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

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