The "APEX" of Animation

Our Metahuman All Rigged up with APEX

The Rat Trap Project – Post *: The "APEX" of Animation

When I started The Rat Trap Project, my goal was simple: learn by doing and share the journey as openly as possible. In the first few posts, I wrote about how the project itself began almost as an experiment—an idea that small teams (or even one person) can still take on ambitious animation if they’re willing to embrace the right tools and workflows.

From there, the story has unfolded step by step. I explored how Metahumans, with their film-level realism, could serve as a foundation for character work. I wrote about the challenges of shading inside Houdini, the back-and-forth between Unreal and Houdini to capture animation, and the tests we ran to make sure facial expressions felt alive. Each post has been about peeling back one more layer—sometimes hitting walls, sometimes finding unexpected breakthroughs.

Now, the next challenge is movement. How do I rig and animate my characters in a way that’s flexible enough to handle both a Metahuman and, eventually, the rat itself? That’s what led me to Houdini’s new APEX workflow.

Discovering APEX

APEX (short for All-Purpose EXecution,) is Houdini’s new take on rigging. Unlike traditional rigs that are often custom-built for each character, APEX is built around procedural techniques. That means you can define systems once, and then adapt them to different characters by using “tags.”

When I first opened the APEX examples, it felt a little intimidating—lots of nodes, lots of wires. But once I understood the philosophy, it clicked: rather than hardcoding a rig for one character, APEX looks for tags (like “left_arm” or “spine”) and builds functionality based on those. It’s a bit like giving Houdini a set of instructions in plain language, then letting it do the heavy lifting.

Putting it into Practice

To test this, I didn’t start with my rat. I started with something more familiar—my Metahuman. Bringing that character into Houdini, shading it, and then adapting the APEX procedural rig was a learning curve, but a rewarding one.

I had to dig into every functionality on the rig—understanding what each control did, how it applied to different kinds of characters, and how to make it feel natural. Along the way, I learned how to adjust my control shapes so the rig felt intuitive to animate. I also needed to define the tags on my Metahuman, especially for tricky parts like the reverse foot setup. That was a breakthrough moment: once the system recognized the tags correctly, the workflow came alive.

What excites me is that this same setup can be applied to other creatures—bipeds, quadrupeds, even birds—without rebuilding everything from scratch. Because APEX is tag-driven, the core of the rig remains the same, while the tags tell Houdini how to apply it to a new body.

Why It Matters for The Rat Trap Project

For me, this isn’t just a technical exercise—it’s about building a sustainable pipeline. I don’t want to hack together a rig that only works for one shot or one character. I want a system that grows with the project, and APEX gives me that.

By proving it on my Metahuman, I’ve gained the confidence that I can now adapt the same system to my rat characters. That means when it’s time to bring those rats to life—skittering, climbing, and acting—I’ll have a rigging system that can handle the job without reinventing the wheel.

Looking Ahead

Every post so far has been about tackling a new piece of the puzzle: from asset creation to shading, from Unreal integration to animation tests. With APEX, I feel like I’ve unlocked a key part of the workflow—the bridge between static models and moving, believable characters.

All of this research was done in Houdini 20.5, but SideFX has already released Houdini 21, which comes with new Metahuman workflows. Over the next week, I’ll be diving into those updates and testing how they change the process. My plan is to take what I’ve learned so far, wrap it into a digital asset, and then use it to build an APEX rig for a Houdini 21 Metahuman.

There’s still more to figure out, of course. How far can I push this system? What happens when I introduce more complex animal motion? Those are the questions for future posts. But for now, I’m excited about deploying APEX in my animation journey—it feels like I’ve reached a milestone, while also standing at the edge of the next big step forward.

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