Hi guys,

I'm creating a small fireworks explosion (just the explosion), as seen in Harry Potter and the Order of the Phoenix (122 - 125)

https//www.youtube.com/watch?v=YrAe0XI7mis

I've created the particle animation of the burst, but now I want those particles to leave a smoke trail. What would be the best solution to create smoke that is so thin, like you see in the video? PyroFX simulations seem to heavy for the little smoke I want, or is that a misconception ?

Thanks!

hey buddy i meet this problem for many times . ive found a method to solve this problem . the source geometry is important .and source volume is a key to the final result .

So you mean that I should copy geometry volumes to the particles and then… turn them in to a pyro solver?

Hey Felix,

I LOVE houdini's volume toolset - it's friggin' awesome! You can do so much with it.

There are a couple of things that you should think about when you do a pyro (or smoke sim).

Firstly - how big is it going to be? Looking at your clip in the movie (from 1:22-1:25) I assume your talking about the fireworks with the trails and the little poofs? Lets first talk about size in general. Say you have a 10cm x 10cm x10cm container - lets say for now we're just talking about the poofs - er one poof in particular and that is the maximum size.

Now lets say (just for argument's sake) that you have voxels that are 1cm x 1cm x 1cm cubed. So your resolution is 10x10x10 which is 1000 voxels yeah? Now think about what happens when you double all the x y and z size of your container - now you have 20x20x20 which is now 8000 voxels!!!

Or say you decide to do two poofs at once and there is 10 cm between them (lets say for argument's sake in the x direction). So now you have a container that is 30cm (10cm for the first poof, 10cm in between and 10cm for the second poof) x 10cm x 10cm. So now you have a container that is 3000 voxels.

And when your doing calculations on every single voxel in the container that takes time - so the smaller the container the better (ie. don't try and do one big smoke sim if you can break it up into smaller parts). Especially if you have multiple machines - instead of doing one big sim that would take 5 hours - you can do 5 small sims that take say 30 mins each. Or if your in a production environment do one 30 min sim over and over again until the vfx sup, cg sup, producer, client and everyone else is happy and then apply those settings to the other 4 sims.

The other thing your going to need to think about is this for fast moving objects (like missile trails or fireworks trails) if you use the same geo that your rendering as a source for your smoke it will look ‘spotty’. The reason - well at frame 1 the missile, firework or fast moving object is lets say at (0, 0, 0), at frame 2 its at (0, 3, 0) which means there will be no smoke source in between (0, 0, 0) and (0, 3, 0). So you will probably have to generate in-between geo that may not look like your initial geometry at all (usually a particle system). In your case it already is a particle system - so you could probably do some minimal adjustments to it to get a good source geo.

Best thing is to provide a really simple example file showing what you've tried already.

Also it is possible to ‘fake’ it using particles and instance geometry onto it and make it look like smoke - but I personally prefer just doing a smoke sim - I find the particle method can be quite finicky and more difficult to get a successful look - but people do do it.

Thanks for the reply! I uploaded the hipnc file. It's just a basic particle simulation. I've got the explosion + the particle trail of that explosion. I have done Houdini for some time now, but I can't say I've made a lot of progression, especially regarding fluid dynamics + pyro sims. I understand what you mean about the voxels, but I wouldn't be able to implement it in a smoke-simulation.

ok, here is the first step - go through it and see if you can understand what I'm doing so far. Don't worry about how it ‘looks’ just yet. The whole thing with visual effects is getting closer and closer to your goal - just see if you understand what's happening. If you don't understand what I'm doing ask some questions.

If you do understand see if you can take it closer to what your looking for. Let me know when you've played around with a little and feel stuck.

Amazing, thanks so much ) ! I studied the scene file and it left me with some questions. I understood everything you did in the ‘explosion’ geometry sop (that is, building the actual volume source by stamping points). The tricky part - I find - is in dop context. I tried to understand it using Houdini Help, but not always with success. First of all did you use an off the shelf PyroFX tool, or have you created these dynamics from scratch? Then I had some more practical questions

- In the Source Volume dop under the ‘Volume Operation’ tab, you changed Source Volume to maximum and Velocity to Add. What does this mean?
- Under the ‘Mask’ tab, you set ‘density’ as a mask(?) for Velocity Volume. How does this affect the simulation?
- In the Pyro Solver under Shape > Dissipation, what’s the logic behind the wave-form of the ‘Remap Dissipation Field’? Shouldn’t it be a declining line, as smoke dissipates more and more over time?