Houdini no longer has SHOP networks. Materials are now built from pure 
VOPs.
Nodes
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Channel nodes create, filter, and manipulate channel data.
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Composite nodes create, filter, and manipulate image data.
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COP nodes provide real-time image manipulation within a 3D space.
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Dynamics nodes set up the conditions and rules for dynamics simulations.
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LOP nodes generate USD describing characters, props, lighting, and rendering.
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Object nodes represent objects in the scene, such as character parts, geometry objects, lights, cameras, and so on.
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Render nodes either render the scene or set up render dependency networks.
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Geometry nodes live inside Geo objects and generate geometry.
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Containers for VOP networks.
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VOP nodes let you define a program (such as a shader) by connecting nodes together. Houdini then compiles the node network into executable VEX code.
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TOP nodes define a workflow where data is fed into the network, turned into work items and manipulated by different nodes. Many nodes represent external processes that can be run on the local machine or a server farm.
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APEX nodes provide operations for building up the functionality of APEX graphs, which are used in KineFX to create character rigs and perform other geometry manipulation.