bits and specks.
[+]Mesher	Mesher	Selects the mesher. Surface Nets gives a connected, smoothable mesh with fewer triangles. Marching Cubes gives a triangle soup without smoothing.
Mesh Smoothing Iterations	Meshsmoothiters	Sets the number of volume-preserving smoothing steps on the Surface Nets mesh, which remove grid stepping and orange peel.
Mesh Smoothing Strength	Meshsmoothstrength	Sets the smoothing amount per step. 0.5 is standard.
Refractive	Refractive	Render the surface as refractive water or glass. When off it is opaque and uses Diffuse Color.
Environment Map	Envmap	Sets the equirectangular map TOP seen through the water, and reflected when the scene has no Environment Light.
IOR	Ior	Index of refraction. Water is 1.33.
Refraction Strength	Refractionstrength	Sets how much the water bends light. 0 is none, 1 is physically based, 2 is strong.
Units per Metre	Unitspermetre	Sets the world units per meter, which scales absorption and scattering. Larger values give clearer water.
[+]Absorption Coefficients	Absorptioncoef	Sets the light absorbed per meter in red, green and blue. Pure water is 0.38, 0.07 and 0.012, so deep water turns blue-green.
Absorption	Absorption	Multiplier on the absorption coefficients. A value of 1 is pure water.
Scattering	Scattering	Sets the light scattered per meter. Water is about 0.002, murky water 0.2 to 1.
[+]Scattering Color	Scatteringcolor	Sets the tint of the scattering particulates. White is neutral.
Phase G	Phaseg	Sets the scattering direction. 0.8 scatters forward like natural water, 0 scatters evenly in all directions.
Reflectivity	Reflectivity	Sets the reflectance at normal incidence. Water is 0.02.
Roughness	Roughness	Surface roughness. Blurs the light reflections, the environment and the refraction.
Ambient Occlusion	Ambientocclusion	Scales the contribution of the Environment Light, from 0 to 1.
[+]Diffuse Color	Diffusecolor	Sets the body color when Refractive is off, as for milk or paint.
Specular Level	Specularlevel	Sets the specular strength of the surface PBR material. Only used when Refractive is off.

PAGEVisualize

Color by Speed	Colorbyspeed	Enable coloring the particles by speed through the ramp.
Max Speed	Maxspeed	Sets the speed that maps to the end of the color ramp.
[+]Color Ramp	Colorramp	Selects the color ramp preset. Custom uses the ramp editor or a TOP.
Open Color Ramp Editor	Opencolorrampeditor	Open the color ramp editor.
Reset Color Ramp	Resetcolorramp	Resets the color ramp.
Custom Color TOP	Customcolortop	Sets a TOP used as the color ramp, overriding the ramp editor.

PAGECommon

Free Extra GPU Memory

Freeextragpumem

Free memory that has accumulated when output memory has grown and shrunk.

INPUTS

Input 0 POP - Particles In

OUTPUTS

Output 0 POP - Particles Out

Output 1 POP - White Water

Output 2 POP - Surface

Output 3 POP - Field