Vellum Part Four

Intro:

Whoop! Time for some more Vellum. :)

Houdini 18.5 Updates:

So As some of you might have heard...Houdini 18.5 is here, and SideFX was very nice to us all. Inside this build of Houdini there are a ton of Vellum updates. Let's go over some.

- The Vellum Brush tool now exists! We can now paint on creases and patterns into our cloth sims, and softbodies.

- The Vellum brush also contains something called velocity dampening.

- For inflation effects you can scale the pressure per-point on your geometry. This helps create more gradual inflation effects.

- Pressure constraints are improved.

- A new Vellum Constraint Property SOP has been added to help edit Vellum Constraints

- NanoVDB support for VDB interaction with Vellum.

-  There is a set minimum amount of stiffness parameter on the Vellum Solver now.

- Free downloadable fabric setups

- A secondary bend constraint pass option has been added. When you are working with very shear fabrics, this tool can help you get their crinkles right.

- Annnnd more :)

Vellum Brush:

Let's talk about our new favorite tool...The Vellum Brush.

As previous mentioned above, The Vellum Brush allows us to paint on creases and patterns into our cloth sims, and softbodies. This tool has two modes. Damped and Settle. In the damped mode, you can sculpt curves into your cloth. In Settle mode, you can edit the way your cloth is draped over other objects. 

It is designed to edit pre-existing setups and add more detail, rather to create a whole new appearance of the sim. Keep in mind, while you are working with this tool, it will pause Houdini, and will not update the simulation until you are finished with your edits. 

Here are some quick tips for working with The Vellum Brush:

  • - Works best with high-res cloth/geometry.

  • - Keep in mind that this brush carries a Post Drag attribute. This means your cloth will settle from it's new shape, into a more relaxed state.

  • - You can use this node to edit and adjust individual frames in your vellum sims.

  • - Keep in mind, if you have Input Frame turned on, the input geometry, constraint, and collision geometry will be time shifted to this frame before brushing.

  • - This node is great for modeling operations.

  • - In Damped mode, Velocity Damping is very high, so there is very little momentum after brushing. 

  • - Damped and Settle mode will affect your parameters differently. Some of the main ones they affect are: Velocity Damping, Live Simulation, Time Scale, Gravity, Substeps, Constraint Iterations, and Smoothing Iterations.

  • - You can click and un-click check boxes of attributes you would like your brush to affect. These attributes are Stretch, Bend, and Volume.

Vellum Solver

Let's do a quick review of the Vellum Solver before we move onto constraints. 

  • The default Vellum Solver is a simplified DOP network to make your vellum solvers and simulation changes more easier. Here you can control your collision and sub-step iterations and numbers.

  • You add specialized forces inside this solver, or just use the default forces on the surface. It even has an internal ground plane that you can work with.

Pin To Animation

This is one of the most common constraint types you will use. It is incredibly useful for getting certain parts of your simulation to stay still. 

This constraint node has multiple options to help you stiffen up your vellum sim. 

Pin to Points: This can be very useful to pin the points of your vellum object together, or to an animated object.

- You can also match the movement of the points controlled by this constraint by turning on Match to Animation.

Plasticity

This attribute always confused me, so I figured It might to some good to explain it.

This parameter attribute controls how your object shifts back into it's resting state. Based on how the threshold is setup the object will either crawl back into it's original form, or seep into a new shape.

  • - It's Threshold Attribute will also scale the amount the object will stretch by as well.

  • - You can also get the plastic of the object to "harden" by scaling up the Scale of the constraint.

Vellum Post-Process

This node is really useful for cleaning and smoothing out your simulations. It works best with a cached out Vellum simulation. It can also visualize problems in your simulation, or popping areas. 

  • It also has the ablity add more passes on how your simulations move. So if you need to blur or add more substeps. This node will let you do that.

  • It can also upres cloth, and give it more subdivisions. 

Tips For Vellum

  • Vellum uses continuous collision detection. This is great to avoid fast moving geometry teleporting through your cloth, but is useless if your points are randomized every frame.

  • Don’t assume your geometry sequence is good.

  • Vellum only cares about points of the previous frame, so won’t mind if your primitive numbers are randomized each frame.

  • Graph color should generate as few sets as possible.

  • The default Vellum pscale is small, but any pscale attribute in the input override it. 

  • You don’t want your pscale to be bigger than your edge length, which could cause self intersections.

  • Vellum handles both external and self collisions. For example, if you have 50 iterations and 5 collisions, it will do 10 iterations between each collision. Increasing this helps with failed collisions, but also consider increasing substeps instead.

  • Having 2 substeps and 10 collisions is about same cost as 1 substeps and 20 collisions, but it will likely be better

  • Simulating cloth or hair far from the origin can cause problems and make the cloth/hair look crinkled.

 

References:

Vellum Brush geometry node: https://www.sidefx.com/docs/houdini/nodes/sop/vellumbrush.html

 

What’s new Vellum: https://www.sidefx.com/docs/houdini/news/18_5/vellum.html

Vellum Constraint Property geometry nodehttps://www.sidefx.com/docs/houdini/nodes/sop/vellumconstraintproperty.html

Vellum Solver geometry node: https://www.sidefx.com/docs/houdini/nodes/sop/vellumsolver.html#dosecondary

Downloadable fabricshttps://www.sidefx.com/docs/houdini/vellum/fabric.html

SideFX ships Houdini 18.5: http://www.cgchannel.com/2020/10/sneak-peek-houdini-18-5/

Stich and sliding constraintshttps://www.sidefx.com/docs/houdini/vellum/slidingconstraints.html

Varying friction: https://www.sidefx.com/docs/houdini/vellum/varyingfriction.html

Tips and troubleshooting: https://www.sidefx.com/docs/houdini/vellum/vellumtips.html

Setting up a 64-bit solve: https://www.sidefx.com/docs/houdini/vellum/64bit.html