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Burning down the house!


For my third year film which I am also a VFX supervisor on we have an opening shot where we want to give the viewers a sense of the danger that will be coming their way. The most dramatic way to suggest danger is with a big old burning house where people run around being all crazy. Most of the compositing of the shot was done by a classmate of mine and I was responsible for the FX side of things, I have a simple crowd sim in the shot but that's not what this post is about. It's about the house I have put on fire.




I started with a mesh of a house that my co-supervisor modeled, then turned it into vdb's and that back into a polygon mesh to get a high poly model to paint over.



The original model doesn't have enough poly's to work with a paint sop

This is the model painted over, I can now decide where the fire will show up.

The most important thing you have to get right for a good pyro sim is a good source. Being able to decide where the fire shows up is a big plus right from the start. But it is very uniform. To add some more interesting detail to the places I have painted over I used a simple pointvop to add some more noise to the already painted areas. By multiplying the color with the noise and putting a very slow time to the offset to get a changing source.



Now the places I have painted over have gotten more detail.

To get the most iterations out of the process I first put the source into pops. There I try to get a firey simulation going to source some of the fuel and temperature from.



I tried my best to make the particles behave a bit like fire by shortening their lifespan and adding extreme velocities.

Afterwards I put the particles through the new pyro workflow, using the pyrosource and volume rasterize attribute nodes. The volume rasterize attributes node does some cool stuff to help out with the sourcing, but it's not nearly as in depth as playing with the volume in vops. So I took the input from the node and put it into vops to get a final source with more detail and noise. I also got some negative fuel values from the volume rasterize attributes node, so the volumevop was necessary to get some good noise.



The pyrosource after putting it through the volumevop. I also decreased the size of the particles based on their life.

After that I simulate the fire and cache the burning fields for that fire to continue simulating the smoke that comes out of the fire. I did this so I could have more control and less waiting time. For example I want the fire to have a resolution of 0.03, but for the smoke sim that would take too long and too much data. So I simulate the smoke with a resolution of 0.1 and merge those two simulations to get the result I want without waiting too long.



That's how I made my simulation. I shaded the fire using rest fields and noise. The only unfortunate thing is that the shot turned out like this in the movie.


But at least I learned a lot and gotten the hang of the new pyro workflow.

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