Guide for Houdini users#

If you know Houdini, most of comfyui_pcg will feel familiar: the same geometry model (points, vertices, primitives, detail), the same attribute names, Houdini's node names and parameter labels, display flags, a Copy Parameter / Paste Relative References workflow and a Copernicus-style image context. The differences come from two places: the plugin is written in Python and numpy rather than being Houdini, and it runs inside ComfyUI, whose graph is evaluated very differently from a Houdini network. This page lists what carries over directly, the systematic differences, and how to find the node you are looking for.

Naming#

• Every node's display name is the Houdini name followed by (PCG): PolyExtrude (PCG), Copy to Points (PCG), Attribute Promote (PCG). Image nodes are prefixed COP: COP Blur (PCG), COP HeightField Terrace (PCG).
• Several Houdini nodes are sometimes combined into one node with a mode menu; for example Twist, Bend and Taper are modes of Twist / Bend / Taper (PCG), and Attribute Remap is covered by Attribute Adjust (PCG).
• Where the plugin's node of the same name does something different from Houdini's, a second node with the suffix · Houdini follows Houdini's behaviour, for example Cross Section Surface · Houdini (PCG).
• Parameter labels and menu entries in the panel use Houdini's wording. The internal parameter names (tx, size_x, rand_seed) are what ch() and saved workflows use; the reference lists both.
• The internal node type is PCG_<Name>, shown on renamed nodes and in saved workflows.

To find the equivalent of a Houdini node, use the Houdini node index: it lists Houdini SOPs and Copernicus COPs with the comfyui_pcg node to use and whether it is a full or partial equivalent. Each node's reference entry has a Compared with Houdini section for known differences.

What carries over#

Systematic differences#

No VEX: Wrangle is vectorised Python#

Wrangle code is Python with numpy, run once over all elements, not once per element. @P is the whole column of positions.

• No per-element if; use where(cond, a, b) or masks. Python's and / or do not work on columns; use &, |, ~.
• Trigonometry is in radians, as in VEX.
• rand(seed) behaves like VEX (same seed, same number) and is combined with the node's Rand Seed.
• point(), prim(), detail(), npoints() and friends read the node's inputs by number, as in VEX; there are four inputs.
• ch("name") in Wrangle code reads a parameter of that Wrangle node (created automatically), not another node's parameter. Wire values in, or reference them from a panel field, to bring outside values into the code.
• There is no @group_name syntax for writing groups and no setpointgroup(); create groups with Group Create and an @ expression.

See Wrangle syntax.

Data model limits#

• Packed primitives, simplified. Pack / Unpack, Assemble and Copy to Points' Pack and Instance exist, the viewport instances them and Export Mesh writes them as glTF GPU instances, but only Transform, Merge, Blast / Delete, Sort and the point attribute / group nodes keep them packed; every other node unpacks its input automatically. There are no packed disk primitives, Assemble's packed pieces carry no path attribute, and there are no intrinsics beyond the transform.
• VDBs are dense grids with VDB semantics. The VDB and Volume SOPs are on the Volumes & VDB page; a VDB here has an active mask and a background like OpenVDB, but stores every voxel, so very large sparse grids cost memory. Grids are axis aligned (VDB Transform resamples a rotation) and there are no frustum grids or VDB Points (points are points).
• NURBS / Bezier curves and surfaces exist with uniform clamped knots only; Convert, Carve, Resample and a few generators support them. NURBS surface modelling (projection, surface intersections) is not available.
• Height fields are images. Houdini's HeightField SOPs correspond to COP HeightField nodes working on an image layer (default C), not to volumes.
• No array or dictionary attributes, and strings created by Attribute Create or Wrangle are limited to 64 characters.
• No metaballs, tetrahedra, agents, KineFX, Vellum, Pyro or RBD solvers.

The group field#

• A pattern is either tokens (names, numbers, ranges, exclusions) or, if it contains @, a single expression. @P.y > 0 top does not mix them as Houdini can.
• Group names do not take wildcards in Group fields (piece* is an error); Group Combine does accept them.
• Edge fields accept only edge-group names (no point-pair or number syntax), and unknown edge-group names are silently ignored.
• In Group Combine, ^ means symmetric difference, while in a Group field ^ means exclude.

ComfyUI evaluates graphs differently#

Viewport and interaction#

Selecting a Transform, a generator such as Box or Grid, or a few other nodes shows a translate / rotate / scale handle in the viewport, and selecting a Curve (PCG) node turns the viewport into a curve drawing tool (see Handles). Handles edit the node's parameters. S gives a select mode: click / box selection of points or primitives becomes a group expression you can copy or turn into a Group Create node (see Selecting); there is no brush tool, and a selection does not feed nodes by itself the way Houdini's viewport state does. The viewer uses Houdini's mouse layout (left tumble, middle track, right dolly; Alt + left tumbles while a handle is active) and the camera bar can lock a camera to the view.

Mapping cheat sheet#

See also#

• Getting started
• Concepts: Geometry, Group patterns, Expressions, Parameters, Loops and time, Images, Viewing and export
• Houdini node index and Node reference