Operators Houdini's procedural nature is found in its operators. Digital assets are generally constructed by connecting sequences of operators (or
OPs). This proceduralism has several advantages: it allows users to construct highly detailed geometric or organic objects in comparatively very few steps; 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 generation in production of
textures,
shaders,
particles, "channel data" (data used to drive animation),
rendering and
compositing.
Operator Categories Houdini's operator-based structure is divided into several main groups: • OBJs – The 'Object' context should be considered as Houdini's 'classic' (or legacy) scene-objects area and contains nodes that usually pass transform information down when wired together (Traditionally these are things like cameras, lights, and geometry subnetworks that contain Sops, but Object would have also been used for rigging in Houdini in the past.) This has nowadays been deprecated in preference to assembling the scene in Lops (ie using USD/Solaris). It is worth noting here that a subnetwork of nearly any category can be created within any other category in Houdini. So, for example a Ropnet inside a Sopnet inside an Object Subnetwork, would be totally valid. • SOPs – Surface Operators – for generic geometry processing, procedural modelling and some types of simulations. • POPs – Particle Operators – used to manipulate
particles systems. • CHOPs – Channel Operators – for procedural animation and audio manipulation. • COPs – Composite Operators – used to perform compositing on footage/image sequences or generate/bake images based on results from the scene. • DOPs – Dynamic Operators – for dynamic simulations for fluids, cloth, rigid bodies (bullet) etc. • SHOPs – Shading Operator – Legacy context for representing/authoring materials for different renderers. Note this context has been replaced by 'matnet' material contexts more recently and SHOPs is now considered deprecated. • ROPs –
render operators – for building networks to represent different render passes and render dependencies. This can include any number of steps of processing and is not restricted to pixel-production. • VOPs – VEX operators – for building sub-networks of nodes of many of the above-mentioned types that execute using a highly optimized SIMD architecture. Examples include the 'Attribute Vop' and 'Volume Vop' Sops, the 'Gas Field Vop' and 'Pop Vop' Dop nodes, there are many other examples throughout houdini that allow one to write code using nodes. • TOPs - Task Operators • LOPs - Lighting Operators - for generating USD describing characters, props, lighting, and rendering.
Operator Wiring and Processing Operators (nodes) are connected/wired together in networks. In some of the contexts such as Sops, Cops and Chops, data can be thought of as flowing through the wires, manipulated by each operator in turn. In other contexts the network connections represent other relationships such as 'attachment' (Dops) or 'ordering' (Rops and Tops). Houdini can generate, process and write out many types of data, including 3D
geometry,
voxel,
bitmap images, particles,
dynamics,
shader algorithms,
animation,
audio, or a combination of these and custom formats. This
node graph architecture is similar (in some contexts) to that employed in node-based
compositors such as
Shake or
Nuke.
Houdini Digital Assets Complex networks can be grouped into a single meta-operator node which behaves like a class definition, this is known as a 'Houdini Digital Asset' or HDA (legacy users might also know the name 'OTL' which stands for "Operator Type Library" - essentially a collection of HDAs in one file). These user-defined node-types can be instantiated in other networks (of compatible category) like any other node. In this way users can create their own sophisticated tools without much need for programming. Houdini can be regarded as a highly interactive visual programming toolkit which makes results much more accessible to artists by connecting nodes, reducing the need to write code.
Hard to Learn Houdini’s toolset is largely implemented through operators, which can result in a steeper learning curve compared to similar tools. It is not always practical to understand every node type; instead, effective use of Houdini involves representing creative outcomes as networks of nodes. Experienced users are typically familiar with a range of common approaches (network structures similar to algorithms) that produce desired results. The flexibility provided by lower-level building blocks supports both artistic and technical workflows. Proficiency in Houdini often requires time, practice, and access to mentorship.
Conversely Within large productions, the development of a procedural network to solve a specific element creation challenge can make automation much easier. Many studios that use Houdini on large feature effects, and senior Houdini artists/TDs at game studios and feature animation companies develop libraries of procedures (usually embodied as HDAs) that can be used to automate the generation of many of the elements for the film or game, with almost no need for regular-artists to edit the underlying (more complex) setup to get an approval for their work.
IO 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 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. ==Rendering==