No Black Boxes Here

Unlocking the Metahuman HDA in Houdini 21

The Rat Trap Project – Post 9: No Black Boxes Here

When Houdini 21 dropped with its new Metahuman workflow, I was excited. Finally — some of my biggest headaches were solved right out of the box. The head and body stayed together (no more mismatched seams), and the materials set themselves up like magic. Plug and play. Easy peasy lemon squeezy.

For a moment, I almost regretted all the time I had spent wrestling with the old workflow. Why did I put myself through all that pain? But then I remembered — those struggles taught me a lot. Without them, I wouldn’t have had the confidence to start poking around under the hood when things got tricky.

And tricky they did get.

At first, everything felt smooth. The new workflow comes with a Face Control Board and its own body FK controls. But these were too limited for me. I decided to apply the multi-character APEX rig to get more functionality. But when it came time to actually animate, the default rig just wasn’t doing it for me. Limited controls, not enough flexibility. Trying to drive facial animation with data from Unreal turned into a nightmare.

So I did what I’ve always done — I opened the "Black Box".

I dove into the Houdini 21 Metahuman HDA, line by line, node by node. There weren’t many tutorials to lean on, so experimentation was the only way forward. And yes — ChatGPT was basically my study buddy, breaking down every piece of APEX script into plain English so I could really understand what was going on. Together, we figured out where the rig needed a little… persuasion.

I managed to bypass the Face Control Board and hook things up in a way that let me bring animation data from Unreal (face or body) straight into Houdini. Its not perfect — there’s still an error with the Extract Controls node that I’ll be writing to SideFX about — but it works.

Two weeks of deep diving later, I can say this: I understand the new Houdini 21 Metahuman workflow. Not just how to use it, but how to adapt it to my own goals. That’s the payoff of refusing to treat tools like black boxes. Open them up, poke around, and learn.

Now, animating my characters across Houdini and Unreal isn’t a headache anymore — it’s fun again. Viva Houdini!!

Looking Ahead

Next week, I’m setting my next "trap:" capturing face mocap in Unreal for my Metahuman, and body mocap with a Perception Neuron suit. The challenge — and the fun — will be taking all that data and animating Bakergirl in Houdini If all goes well, I’ll have the full animation pipeline humming: from capture straight into performance-ready characters. Can’t wait to see how this one turns out

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