Controlling Fluid Simulations with Custom Fields in Houdini Master ...

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Figure 6: The various microsolvers available in Houdini's dynamics context.an upres technique which greatly increases the detail in the nal high resolutionresult. To handle these high resolution simulations they allow distributedsimulation over a network of computers. These developments and techniqueswill be explained in greater detail in the next section as they are important tounderstand the proposed workow and tool.4 Technical BackgroundHoudini's node based architecture is open and not a black box. For example thepreset pyrosolver is a digital asset, which means it can be unlocked, modiedto support extra functionality and saved as a new custom solver. ThereforeHoudini is very well suited to get more control over uid simulations. Howeverin order to determine where to insert extra functionality and make modicationsan explanation of some of the current uid techniques and their implementationin Houdini is required. Some of the equations of uids will be explained forcompleteness and some of the equivalent microsolver networks will be shownalongside.4.1 What are microsolvers?Microsolvers are nodes inside of DOPS that perform specic tasks related tothe uid solving process. Instead of solving various aspects of a uid simulationin one big solver node as is done with the preset smoke- or pyrosolver, amicrosolver performs only a specic mathematical task. By wiring these microsolverstogether more complex operations can be performed, this is how thebigger smoke and pyro solvers were created. Some microsolvers are digital as-8
sets, which means they are built from a network of smaller microsolvers inside.There are currently around 60 microsolvers in Houdini 10 as can be seen inFigure (6). It is beyond the scope of this thesis to cover all of them so instead afew of the more important ones of which the equivalent often shows up in uidequations will be covered. Microsolvers are wired together by using a mergedop, the order of the inputs is very important as it will dene which microsolvercontributes to the nal result rst. The order of operations works from left toright, top to bottom. This is quite dierent from the pure top to bottom nodeow inside of the surface operators context (SOPS).4.2 How do volume elds work in Houdini?4.2.1 Dening scalar and vector eldsVolume elds in Houdini come in two types: scalar elds and vector elds. Theyare dened as primitives in SOPS (surface operators) but can also be createdin DOPS (dynamic operators) and represent an attribute of the volume, themost commonly known is the density scalar eld. Other custom elds can bedened using a volume sop which will dene an empty volume primitive or aniso-oset sop which will dene a volume based on the closed mesh of an inputgeometry. One of the more powerful ways to dene custom volumes is by usingthe volumevop. This runs CVEX (Houdini's vertex expression language) overa set of volume primitives, the operations can be dened through code or bybuilding a CVEX VOP (vertex operators) network. Not only do you have a lotof low level mathematical functionality, using VOPS will compile the resultingnetwork in vex code which will make it a lot faster to run in comparison tonormal expressions which would have to be interpreted. This volumevop is usedas part of the tool that was developed to dene custom elds in SOPS.In DOPS elds can be created using a (SOP) Scalar Field node or a (SOP)Vector Field node. The (SOP) part allows you to reference elds from SOPS,but sometimes temporary elds are needed to store results of calculations andthe eld is created without the need to reference it from SOPS as it will be lledup with data that is calculated from other elds in DOPS. To quickly create anempty eld based upon the properties such as the dimensions and resolution ofanother eld, a Gas Match Field microsolver can be used.A scalar volume eld such as density will contain a single primitive, whereasa vector eld such as velocity will contain 3 primitives, one for each component.By convention the velocity eld is named vel, the components are vel.x, vel.yand vel.z.4.2.2 Naming volume primitivesIt is important to give the correct names to scalar elds before merging theminto a single vector eld. Some elds are recognised by the preset shaders, suchas the color eld (Cd), but are not yet supported by the preset uid solvers.By modifying the preset shaders you can have custom elds inuence the look9
Page 1 and 2: Controlling Fluid Simulations with
Page 3 and 4: 1 AbstractThere are various dierent
Page 5 and 6: Figure 2: High resolution 2d uid co
Page 7: Figure 5: The node based operator g
Page 11 and 12: • as an axis aligned plane that s
Page 13 and 14: Figure 7: The node tree for the upd
Page 15 and 16: Figure 8: The node tree showing the
Page 17 and 18: Figure 9: The node tree in Houdini
Page 19 and 20: 4.4 Up Res techniqueIn Houdini 10 a
Page 21 and 22: • The nal option is with a Gas Fi
Page 23 and 24: Figure 12: Premultiplication proble
Page 25 and 26: Figure 14: The preset smoke solver
Page 27 and 28: Figure 16: A helix curve emitting d
Page 29 and 30: Figure 19: A few stages of the adve
Page 31 and 32: Figure 21: Top: The stages of the a
Page 33 and 34: and pyrosolver work took time. This
Page 35: [10] Christopher Horvath and Willi

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