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A single basis curves primitive may contain any number of individual curves. Renderers may render curves as tubes or ribbons, using the
width array property (which must have the same length as the
points property defining the vertex positions of the curves).
Creating vs. editing prims
This node belongs to a class of nodes that create or edit USD prims directly. These nodes operate in Create mode or Edit mode. This is controlled by a Create primitives checkbox or a Create/Edit popup menu. In create mode, the node creates new prims. In edit mode, the node changes the attributes on an existing prim. The Edit mode has two variations. Edit will not modify primitives which have a
houdini:editable attribute set to
false. Force edit will modify a primitive regardless of the existence or value of this attribute. This attribute can be set on a primitive using the Configure Primitives LOP.
Parameters that correspond to a USD attribute have a pop-up menu to the left that controls how the node authors the attribute.
In addition to that, any connectable USD attributes (i.e., the ones in the
inputs: namespace) will have menu items that allow disconnecting them from their sources.
Pop-up menu item
Set or Create
Sets the attribute to the given value, whether it previously existed or not.
Set If Exists
Only set the attribute to the given value if it previously existed.
Use this mode to make sure an attribute is only set on primitives of the correct type. For example, only
Makes the attribute appear to not exist, so it takes on its default value. (If the attribute doesn’t already exist on the prim, this does nothing.)
Deletes the attribute input connection to its source. Input connections take precedence over attribute values, so disconnecting an input allows the attribute value to take effect.
Ignore this parameter, don’t create or change the attribute in any way.
Whether this node should create new prims, or edit existing prims. In addition, the
Force Edit option can be chosen to cause this node to ignore the
houdini:editable attribute on prims, and always edit the specified attributes. This is in contrast to the
Edit mode which will trigger a warning and not set attributes on prims with the
houdini:editable attribute set to
In create mode, this lets you control where in the scene graph to create the prim(s).
The default is usually
/$OS. This creates a primitive at the root level with the same name as the node (for example,
/tube1). This is a useful default for preventing naming conflicts, but terrible for organization. You should try to remember to change the Primitive path to a better value when you create prims.
For example, instead of naming models after the node that created them, you might want to name them after the geometry inside, and organize them under a
The “Create primitives” section contains basic controls for how to create the new prim(s).
In edit mode, the node has a Primitive pattern parameter. This lets you specify the prim(s) the node should operate on. You can click the select button beside the text box to select the primitives from the scene graph tree. You can also use primitive patterns for advanced matching, including matching all prims in a collection.
Initialize Parameters For Edit
In edit mode, changes the state of all control menu parameters to
Do Nothing, so that this node will not apply any changes. Also grabs the current values of each property from the first Primitives match, and sets the values of the corresponding parameters to match. This means that changing any parameter’s control menu to
Set or Create mode will set the property to its current value, making it easier to apply changes to an existing value rather than setting a brand new value.
This section only appears when the node is creating primitives.
If you want to create a new cube primitive at
/world/objects/cube1on an empty stage: Set Primitive Specifier to “Define”, and the Parent Primitive Type to “Xform”.
If you want to override the radius of a sphere at
/world/objects/sphere1: Set Primitive Specifier to “Over”, and the Parent Primitive Type to None. This makes sure the primitive types of any existing ancestor prims are not be modified by this node.
The number of primitives to create.
Set all created prims to have this kind.
The USD operator to use when creating the new prims.
Authors a completely new prim. Use this if you want to create a brand new prim or replace an existing prim.
Authors an override of an existing prim. Attributes not explicitly authored on this prim will get their values from the existing prim on the lower layer.
Define a primitive class. This is usually not necessary unless you are doing deep USD magic.
If the Specifier is
Over, this parameter will cause some ancestor primitives to be authored with a specifier of
Class. This makes it easy to create an
Define within a
Class without having to use two separate nodes. When the Specifier is
Class, this parameter is disabled because the entire primitive hierarchy is already authored as
Parent Primitive Type
If any parents of a path in Primitive paths do not exist, this node will automatically create them. In this case, it will create parent nodes of this type.
Sets a “default” color for this shape, which will be used in the viewport if the prim does not have a shader/material. You can write shaders to take this
displayColor parameter as an override.
Sets the opacity of the Display color. This is a separate attribute (instead of specifying the display color as RGBA) so it can be independently overridden, and also because shaders don’t usually accept RGBA parameters.
Each pair of connected vertices defines a separate linearly interpolated line segment.
A more complex interpolation function is used to determine the path of the curve, as specified by the Curve basis parameter.
If the curve type is cubic, this option determines the path that the curve follows between the points that define its shape. In all these bases, the position of the curve is a calculation involving a sequence of four points.
Use bezier interpolation of the vertices to generate the path of the curve. This is essentially equivalent to the Bezier curve option of the Curve SOP.
Use B-Spline interpolation of the points to generate the curve. This is essentially equivalent to the NURBS curve option of the Curve SOP.
Use Catmull Rom interpolation of the points to generate the curves.
Controls the behavior of the start and end of each curve.
The curves points define the start and end points of the curve. Not all cubic curve types will necessarily actually touch the start and end point positions, though bezier and linear curves will.
The points of the curves are rendered as a loop. For linear curves, the last point is simply joined to the first. For cubic curves, addition points from the start of the curve may need to be reused to compute the path of the curve.
For BSpline and Catmull Rom curves, this wrapping option tells the renderer to add “phantom points” to the curve which cause the visual representation of the curve to start exactly at the first vertex position, and end at the last vertex position. The renderer is expected to compute these phantom point positions automatically.
For renderers that distinguish between single-sided and double-sided surfaces, controls whether this prim is double-sided. Making surfaces double-sided is necessary for flat thin surfaces, or open surfaces where the camera can see both the inside and outside of the surface.
How the transform parameters below apply to the primitive. (Because each USD prim can contain multiple transforms, and be affected by parent transforms, there are many different ways to apply new transforms.)
Apply the parameters as a new, most local transform.
Apply the parameters as a new, least local transform.
Overwrite or Append
If a local transform exists with the name
xformOp:transform, overwrite it with the parameters. Otherwise, apply it as a new, most local transform.
If this replaces an existing transform, that transform’s order in the local transform stack is not affected.
Overwrite or Prepend
Like “Overwrite or Append”, but least local.
Apply Transform in World Space
Apply the parameters as a new, most local transform. However, prior to applying the transform, it is updated so that the result of the transform appears as if the transform was being applied in world space, before any local or ancestor transformations were applied to the primitive.
Replace All Local Transforms
The transform specified in the remaining transform parameters is always applied as a new transform. Prior to applying this transform, all existing transform operations on the local transform stack are cleared so that only the supplied transform remains on the local transform stack (though ancestor transforms still have an effect).
The order in which Houdini applies translates, rotates, and scales, and the order in which it applies rotation.
Movement amount along XYZ axes.
Degrees rotation around XYZ axes.
Non-uniform scaling along XYZ axes.
Scales the object uniformly along all three axes.
Moves the local origin of the object.
Rotates the local origin of the object after translation.