Prior to the Otis Muscles and Tissue system, the Vellum system had a multi-pass approach for the muscle, tissue, and skin simulation. Each layer required independent setup, caching, and constraint management. See Vellum muscles and tissue overview for more information.
Otis builds upon these concepts by introducing a single-pass system where all the simulation layers interactively live. These layers include the muscle and bones, tissue core, and tissue solid (fat). The single-pass framework simplifies the setup process and reduces the need for complex manual tuning. See Introduction to Muscles and tissues for more information.
Migrating characters from Vellum to Otis ¶
The Otis Muscles and Tissue system introduces fundamentally different achitecture compared to the legacy Vellum-based workflow. While you can migrate existing Vellum projects to Otis, this process it not automatic and requires manual updates to the node network, paramater values, and geometry setup.
The recommended method is to take your character and setup the muscle and tissue simulation again using the Starter Recipes provided. This ensures you are using all the correct nodes, solid default starting values, and your geometry is setup properly to create the auto-generated simulation layers. The geometry preparation process for both Otis and Vellum are similar. You need your muscles and bones geometry, animated bone surface, and render skin geometry to get started.
If you plan to migrate your characters manually, make sure to check the following:
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Replace any nodes with the Vellum suffix with the Otis suffix.
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Existing Muscle Flex setups will still work using an older version of the Muscle Flex SOP, as it is independent of the new Otis workflow.
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Your parameter values in Vellum such as damping or stiffness do not tranfer to Otis. You will have to reconfigure your values to match the visual output within Otis.
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Replace your solver nodes with an
Otis Muscle and Tissue Configure SOP followed by an 
Otis Solver SOP, with both outputs of the former connected to both inputs of the latter. Connect your muscles into the first input, the animated (and triangulated) bones into the second input, and the tissue into the third input of the Otis Muscle and Tissue Configure SOP. -
You should expect that simulation results will look different even when using the same animation, geometry, and default parameter values.
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The Otis simulation layers and solver system behaves compeletely different than the Vellum system.
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Note
Please note that though the Otis Solver is a SOP node, it is not available in Houdini Core. However, a .hip file containing this node can still be opened in Houdini Core, but the node’s parameters will be greyed out.
Warning
The Otis Muscles and Tissue system is not backward-compatible with Vellum nodes and its parameter value ranges. It’s recommended to replace them with Otis specific nodes and its parameter value ranges.