Why though?

I was playing God of War Ragnarök during Christmas break, and I really liked how the mystic gateway animation felt. During one of the play sessions, the developer side of me woke up and thought, "Hmm, I wonder if they just reversed an RBD sim for this."

Like any technical artist out there, we like to tinker, so I thought, "Let's try it out! I'm curious how this was made." And I did. I created a fast prototype in an hour or two, proving that reversing a gravity RBD sim would be enough. And that was it. I moved on to the next thing that caught my attention.

One month later, I stumbled upon the reference photo I took for this test and decided I could spend a bit more time on this and maybe write a blog about my approach to it.

While this won't be anything fancy or uncommon, I think it's interesting to see how others approach problems and how it can sometimes help with other unrelated things.

Before I ramble onwards, two disclaimers:

• This past year I've been learning Houdini, so my breakdown will focus on a Houdini workflow(although similar concepts exist in other DCCs as well).
• This is a fast prototype, where i don't care about polish or quality, its just to prove a concept.

First step, references!

Break down part

Looking at the images I noticed:

• There are 2 states (Active and Inactive), we can also see that there's an archway shape and some rocks following this shape.
• Rocks are following the normal of the curve towards the inside and bend with the curvature.
• There are gaps between the rocks and the wood frame and that makes our life a bit easier to recreate this.

This gives me plenty of information on how I can start, but lets also look at the animation and see what our model needs to support in terms of movement.

• There are 2 big rock pillars intact and not moving (left and right, one a bit uneven)
• Each individual plank fracture independently, indicating it's probably split into its own element.

To-Do List:

I need a rock asset, an archway made with it, a wooden door made of vertical planks, a fracture for the door and an RBD sim.

Rock

This was easy, and I cheated a bit, I downloaded a rock from Megascans. While Houdini can easily generate some, I thought this is not the real focus here.

Archway

For the archway, I created a spline that i mirrored and remeshed, where a copy to points was used to duplicate and scatter the rock asset. To make it look less repetitive I'm adding some random scale and a random rotation, while still trying to maintain the rocks rotation towards the curve interior.

Wooden Door

Since I knew I would need to fracture the wooden door, I wanted to have separate planks created from the start, so I don't have issues with newly created faces. For plank creation, I created a shape from the archway initial curves, bridged the missing gap, and using a boolean from the door shape with boxes created as planks it results in planks following the archway shape.

Wood Fracturing

Fracturing the wood planks, I went with the old approach of scaling down the mesh into a Voronoi fracture and then scaling it back. This workflow creates some wood-like splinter fragments. Afterward, I handled the cluster creation manually by grouping up splinters that are close by.

Simulation

Tearing down using a RBD Bullet solver, just an RBD Solver with gravity on.

And now final step, reversing the simulation of the RBD solver.

This was a fun thing to tackle, especially since I didn't have to worry about the final quality/polish of it being in a real production environment.

I want to emphasize that having a quick prototype does not actually represent the entire amount of work needed for an asset to be used in a shipped title. It's more than just having a working animation; it's about visual, gameplay polish and production iteration. It needs to work with gameplay, collision, metrics, combat, VFX, and audio setups. It needs to work in a full game environment and, more importantly, be iteration-friendly.

I always like to find things that I could have done differently or improve on, and for this mini-project, it's no different. This is what I would have done differently or had a different approach if it were in a real production environment:

• Rock generator that adds that visual noise as repetition is still pretty noticeable in my scene(or more rock assets to scatter) even if i tried to hide it with rotation/random scale etc.
• I would like to find a nice way to check intersection between the rock points, as the current setup i was just lucky with the seeds and scale of rocks that i used, my current setup was NOT production friendly, and would have required more work if an art director or shot supervisor would have requested lets say...add 1 more rock. *flip table*

What are your thoughts on this approach?
/Andrei

P.S: Adding also the Houdini network view.