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Since | 17.0 |
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 |
Meaning |
---|---|
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 |
Block |
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.) |
Disconnect Input |
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. |
Do Nothing |
Ignore this parameter, don’t create or change the attribute in any way. |
Parameters
Action
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 false
.
Primitive Path
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 /Models
branch.
The “Create primitives” section contains basic controls for how to create the new prim(s).
Primitives
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.
Create Primitives
This section only appears when the node is creating primitives.
For example:
-
If you want to create a new cube primitive at
/world/objects/cube1
on 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.
Primitive Count
The number of primitives to create.
Primitive Kind
Set all created prims to have this kind.
Primitive Specifier
The USD operator to use when creating the new prims.
Define
Authors a completely new prim. Use this if you want to create a brand new prim or replace an existing prim.
Over
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.
Class
Define a primitive class. This is usually not necessary unless you are doing deep USD magic.
Class Ancestor
If the Specifier is Define
or Over
, this parameter will cause some ancestor primitives to be authored with a specifier of Class
. This makes it easy to create an Over
or 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 Class
primitives.
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.
Axis
The axis the spine of the cone is aligned with.
Height
The length of the “spine” of the cone (from the point to the base).
Radius
The radius of the base of the cone.
Color
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.
Opacity
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.
Double Sided
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.
Transform
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.)
Append
Apply the parameters as a new, most local transform.
Prepend
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).
Transform order
The order in which Houdini applies translates, rotates, and scales, and the order in which it applies rotation.
Translate
Movement amount along XYZ axes.
Rotate
Degrees rotation around XYZ axes.
Scale
Non-uniform scaling along XYZ axes.
Uniform scale
Scales the object uniformly along all three axes.
Pivot translate
Moves the local origin of the object.
Pivot rotate
Rotates the local origin of the object after translation.