SteveSayer Posted July 22, 2015 Share Posted July 22, 2015 Hi, all. Hope someone can help out with this--I've hit a bit of a wall. I am working on a fan-type effect, where I want to cache out a smoke object, then use that cached simulation to advect some particles. But I want to be able to animate the position and rotation of that fan. So I want to simulate the 'airflow' of the fan at the origin, cache that, then move the resulting cache around in my scene and have it blow the particles around appropriately. However, just transforming the cached velocity fields, then reading them into DOPs with the gas advect particles node doesn't have the expected effect. At a guess, it seems like the container is being transformed, but the cached velocity vectors aren't being transformed. So turning the fan to face left instead of right makes the particles to the left move, but they move in the wrong direction. I've attached an image that illustrates what happens--which is quite strange. This happens whether I rotate the object at the object level or the SOP level. Any thoughts on how to approach this? Thanks for any suggestions! Quote Link to comment Share on other sites More sharing options...
Diego A Grimaldi Posted July 22, 2015 Share Posted July 22, 2015 Have you tried using VDB instead? My friend posted a file in his blog on how to do particle advection with VDB in his blog here: http://pepefx.blogspot.ca/2014/07/ink-in-water.html 1 Quote Link to comment Share on other sites More sharing options...
SteveSayer Posted July 23, 2015 Author Share Posted July 23, 2015 Hi, Diego. Thanks for the suggestion. I'll have to dig in and see if I can figure out his approach. From a quick skim I'm not sure if he's rotating (or animating) the VDBs. I'm interested in the more generic solution, too, though--eventually I'd like to advect not just particles but one smoke object with another. So think, like, a wisp of smoke is rising from an ashtray... and I want to move and rotate my previously-simmed fan effect so that it sweeps through that wisp and blows it away with appropriate motion. Other suggestions therefore still very welcome! Quote Link to comment Share on other sites More sharing options...
SteveSayer Posted July 31, 2015 Author Share Posted July 31, 2015 Answering my own question... in case anyone else can benefit. The solution seems to be to use a 'transform' node in a volume VOP to transform the vectors from world space into object space. Feels a bit convoluted but seems to work. Quote Link to comment Share on other sites More sharing options...
anim Posted July 31, 2015 Share Posted July 31, 2015 seems that if you define your vector type properly for VDB vel volumes, it should be transformed automatically: Monday, February 3, 2014 Houdini 13.5.70: Transforming vector-valued VDBs now also have it applied to the voxels as well, depending on their vector type. This breaks backwards compatibility. Quote Link to comment Share on other sites More sharing options...
SteveSayer Posted July 31, 2015 Author Share Posted July 31, 2015 Ah, good find, Tomas. I've always wondered exactly what the difference is when you have the option of several different types of vector. Guess this is one of the major ones. I'll have to experiment with it. Thanks! Quote Link to comment Share on other sites More sharing options...
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