Houdini LOPS Variants: Build a Tree That Catches Fire

You’re building a forest fire. Some trees burn. Some don’t. Layout wants one asset they can place a hundred times and flip — not two files, not a Switch that dies the moment the USD leaves Houdini.

You remember a Jungle Book talk where fire lived on props as options. That option, in Solaris, is a variant.

The scene starts quiet

You don’t start with flame. You start with a root.

A Primitive drops /testTree01 into the stage. Configure Layer marks it default. Then a SOP Import hauls the tree in from the workarea, Material Library wraps bark and leaves, and Graft Stages stitches that clothed geo under the root. Hierarchy. Pulse. Still no fire.

Then the Volume appears — /flame — and another Material Library binds the pyro look. You graft it onto the same prim path. Same tree. Second composition. Heat rides the hierarchy now.

Only then do you wire both grafts into Add Variant: left is tree alone, right is tree plus flame. You name the choices — no_fire, on_fire — and the display flag lights green. Two versions. One asset.

Walk the network yourself

Use Next to reveal each beat. Pan and zoom the graph. By the last step the wires should match the chained-graft setup — Ben Skinner’s cleaner prep, no Merge required.

Why this beats a Switch

A Switch flips cooks inside Houdini. Variants travel with the USD. Save the asset out and the dropdown still exists in usdview, Maya, Katana — anywhere that speaks composition. Lighting can set more trees on fire at the end of the line without kicking the shot back to FX.

Append a Set Variant LOP, pick fire_toggle (or whatever you named the set), choose on_fire, watch the stage update. Duplicate the prim, set a subset on fire. The forest starts choosing its own fate.

Credits

This walkthrough follows Matt Estela’s LOPS Level 2 Variants notes on CGWiki, including the chained-graft prep and the reminder that variants exist so the option ships with the file — not only with your session.