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

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Figure 4: The dierent states of target driven smoke using custom elds. Imagetaken from [7]3.3.2 Target driven smoke animationCustom elds can also be used to create target driven smoke animations, as in[7]. They introduce a driving force term that causes the uid to carry the smoketowards a particular target and they also add a smoke gathering term whichprevents the smoke from diusing too much due to numerical dissipation. Thecore concept behind this technique is using the gradient of a blurred densityeld of a target shape as direction vectors to advect the velocity eld of thesmoke. The resulting vector eld looks like all the vectors are pointing towardsthe target shape. Because the start and end shape might have dierent densitiesor the smoke might lose density along the way, density is added as it interpolatesbetween the two shapes as can be seen in Figure 4.3.4 A broader publicThe above approaches use custom elds and geometry to gain more controlover the simulation, but they are separate or proprietary software solutions.Because they are custom built they have the advantage that they will t withinthe pipeline of a particular company and are able to explore new hardwareplatforms such as the GPU. The drawback is that these tools are not veryportable and useable by a broader public.3.4.1 Node based dynamic simulationsOne company that is trying to change the way uid simulations are set upis Exotic Matter with their software called Naiad [5]. The creators are MarcusNordenstam and Robert Bridson who have both contributed greatly to thedevelopment of uid solvers over the past ten years. Robert Bridson has publishedseveral insightful papers and also the book Fluid Dynamics for ComputerGraphics [2]. At its core Naiad is a dynamics solver and a simulation framework.Naiad allows for the creation of a description of a simulation scene. Thisdescription can then be simulated. The description can be made through a nodebased interface which resembles that of node based compositing software as can6
Figure 5: The node based operator graph in Naiad, by Exotic Matter. Imagetaken from [5]be seen in Figure 5. They are trying to establish a Nip (Naiad Interface Protocol)format which is similar to a RenderMan RIB format, but then for dynamicsimulation. This has not been done before and will hopefully help establish astandard for dynamic simulations. This is also one of the few systems whichuses a node based approach for uid dynamics, the other major node baseddynamics system is part of Houdini.3.4.2 A brief history of Houdini's dynamic contextSince Houdini 8 node based dynamic operators (DOPS) were introduced andsince Houdini 9 uids and gas microsolvers were added. Houdini 10 brought afurther development of a preset pyrosolver which can produce high quality pyrotechnicresults combined with an extensive pyroshader. They also introduced7
Page 1 and 2: Controlling Fluid Simulations with
Page 3 and 4: 1 AbstractThere are various dierent
Page 5: Figure 2: High resolution 2d uid co
Page 9 and 10: sets, which means they are built fr
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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