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

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and create extra output variables that can be rendered to separate image planesfor greater control in a compositing application. Another useful eld to haveas part of the simulation is the rest eld, which basically carries a set of uvcoordinates that can be used to map textures to the uid.Fields can be given any name, but only certain specic elds will be pickedup by the Mantra render engine, for example if the vel eld is detected andmotion blur for rendering is turned on, the information of the velocity eld willbe used to apply motion blur on a fast moving uid simulation.4.2.3 Rest eld interpolationIn the preset smoke solver a single rest eld is advected along with the otherelds. This rest eld is then used inside a volume shader to map various kindof noise to the uids and increase the detail of the smoke as is also describedin [17]. When this rest eld is advected far away from the initial position, thetextures will become distorted which can give undesirable results. In the newpyrosolver in Houdini 10 a second rest eld (rest2) is introduced to solve thisissue. The initialization of both elds (rest and rest2) is staggered in time andthe pyro shader continually interpolates between them. For example if the resetvalue on the pyrosolver is set to 50, the rest eld will be re-initialized every50 frames, starting at the rst frame. The rest2 eld will also be re-initializedevery 50 frames, but will start at frame 25 (half of the reset value). The pyroshader has a corresponding Reset Rate parameter that should be set to thesame value. This will cause the shader to interpolate between the two rest eldswithout showing any popping when the rest elds are reinitialized.r e s t : +−−−−−−−−−−−−−+−−−−−−−−−−−−−−−+−−−−−−−−−−−−−−− ( e t c )r e s t 2 : −−−−−−+−−−−−−−−−−−−−−−+−−−−−−−−−−−−−−−+−−−−−−− ( e t c )At each + in that graphic, the eld resets. The pyro shader then constantlycross-fades between the two elds, such that a given eld has full contributiononly at the reset frame [12].4.2.4 Volume eld viewport visualizationWithin DOPS there are dierent way to visualize the information stored ina eld. This visualization can greatly help to visually debug the values of aeld. To be able to enable this visualization for a specic eld, a Scalar/VectorField Visualization node needs to be attached to the data input of a eld. Thisvisualization can consist of:• showing the eld as smoke (useful for density),• as straight lines depicting both length and direction of the values of avector eld (generally useful for a static or dynamic velocity eld),• as streamers, which are curves depicting the ow of a vector eld, (usefulfor viewing the interpolation of the velocity between voxels)10
• as an axis aligned plane that shows a coloured slice in the volume, mappingthe values of the eld to the colours on the plane. (useful for temperature)Within SOPS a volume will generally be visualized as smoke when the values ofthe eld are positive. This means as well that when you are trying to visualizea eld that has negative values, such as velocity or temperature, nothing willbe shown in the viewport in SOPS, even though the data is actually there. Aquick check can be performed by taking the absolute value of the data insidea volumevop just for visualization purposes to make sure the data exists. Atthis point visualizing your custom elds in dops will give you more options asdescribed above. The reason why this section on visualizing volumes in SOPS isincluded is because it is not in DOPS, which means no solving needs to happento visualize the eld which makes it faster to shape volume elds independentof the previous frames. Also it is important to understand negative values arenot displayed in the viewport as this clarication will hopefully avoid confusionlater on when the tool is used and there seems to be no output.4.2.5 Houdini specic trainingSidefx, the creators of Houdini, provide a great set of masterclass tutorials onuids and the pyrotools on their website which will help when learning how touse microsolvers and understand volume elds in Houdini [15].4.3 The equations of uidsA signicant amount of research has already been done in the area of computationaluid dynamics (CFD) for simulating smoke, re or various types ofliquids. A few of the equations will be covered to help explain the componentsthat make up a simple smoke solver in Houdini.4.3.1 The incompressible Navier Stokes equationsThere is a consensus among scientists that the Navier Stokes equations are avery good model for uid ow [17]. That is why they tend to be used as afoundation for a lot of solvers, that will then in some way modify or extendthese equations:∂⃗u∂t + ⃗u · ∇⃗u + 1 ∇ρ = ⃗g + v∇ · ∇⃗u (1)ρWhere:⃗utρgvvelocitytimepressurebody forcesviscosity∇ · −→ u = 0 (2)11
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 and 8: Figure 5: The node based operator g
Page 9: sets, which means they are built fr
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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