= Trim NURBS surfaces =

== Create or edit profile (trim) curves ==

Houdini supports _curves on surfaces_ (also known as _trim curves_ and _profile curves_). Profile curves are 2D curves that exist in the parametric (UV) space of their parent surface. Houdini draws profile curves as dotted lines on the surface.

tasks>>
    Create a profile curve on a surface:
        * Use the [Project surface node|Node:sop/project] to project a curve onto a surface. Note that you cannot project [quadratic primitives|primitive_shapes].

        * Use the [Surfsect node|Node:sop/surfsect] to create profile curves where two surfaces intersect.

    Trim a NURBS surface with a profile curve:
        # Create one or more profile curves on the NURBS surface you want to trim.
  
        # Select the surface you want to trim.
        
        # In the viewer, use the [tab menu|/baiscs/tab_menu] to choose the [Trim node|Node:sop/trim].
        
        # In the parameter editor, choose whether you want to trim inside or outside the areas defined by the profile curve(s).

    Extract a profile curve to a 3D curve:
        # Add a [Profile node|Node:sop/profile] and in the parameter editor click __Extract__.

        # By default, the operator deletes the surface it extracts the curve from. Turn on __Keep surface__ to prevent the operator from deleting the surface.

    Transform a profile curve:
        # In the [display options|/basics/view], turn on display of Profile Numbers.

        # Note the number for the profile curve you want to delete. This will be in the form `2.0` (for profile curve 0 on surface 2).

        # Add a [Primitive node|Node:sop/primitive]. In the parameter editor, turn on __Do transformation__ and set the __Source group__ to the ` <<x>>.<<y>> ` profile number of the profile curve you want to transform.

        # Use the __Translate__, __Rotate__, and __Scale__ parameters or the manipulator in the viewer to transform the profile curve.

    Delete a profile curve:
        # In the [display options|/basics/view] or the toolbar on the right side of the viewer, turn on display of Profile Numbers.

        # Note the number for the profile curve you want to delete. This will be in the form `2.0` (for profile curve 0 on primitive 2).

        # Add a [Delete node|Node:sop/delete]. In the parameter editor, turn on the __Enable__ checkbox below __Number__.

        # In the __Pattern__ parameter, enter the `<<x>>.<<y>>` profile number of the profile curve you want to delete.

== Notes ==

- Houdini has polygonal, NURBS, and Bezier profile curves. Each profile curve type is very similar to its 3D equivalent. For example, a NURBS profile can be open or closed, clamped or un-clamped, rational or non-rational, with order between 2 and 11.

- The entire profile or part of it can move outside the surface boundaries, and become clipped at the surface edge. A profile curve that is partially or completely clipped is still valid and can be moved back onto the surface. See the Profile and Primitive sops for ways to manipulate a profile.

- You can check whether a node's geometry has invisible profile curves by pressing ((MMB)) on the node tile. The pop-up info window shows the number of visible and hidden profile curves in the geometry.

- A profile can co-exist with 3D primitives in primitive groups. A resulting mixed group would specify its elements like this: `1 0.2 6 2.9-2.17 4.*`.