Houdini is a high-end 3D animation package developed by Side Effects Software which is headquartered in Toronto, Canada. It is a rewrite of the PRISMS ecosystem of standalone tools. Its chief distinction from other packages is that it has been designed as a purely procedural environment. A version of the product, called Houdini Apprentice, is available as a free download for non-commercial use.
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Houdini covers all the major areas of 3D production, including:
Houdini is an open-environment and supports a variety of scripting APIs. Python is increasingly the scripting language of choice for the package, and is intended to substitute its original CShell-like scripting language, Hscript. However, any major scripting languages which support socket communication can interface with Houdini.
Houdini tasks the Graphics Processing Unit (GPU) rather than the Central Processing Unit (CPU) which is not typical to most 3D apps. As a result, Houdini specifies the requirements as an ATI FirePro or an Nvidia Quadro graphics card which is more stringent than the requirements for most 3D software. Due to this difference in function from typical 3D software, the CPU requirements are far more open. In addition, Houdini also requires a 3 button mouse (scroll wheel optional) and recommends a wacom tablet for painting tools. Houdini runs on Windows, Mac and Linux.
Houdini's procedural nature is found in its operators - digital assets are generally constructed by connecting sequences of operations (or OPs). This proceduralism has several advantages: it allows users to construct highly detailed geometric or organic objects in comparatively very few steps compared to other packages; it enables and encourages non-linear development; and new operators can be created in terms of existing operators, a flexible alternative to non-procedural scripting often relied on in other packages for customisation. Houdini uses this procedural paradigm throughout: for textures, shaders, particles, "channel data" (data used to drive animation), rendering and compositing.
Houdini's operator-based structure is divided into several main groups:
Operators are connected together in networks. Data flows through, manipulated by each operator in turn. This data could represent 3D geometry, bitmap images, particles, dynamics, shader algorithms, animation, audio, or a combination of these. This node graph architecture is similar to that employed in node-based compositors such as Shake or Nuke.
Complex networks can be grouped into a single meta-operator node which behaves like a class definition, and can be instantiated in other networks like any compiled node. In this way users can create their own sophisticated tools without the need for programming. In this way Houdini can be regarded as a highly interactive visual programming toolkit which makes programming more accessible to artists.
Houdini's set of tools are mostly implemented as operators. This has led to a higher learning curve than other comparable tools. It is one thing to know what all the nodes do - but the key to success with Houdini is understanding how to represent a desired creative outcome as a network of nodes. Successful users are generally familiar with a large repertoire of networks (algorithms) which achieve standard creative outcomes. The overhead involved in acquiring this repertoire of algorithms is offset by the artistic and algorithmic flexibility afforded by access to lower level building blocks with which to configure shot element creation routines. In large productions, the development of a procedural network to solve a specific element creation challenge makes automation trivial. Many studios that use Houdini on large feature effects, and feature animation projects develop libraries of procedures that can be used to automate generation of many of the elements for that film with almost no artist interaction.
Also unique to Houdini is the range of I/O OPs available to animators, including MIDI devices, raw files or TCP connections, audio devices (including built-in phoneme and pitch detection), mouse cursor position, and so on. Of particular note is Houdini's ability to work with audio, including sound and music synthesis and spatial 3D sound processing tools. These operators exist in the context called "CHOPs" for which Side Effects won a Technical Achievement Academy Award in 2002.
VEX (Vector Expression) is one of Houdini's internal languages. It is similar to the RenderMan shading language. Using VEX a user can develop custom SOPs, POPs, shaders, etc. The current implementation of VEX utilizes SIMD-style processing.
Houdini is bundled with a production-class renderer, Mantra, which has many similarities to PhotoRealistic RenderMan in its scope and application. Micropolygon rendering is supported, allowing high-quality displacement operations as well as traditional scan-line and raytracing modes. Shaders are scriptable and composed in their VEX language, or by using VOPs; their node-based interface to programming VEX. Mantra (as does Houdini itself) also supports point-clouds, which can be similar in application as brickmaps in Renderman. This allows more complicated light interactions, such as sub-surface scattering and ambient occlusion, to be produced with lower computational overhead. Mantra can perform extremely fast volume rendering, and also Physically Based Rendering - a technique which attempts to more accurately model the interactions of light.
Historically, Houdini's main strength has been its particle animation system, and its major weakness its unintuitive animation tools. As a result, its use was relegated to special effects. However, in more recent versions these tools have been vastly improved and it has been used in various feature animation productions, such as The Wild, a major Disney feature film as well as the feature animation Ant Bully.
Derivative Inc. is a spin-off of Side Effects Software that markets a derivative of Houdini called TouchDesigner. Tailored toward real-time OpenGL-generated animation, it was used on rock group Rush's 30th Year Anniversary Tour to produce dynamic graphics driven directly by the musicians.
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