Node Nuggets 006 – Match Size (Houdini) (Patreon)

As we get lost in the complexities of 3D apps, we sometimes forget that fundamentally they’re there to make our jobs easier! With that end in mind, let’s take a look at an underrated and helpful Houdini tool, Match Size.

As the name suggests, Match Size can be used to set the size of one piece of geometry to match that of another, but it can do a lot more too.

Let’s say we want to create a lookdev setup for producing turntable renders. We’ll need a fixed camera or two, a backdrop/set, some lighting, and a model placed front and center.

We’ll want the model to sit on the backdrop. Instead of sliding it up and down manually, trying to eyeball it, we can use one of Match Size’s Justify parameters. Justification works similarly to how it does in a text editor, letting us line things up based on edges or centers. Here we justify the minimum Y (vertical) position of the model with the minimum Y ("Same") of the backdrop. Instant and perfect alignment! (I turned off Z justification to let me manually position the model front to back.)

Replace that model with another? It will automatically be aligned too.

A quick note about what Match Size is doing under the hood: it’s comparing the bounding boxes of each model. A bounding box is the smallest box that can fully contain a mesh. More elaborate meshes will sometimes have bounding boxes that contain a lot of empty space.

Now that we’ve got our first model set up, we’re going to want to reuse the setup with as little work as possible. We should be able to bring in any model from any source and have it positioned perfectly.

Let’s try. This new model is a bit smaller and as a result our framing and lighting will be thrown off.

No problem, just connect the two models to a Match Size, check the Scale to Fit parameter and the new model will now not only be justified, but sized perfectly!

This is a big time saver when working with lots of models from many different sources. Scale can be a major problem even with the same model as a result of switching geometry formats or using different DCCs to edit it.

(And yes, it is kind of amusing that Match Size, by default, has the “match size” part turned off. Good old Houdini!)

Finally, note that there is a Match Size node in LOPs/Solaris. It works mostly the same as the SOPs version we’ve been looking at. There’s just one complication: because we’re now in USD land, the node looks for stage inputs containing many prims, then expects the user to specify which prim to modify and which one to target.

Since the inputs are a bit different compared to SOPs it can lead to some initial confusion. Once you get things wired in correctly and select the right prims then the rest will work as expected.