Building Bliss - Work in Progress #5

Shot 3 Full Render Preview

I've been hard at work continuing Building Bliss after I stopped work on it many months ago. I've made a LOT of progress and I'd like to share it, starting with shot 3's full render!

Shot 4 Viewport Preview

This shot has been one of the hardest I've done to date, requiring an incredible amount of simulation through Houdini. This WIP will mainly entail how I've been going through this shot using Houdini.

Car Tire Pyro Simulation

I'm starting to move away from EmberGen as my primary source for volumetric explosions and fog and towards a more production-friendly workflow using Houdini's pyro system. Below is a very short and sweet tire smoke simulation that you see coming off the humvee in the beginning of shot 4.

The node network for this shot isn't as complicated as it looks. This is a very basic pyro setup aside from a couple of additions, like a point velocity node so that I can randomize the emitter velocity attribute for a bit more randomized emission.

To prevent "stepping" (gaps between emission pulses) I am interpolating the emitter mesh back two frames This allows for consistent emission without gaps in the smoke. I'm also grouping only the bottom of the tires so that they are the only parts of the mesh that emit smoke.

Building Fracture Pyro Simulation

Now for the real meat and potatoes of the pyro work for this shot: the building fracture/split. When Sara detaches about 3/4 of the building, it needs to have a huge, smokey impact. Thusly I stuck a big smokey explosion right at the fault line.

The setup for this pyro sim was even more simplistic than the tires, but the real difficulty came when trying to get the speed to look correct.

The pyro solver required a few extra settings to get it to look the way that I wanted. For example, I animated a "divergence" (aka pressure) to speed up the expansion of the smoke during the initial explosion and have it slow down over time.

I was also getting smoke that was looking too smooth during the initial emission. I solved this by dropping in a "gas disturb" node that breaks up smaller details of the simulation. This node applies noise pre-solve so any smoother areas are broken up.

Glass Explosion RBD Simulation

On top of the pyro simulations, I've also been experimenting around with Houdini's RBD pipeline. It's about 200 million times faster than Blender's RBD system and is exceptionally more robust. I used this pipeline to create a simulation for a glass explosion that occurs once at the same time as the smoke simulation.

The setup for an RBD sim is a bit more involved than pyro as it can require sim additional attributes before simulation can happen.

In my case, that additional attribute was a velocity attribute. RBDs can rely on external sources to inherit their velocity, such as a hammer hitting a stationary pile of blocks. In this case, though, I needed a high initial velocity force enacted by nothing more than a preset vector. This meant I had to create a custom vector value, inheriting the normals of the front of each glass pane so they would explode outward. This snippet creates a new vector attribute named cVel, inheriting the normal (@N) vectors.

During the fracture, each piece will inherit this attribute that I then later convert to a velocity attribute before solving. This snippet calls the velocity attribute (v@v) and sets it equal to the cVel attribute I created before I fractured the windows.

I then use a point velocity node to add some noise to the velocity vectors so that the glass would explode outward more randomly.

After that it's just plug and play with the RBD solver! I didn't really mess with any more settings aside from adding a collider for the structure in front of the main building so that there wouldn't be any glass shards clipping through.

And that's all for this WIP! I hope you all enjoyed a bit more of a technical breakdown. I wont be doing these kinds of WIPs every time, but the amount of sim work called for it. Stay tuned for more!