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

More documents

Recommendations

Info

Figure 11: The parameters of the metaball-based volume attribute transfer tool.5.2.2 A metaloop based approachThe original way of dening volumes through metaballs involves copying metaballsonto the points, converting them to a fog volume using an iso-oset withthe Mode set to Meta Balls. The problem is that this only uses the density ofthe metaballs and none of the potential other attributes.The volume attribute transfer tool is based upon this workow but againmakes use of vops functionality to perform the lookup of the attributes manually.Similar controls are provides as with the pointcloud approach, althoughthis tool was further developed and presets were added to make it easy for usersto transfer popular attributes that can be used from a drop down menu. Theinterface of the tool can be seen in Figure (11). This tool also allows the possibilityto dene a per point transfer radius by using the pscale attribute if it ispresent on the template geometry.Inside of the digital asset, expressions are used to help dene the presetsettings. Metaballs are copied onto the template points and are then referencedby a volumevop. Inside of the volumevop a while loop is created that will loopthrough all the metaballs, lookup the value of each one at a given position andadd the result together. This is the default density calculation. When trying toadd a dierent attribute together the results were very blocky, the reason whythis was happening was because the attribute would be dened as a solid box ofa constant value within the bounding box of a metaball. This is a problem verysimilar to 2d sprites that have not had there alpha channel premultiplied. Sothe solution was to multiply the attribute with the density value of the metaball,this problem is represented in Figure (12). The density needs to be multipliedinside the deepest level of the metaloop, just before the attribute is added to theresult of the previous metaball, not as a post multiplication operation becausein overlapping areas the masking will be incorrect, the post multiplication eect22
Figure 12: Premultiplication problems: The image on the left shows the attributein grey and the density of the metaball in white.The image on the rightshows the correct premultiplied values.is shown in Figure (13), the artifacts are not that visible in a still frame but willcause jittering during animation.At this point the values are simply added together, which works well fordensity, but not so well for velocity or color. For example in the case of particlesbeing emitted from a single point in space and then spreading out from thatpoint over time, what would happen is the density would be accumulated andlook good, but because of the dense number of particles at that single point, thevelocity eld would grow extremely large and when this entire resulting volumeis then rendered by mantra with motion blur turned on, huge streaks are createddue to the high velocity accumulation.So more control was needed to dene the kind of mathematical calculationthat takes place. Therefore an averaging operation and a switch is added, so theuser can choose whether to accumulate the values or to average them. This is nottrivial and this choice should be provided to the user. A temperature eld forinstance might require either accumulation or averaging, both could work andwill give dierent results. The presets try to alleviate this issue for some of thepopular attributes based upon what works better for those attributes in mostscenarios so the user does not need to worry too much about this. However ifthe accumulation of velocity attribute were required, the user can set the presetto other and dene v as the attribute, vector as the type and add as theblend operation as the other preset allows any custom attribute to be used.Because the copy sop inside of the asset transfers the template point attributes,the special attribute pscale can be used. When a pscale attribute isdened on the template points, the metaballs will be scaled based on the pscalevalue. This tends to give good results, however when the pscale goes to zero,jittering in the volume might occur if the points are moving. It would be as ifthe small metaball would only occasionally be picked up by a voxel when it is inclose enough proximity. A better way of dealing with this problem is by usingthe weight attribute instead of the pscale. This will not modify the radius, butwill instead result in more of a fading eect. The best results can be achievedwhen both are used together.23
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 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: • The nal option is with a Gas Fi
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

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?