Houdini 20.0 Tutorial: Mastering Vellum Simulation

Let's start with a little history. Vellum is the latest major solver integrated into Houdini since version 17 (released in 2018). It is based on a particle simulation approach called Particle-Based Dynamics.

In this system, a mesh (for example, cloth or rope) is broken down into individual points, called particles. Each particle is simulated independently, while constraints are added between them to reproduce realistic physical behavior such as elasticity, stiffness, and flexibility.

In short, Vellum isn’t just for clothing simulations — it can also be used for curves, soft bodies (like balloons), or even particle simulations for snow, sand, or liquid effects. It’s an efficient, flexible method, particularly well-optimized for GPU processing.

So why are there two methods (SOP and DOP)?

The reason is simple: Vellum was first introduced through the DOP context in Houdini 17, then later optimized with a more flexible, modular SOP version, making it easier to prototype and iterate.

This distinction is important — both approaches are fully valid and seamlessly integrated within Houdini.

The SOP Method: Fast, Clear, and Efficient

The SOP context lets you build a Vellum simulation directly inside a Geometry node, without switching to the DOP context.

With just a few nodes — Vellum Constraints, Vellum Solver, and some quick adjustments — you can create a complete setup. It’s modular, visual, easy to organize, and fast to iterate on.

Even better, the presets are generous. For example, a single Vellum Configure Cloth node automatically prepares the right physical attributes, generates constraints, and sets key properties such as density, thickness, collisions, and gravity.

The solver itself includes handy built-in parameters (like timescale, substeps, constraint iterations, and visualization options) — all without the need to build a complex network.

For quick simulations, artistic tests, or single-geometry setups, the SOP method is often smoother and faster to work with.

The DOP Method: More Complex, but More Granular

The DOP context (Dynamic Operators), on the other hand, involves explicitly moving to a dedicated simulation network.

You need to export the geometry and constraints via Null nodes, import them into a DOP Network, and then add a Vellum Source, Vellum Object, and Vellum Solver, connecting them to collision objects or external forces.

It takes more time to set up, but it offers greater precision and flexibility, particularly in complex pipelines, distributed simulations, or cases where the simulation needs to be triggered, animated, or modified dynamically.

So, SOP or DOP?

In summary, the SOP workflow is more direct and accessible, while DOP provides deeper customization.

For many projects, the SOP workflow is more than enough, especially when the goal is prototyping, experimenting, or working solo on a specific effect. It allows you to move faster while maintaining control.

However, once simulations become more demanding, need to interact with other solvers, or must fit into a rigorous pipeline with dynamic adjustments, DOP becomes the better choice.

The best part? You don’t have to choose — both methods are compatible, and what you learn in SOP can be applied directly to DOP-based projects.

Understanding Cloth and String Constraints

Vellum relies on physical constraints to simulate the behavior of soft or flexible objects. Two main types of constraints are most commonly used:

• Cloth, for surfaces (e.g., fabrics, skin)

• Strings, for lines (e.g., ropes, hair)

Each point in the geometry is connected to others by “links” that limit its stretch and bend capabilities. You can also add weight, friction, or air resistance (drag) to fine-tune realism.

In the Houdini 20.0 Vellum training, you’ll explore how these constraints affect simulations, how to tweak them effectively, and—most importantly—how to avoid common pitfalls like the dreaded fabric that collapses in on itself 🫠 or a rope that stretches way too far.

Going further: soft bodies, balloons, plasticity…

Not too much of a headache yet? 😅 If you’re starting to feel eye strain, that’s normal. Houdini’s Vellum simulation is dense, and nothing beats a step-by-step demo.

The full (and free!) online course, led by expert Vincent Fortin in collaboration with École NAD-UQAC, runs 9 hours and includes 29 modules.

What you’ll learn in this training:

• SOP vs DOP

• Cloth and String constraints

• Attachment constraints

• Soft bodies

• Balloons

• Plasticity

• Detachment – Attach to geometry

• Detachment – Pin to animation

• Detachment – Weld

Register for the EXPERTS training, it’s free!

The Houdini 20.0 Vellum course is free and exclusive to professionals working in a Québec studio that is a member of one of our three partner associations: La Guilde du jeu vidéo du Québec, BCTQ, and Xn Québec.

The training is also available to recent graduates of a digital creation program from a public institution in Québec. You have up to one year after graduation to take EXPERTS courses.

Recent graduates can contact experts@polesynthese.com to activate their account.