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In the contest for world domination in visual effects, Side Effects Software is an underdog. In terms of sheer numbers, it is miles behind market-leading vendors such as Alias, Discreet and Softimage, and the pool of Houdini-trained talent is proportionately small. However, the list of studios that do use Houdini is impressive, and growing, and includes luminary companies such as Sony Pictures Imageworks, Rhythm & Hues, Disney and Digital Domain.
While Houdini is a power-packed application (prices range from $1,299 to $17,000, depending on feature set), new users will quickly discover that its limits are more in the density of the interface than in what it can do. For this reason Houdini is commonly regarded as a "technical director's" package. A huge amount of learning goes into mastering the application, but a skilled TD can readily build projects that lend themselves well to multiple iterations of art direction and cycles of manipulation by effects supervisors and other artists. In production environments, this iteration loop is such an integral part of effects development that it can eclipse other issues, such as the quantity of user-friendly out-of-the-box features, or effects bells-and-whistles.
Houdini projects are based on visual networks of operators, or "Ops," which are containers for geometry, particle systems, animation, textures or actions anything you can represent in a 3D scene. These networks allow an artist to build intuitive flow-diagram relationships, and the ease of manipulating this flow, and inserting new logic at any step, is the hallmark of Houdini. Everything is procedural, and can be manipulated by selecting the right node and modifying the data it contains.
To build a simple demo file rocket, for example, you would chain together a series of SOPs, or Surface Operators. (One of the idiosyncrasies of Houdini is the necessity of starting with a default geometry object, which is then replaced by the type of geometry you actually want to use.) You start with a Nurbs tube SOP then add a cap SOP for the nose cone. The fins are defined with a chain of SOPs: curve, extrude, xform, xfrom, fuse, polybevel and copy. Finally, the rocket body and fins are combined with a merge SOP and transformed with another xform SOP.
There is nothing particularly daunting or difficult about working with Houdini's Ops in many ways the interface is comparable to using 3ds max's history stack, or to manipulating nodal dependence diagrams in Maya's Hypergraph. As in Maya, which lets you move easily in and out of its Hypergraph or Hypershade editors to see projects in an outline or other views, Houdini gives you ways of looking at projects without necessarily having to decipher long chains of procedural relationships. Still, to work in Houdini, you're going to have to buy into its flow-chart workflow, whether or not you think you have any need for parametric control at every step the way.
One concession Side Effects Software makes to the idiosyncrasies of different production environments is that the interface is highly malleable and customizable. Various production layouts can be saved as "desks," and these can be quickly enlisted to present important components in a useful way. The interface overall is fast and easy to use once you master the various hot keys and come to grips with the fact that all tools, however unrelated, live in large textual lists, and the only way to effectively access them is to hit the tab key and type their names to select them from the list. In practice, this approach is pretty fast, but it makes for tough learning, especially when you don't know what tools are available for a certain type of task, much less what it might be named.
While character animation is a core feature of other applications, Houdini has received relatively little attention for its character tools. In fact, Houdini offers a complete set of character set up and animation features, including powerful FK/IK controls and deformations. While character animation is not Houdini's calling, the tools are capable of producing character performances with subtle levels of detail. By themselves, the character animation features are probably not a compelling reason to invest in Houdini, but they do complete the pipeline for projects that need character work to go with their effects animation.
Model building is probably the area that suffers most from Houdini's procedural workflow. While the software does have a reasonable set of Nurbs and polygonal modeling tools, modeling itself requires constant switching between many types of tools, snap settings, and tool variations, and pushing and pulling points and surface elements on various axes. This all implies a fast-and-loose workflow that is at odds with Houdini's add-a-SOP-manipulate-and-repeat methodology. Furthermore, Houdini's modeling toolkit is considerably smaller than those in competing products. Most significantly, Houdini lacks for subdivision surfaces as a model type, even though they have become the new standard in animation environments. (Side Effects does point out that you can render polygons as sub-d's in rendering.)
The one area where Houdini has made its greatest inroads and truly excels is in the creation of dynamic effects. Like Maya, and other high-end apps, Houdini has a robust set of rigid and soft body dynamics and particle effects. What sets Houdini apart in this realm is the easy of creating complex effects, thanks to the Ops workflow. While complex effects in Maya, for example, generally require heavy levels of scripting, effects in Houdini can often be achieved by simply adding operators where needed along the chain. There's nothing wrong with scripting per-se, but effects that rely heavily on scripting also rely heavily on TDs who can tear apart and rewrite scripts every time a change needs to take place in an effect. As in other parts of Houdini, changing what's already been done is relatively easy by comparison. Also, importantly, effects can be easily packaged and exported to other projects where they can be modified to meet the demands of the new scene.
Houdini comes ready to generate some sophisticated effects right out of the box. And, in particular, it offers powerful features for creating flocking behaviors and working with instanced geometry. Key to this is the capability to manipulate the forces between particles.
As with any effects system, greater degrees of complexity call for the direct application of math supplied by well-schooled TDs. Past versions of Houdini limited scripts to single-liners that could be placed within data. One of the appreciable changes in version 7 is a multi-line script editor that will remind many artists of Maya's Script Editor.
Houdini's effects system also incorporates a rich set of soft and rigid-body dynamics and good control over using scene geometry as both particle emitters and collision objects. Particle systems are able to preserve the momentum imparted by emission objects, another important aspect of creating compelling dynamic effects.
Houdini, like other high-end production packages, features a powerful set of rendering features. Mantra, Houdini's built-in renderer (network licenses are available separately), is a powerful renderer particularly for particle effects, since it is adept at volumetric rendering. This is key for creating realistic smoke and related effects, and one of the principle challenges of particle dynamics.
Houdini supplies a shader language, VEX, that is a robust environment for creating shaders in Mantra, mental ray and RenderMan format, and its support for all rendering environments is excellent.
Houdini's innovative visual interface for creating RenderMan shaders makes it a unique environment for harnessing the power of the renderer. In other applications, RenderMan is very much the domain of code-savvy TDs, limiting its utility for the texture artists who ultimately need finer control over shaders and their properties.
Unfortunately, even at the rarified $17,000 price point, Houdini offers no native support for hair or fur shaders, although it does offer tools for procedural plant and tree generation. Houdini is also conspicuously lacking a computational fluid dynamics simulation engine, although third parties, such as NextLimit's RealFlow offer plug-in support for Houdini.
However, $17,000 does buy you a compositing system built in the 3D environment, and compelling arguments can be made for the capability to composite effects and animation data within the 3D environment. Background plates, for example, can be integrated into textural, lighting and dynamic effects.
One of the challenges of working Houdini into a pipeline will be deciding which version of the software to buy. The company provides a range of versions, each with a selection of Houdini Master's all-you-can-eat menu of functionality. Side Effects Software offers an apprentice program that allows you to download and work with a restricted version of the application, which is an excellent introduction to Houdini's power.
Thus, Houdini is a compellingly powerful effects animation package that can fit into almost any production environment. Its procedural power is particularly evident in creation of dynamic effects and particle systems, and version 7 is the cleanest, most capable version yet.
Sean Wagstaff is a freelance technical director who creates special effects for film and games, most recently for Double Fine Prods. and The Orphanage.