Computer Animation Ying Zhu Georgia State University

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Presentation transcript:

Computer Animation Ying Zhu Georgia State University Fluids

Computational Fluid Dynamics Blender uses the Lattice Boltzmann method (LBM) to simulate fluids For more details see http://en.wikipedia.org/wiki/Lattice_Boltzmann_methods http://math.nist.gov/mcsd/savg/parallel/lb/

Creating fluids The process Model the scene (objects, materials, etc.) Specify the “Domain” in which the fluid will flow Select the objects that may collide with the fluid and specify their functions Create the fluid sources and specify their parameters Bake the fluid simulation Play back the fluid simulation (with Alt + A, etc.)

Domain A “domain” is a bounding box for fluid simulation Use the bounding box of the object regardless of its shape Only one fluid domain is allowed for each scene All fluid objects must be in the domain No fluid will be simulated outside the domain Multiple fluids should not collide with each other in their initial position

Domain Add a cube to the scene, move and resize the cube to cover the area that you want the fluid to be simulated In “Object”, “Physics buttons” panel, “Fluid” tab Press “Fluid” button Press “Domain” button

Domain Parameters

Domain parameters: resolution Setting the resolution higher will give you better visual quality But it will require more memory usage Also the computation will be slower Adjust the resolution number based on your computer’s memory capacity Otherwise the computer may crash You can see the “Req. BAKE Mem.” number changes as you adjust Resolution

Domain parameters: time Start and End Time There are two time periods related to fluid simulation Simulation time: the Start and End time (in seconds) in the “Fluid” tab Animation time: the Start and End frames in the Anim(ation) tab under the Scene (F10) panel

Simulation time and Animation Time The “simulation time” is used by fluid physics solver to calculate the behavior of fluid The simulation is then spread over the frame numbers specified in the Anim(ation) tab in the final rendering Animation Time == Simulation Time: Slow motion Animation Time > Simulation Time: Slow motion Animation Time < Simulation Time: Fast Forward

Fluid Baking and Animation Time Frames When baking fluids, Blender will ignore the Start frame in the Anim tab Blender always bakes fluid from frame #1 to the End frame number specified in Anim tab

Simulation time and Animation Time Suppose in the Fluid tab (under Domain) you have Start Time = 0.0 (second), End Time = 10.0 (seconds) The fluid solver will simulate fluid for 10 seconds This means the simulation lasts 250 frames long The standard frame rate is 25 frames per second If in the Anim tab, the End frame number is 250, then the simulation will be played in regular speed when you play the animation

Simulation time and Animation Time If in the Anim tab, the End frame number is larger than 250, then the simulation will be played in slow motion when you play the animation If in the Anim tab, the End frame number is smaller than 250, then the simulation will be sped up when you play the animation

Domain parameters: bake Press the “BAKE” button to bake the fluid simulation Unlike particles and clothes, fluid baking is not automatically started when you press Alt + A You must BAKE first, and then press Alt + A to play back the animation You should specify the Path for storing files created during baking in the Fluid tab (default is /tmp) The baking progress is displayed in header bar

Domain Parameters: viscosity and real-world size Viscosity: resistance to flow in a fluid In the Fluid tab, under “Domain”, press “Ad” button, and adjust the viscosity Can choose predefined settings for water, oil, and honey Can manually adjust viscosity “Realworld-size”: the size of the Domain box in meters Adjust this number to match the size of the fuild in your virtual world

Domain parameters: Particles In the Fluid tab, under “Domain”, press “Par” button Can adjust Tracer Particles: generate particles to trace where the water level used to be Generate Particles: generate particles when water is splashed Compare the results with and without particles at http://wiki.blender.org/uploads/9/90/Manual-FluidSimParts.jpg

Fluid parameters Volume init Volume: the space enclosed by the object is treated as fluid (only works for closed mesh) Shell: the surface (and a thin shell surrounding it) of the object is considered as fluid (works for both closed and open mesh) Both (volume and shell)

Interacting with obstacle Select the object to be collided with fluid, in Fluid tab, press “Fluid” and then “Obstacle” Volume init: which part of the object will be colliding with the fluid Boundary type Noslip: the obstacle is sticky Free: the obstacle is slippery Part: somewhere in between Moving object must be “Noslip”

Inflow object Select an object, in Fluid tab, press “Fluid” and then “Inflow” There is no need to create a separate Fluid object This object will constantly inject fluid into the domain Can be used to simulate tap, fountain, etc. Adjust Inflow velocity (e.g. Z = -0.2) to change the volume of inflow fluid Enable “Local Coords” if the Inflow object is animated

Outflow object Select an object, in Fluid tab, press “Fluid” and then “Outflow” The fluid will disappear when it touches the Outflow object In other words, the Outflow object “kills” the fluid Can be used to simulate sink, etc.

Control object Make an object attract or repel fluid E.g. object with magic force Can make the simulation really slow Time: the time period (in seconds) the control object is activated Attraction force Velocity force How much influence of the control object velocity has on the fluid

Control object Example http://www.youtube.com/watch?v=WruTNnF6Ztg

How to create material for fluid? Select the Domain object Go to “Material Buttons” panel In “Mirror Transp” tab, press “Ray Transp” and set IOR (Index of Refraction) to 1.33 (one tutorial suggests 1.17) Press “Ray Mirror” and set “RayMir” to 0.11 Set Alpha to 0.109 Set Color to R=0.17, G=0.50, B = 0.89

Animating fluid parameters Blender has a special IPO Curve for Fluid Simulation Select the fluid Domain object, open an IPO Curve Editor window Select FluidSim type FluidSim IPOs cannot be keyframed by simply using the I key You must manually set values by pressing Ctrl + LMB in the IPO window.

Parameters that can be animated For fluid Domain object Fac-Visc: changes fluid viscosity Fac-Time: change the speed of the simulation 0 freezes the simulation 1.0 is normal speed < 1.0 slow motion > 1.0 fast forward GravX/GravY/GravZ: change gravity over time E.g. in space or in a speeding car (?)

Parameters that can be animated

Parameters that can be animated For Fluid, Inflow, or Outflow objects VelX, VelY, VelZ: animate fluid velocity Active: takes two values -- Zero or non-Zero Zero: the fluid disappear Anything above zero: the fluid appears Can simulate dripping by animating the Active parameter

How to make fluid simulation start later than frame #1? Note that fluid baking always starts from frame #1 Create a FluidSim IPO curve for the Fluid object’s “Active” parameter Make the fluid Active at the desired frame number

Parameters that can be animated For Control objects AttrForceStr: attraction force strength AttrForceRa: attraction force radius VelForceStr: velocity force strength VelForceRa: velocity force radius

Tutorials http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro/Realistic_Water_using_Fluid_Sim_and_Yafray