mestela
The attributedelete just clears out all the unnecessary attributes made by the pop sim, leaving just the ones I want.
Re the other questions, easier to do bit of a walkthrough. Couple of high level goals here:
1. have each particle be a little puff of smoke that starts invisible, ramps up in density quickly, then fades back to invisble just before it dies.
2. transfer the particle velocity into the pyro simulation.
Re 1, houdini particles have a hidden bonus attribute @nage, meaning Normalized Age. It's 0 when a particle is born, smoothly increases over time, and is 1 when a particle dies. I run that through an attribute remap so that it does the quick ramp up, hold, then ramp down.
Except... for some reason @nage isn't available to the attribute remap sop. Seems it doesn't support bonus attributes (in the same way that it doesn't show @ptnum I guess). As such I have to convert @nage from a sneaky bonus attribute to a Real Boy attribute. That's the the wrangle is for, it just copies @nage to a new attribute, @density.
2. The pyro solver sop only accepts volumes as input. The attribute rasterize sop converts point attributes to volumes. But the way volumes work, each volume can only store a single thing, so the density volume stores density. A v or vel volume stores velocity. A temperature volume store temperature etc.
On the attribute rasterize I specify the attributes I want, density and v, I get two volumes named @density and @v. The houdini viewport does its best to display what we just made; displaying density makes sense, if density is 0 its invisible, if its 1 its full of smoke, easy enough.
But displaying velocity makes less sense. The viewport just makes a dumb guess and says 'ah jeez.. its like fancy density right?', and displays the blocky weirdness. Positive velocity values become really bright, negative velocity values go super dark, its a mess. When you understand what's going on it makes a little more sense; look from a top down view, you'll see the bright and dark patches correspond to the swirl of the tornado (ie if the particles are moving in a +xyz or -xyz direction).
If it really bothers you, you can put down a primitive sop, use the dropdown at the top to isolate to the @v volume, jump to the primitive tab and set the display mode to invisible.
Wow I am so grateful you took your time to break this down. I learned a lot from it. Not sure I would immediately be able to use @nage attribute or some concepts you talked about but at least I understand they exist. Btw I see the @age attribute, I wonder if @nage is life span of a particle from 0-1 what would the @age be? I had issues with perticles lasting long enough so that the smoke swirls would be greater and thicker twisting around the tube. But I must say you came as close to what I envisioned as a great tornado starting point as possible. In only 8 nodes you were 80 percent done with a main body of a tornado.
Thank you again.