houdini tutorial database

Directable Particles

Directable Particles

cmiVFX has released their newest lesson offering another glimpse into magical world of visual effects. The flows of power are often visualized with streamers of light chaotically traversing across the scene. How does one create chaos but also maintain the control a director will require? In this lesson we will teach one way this is done in a production environment. The workflow offered here is tested and proven effective within the film industry. This new, exclusive effect technique can succeed in scenarios where conventional particle control systems fail. Additionally, many lessons stop with mere particles in place. Martin Sawtell will teach you one way in which those particles may be used after the simulation is done. Martin answers the question, "Particles are simmed, now what?". He demonstrates a typical way to pass the results of a particle effect on to the compositing team for further refinement. This courseware will give you a clear idea of what is required of a VFX professional and how excellent results are achieved under the pressure of a real deadline.

This video introduces viewers to the powerful world of VOPs within Houdini, and how it can be used to direct particles via it's point cloud features. Exploring VOPs implementations in shading & rendering, geometry and POPs it presents an entire workflow that brings them together to provide you with an intuitive way to control particles. Central to this workflow is the process of developing a "point cloud advect" Digital Asset that can be re-used in any particle system- allowing the user to push particles around with arbitrary geometry that could come from SOPs, another sim or even a Mantra render.

Emitter Setup
Half of any particle system is it's Emitter! In this segment a novel way to generate source points is presented- using mantra to write a point cloud using a noise pattern in a shader (which is in turn reading in sops geometry as a point-cloud). The chapter also covers the basic task of setting up the guide curves that the particles will follow later. If you are new to the concept of VOPs, point clouds, and their implementation in Houdini via the geometry and shading contexts this provides a good introduction.

Particle Tendrils
Getting on to the fun stuff (particles) this chapter introduces POP VOPs and how they can be used to open a point cloud and then advect particles based on variables like their distance from the sampled geometry and it's control attributes. It also shows how to generate useful vectors on the geometry for pushing the particles around, combining the vectors, troubleshooting the process and setting it up for re-use in other particle sims. By following the tweaking process a workflow is beginning to emerge which you can apply to many problems that require directable particles.

Building Tools And Adding Features
So you now have a cool way to push particles around... So what? Time to turn it into a tool, that's what. Building a nice interface, writing to an OTL (digital asset) and taking into consideration that other people/houdini sessions might be using the asset while you are editing it are all covered. The particles are also further tweaked to bring in some nice noise (as a feature on the tool and separately on the geometry) plus a suction funnel is added to drag the particles into the end of the teapot.

Simming And Render Settings
In theory and motion the particles look good, but that's only in the viewport. A practical method for splitting the sim up for distribution using python is presented, as well as shading, lighting and rendering the particles with AOVs that Nuke will be able to read. It's not difficult to give nuke an EXR that has the particle's age, each light and other shading variables encoded in different image planes. While rendering and comping the particles isn't the focus of this entire lesson it's worth showing some common methods for getting what you need later into nuke.

So we've covered the workflow in a linear, detailed fashion so far but it's worth branching out and demonstrating different applications of the tool and related VOPs workflows. A very quick and dirty video that aims to give you a starting point and inspire you to do things like flow particles over a surface, use VOPs to define forces in a FLIP sim, advect particles with a pyro sim and naturally make some cool noised motion. At the end of this sequence of videos you'll have the know-how to build your own tool that can write velocity/force, position, and arbitrary attribute modifications to particles, geometry, shaders and fluids, unleashing in your mind the possibilities that VOPs and VEX offers in Houdini. Once you see the possibilities you will lose sleep.