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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.
Editing the camera in the viewport
Enter the Camera state
Select the Camera node in the network, then in the viewport click the Handles tool, or press Enter.
⇧ Shift + T
Depth of field
⇧ Shift + F
Cameras in USD are always “Y up”, regardless of the stage’s orientation, and look down the -Z axis.
The parameters on ths node are in millimeters. USD expects camera values to be in “tenths of a world unit”, which means the parameters and values on the prim will sometimes be different.
Cooking this node can generate many USD time samples, rather than just a single time sample at the current time. This can be equivalent to having a Cache LOP following this node, but it will evaluate much faster, and does not cache data from any other nodes. This allows animated data to be authored to USD without introducing a node time dependency which would then cause all following nodes to also be time dependent. This can vastly improve playback performance of some LOP Networks.
In all sampling modes, if a parameter on this node does not vary with time, and does not rely on other time sampled data from the stage, only a single default value will be generated in USD for the corresponding attribute. USD time samples are only generated for parameters that may vary over time.
Sample Current Frame
A single time sample will be generated for the current time.
Sample Frame Range If Input Is Not Time Dependent
If the input to this node is time dependent, this node behaves as if it is in
Sample current frame mode. Otherwise it behaves as if it is in
Sample frame range mode.
Sample Frame Range
The Start/End/Inc parameter is used to generate multiple times at which this node’s parameters are evaluated, and a USD time sample is created for each attribute at each one of these times.
When the Sampling behavior is
Sample frame range, this parameter controls the number and spacing of base time samples to be generated by this node. The default values of this parameter are
@ropinc. These values correspond to the start, end, and step size of the global Houdini animation settings when interacting with Houdini. When using a ROP node to generate a range of frames, these values correspond to the start, end, and increment values specified on the ROP node being executed. This default ensures that a USD file written to disk will contain time samples for exactly the frame range requested by the ROP (regardless of the Houdini animation settings).
For each primary sample generated by this node, these parameters can cause additional samples to be generated aroudn that primary sample time. This is most often used to ensure that accurate data exists at exactly the camera shutter open and close times, as well as at the primary sample time.
Controls the method used to specify the shutter open and close times relative to the primary sample times.
The Shutter open/close parameter values provide exact offset values relative to the primary sample time.
Use Camera Prim
The Camera prim parameter provides the scene graph path of a camera primitive from which the shutter open and close times are extracted to provide the offset values relative to the primary time sample.
When the Shutter mode is
Specify Manually, these two offset values are added to the primary sample time to indicate the shutter open and close times. The open time should be less than or equal to zero, and the close time should be greater than or equal to zero.
Scene graph path of a camera prim on the input node’s stage. The shutter open and close attribute values are read from this primitive.
The number of subframe samples to create for each primary sample. These samples are evenly distributed between the shutter open and close times. Note that such an even distribution may or may not create a sample at exactly the primary sample time.
Always Include Frame Sample
Enable this option to force a sample to be created at exactly the primary sample time. If the Samples values together with the shutter open and close times already place a sample at the primary sample time, turning on this option will have no effect. Otherwise, this option will cause an addition sample to be added. This means that the actual number of samples per primary sample may in fact be one more than the number specified in the Samples parameter.
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.
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.
USD currently does not have look-at constraints. This node calculates the per-frame transform to fulfill the constraint and applies it to the object.
See the Light tool help for how to set up the look-at constraint interactively in the viewport.
Enable Look At
Turns on the constraint that orients this object to look at a point or another object.
Look At Position
If Enable look at is on and, this is a point in space this object will look at. If Look at primitive is not blank, this is an offset from the centroid of the target object (in the local space of that object).
Look At Primitive
The path to a USD prim this object should look at. If this is not blank, Look at position is an offset from this centroid of the target object.
Up Vector Method
Controls how the up vector is set.
Use (1,0,0) as the up axis.
Use (0,1,0) as the up axis.
Us the value of the Up Vector parameter as the up axis.
When Up vector method is “custom”, this is the up axis to use for the look-at constraint.
A twist rotation (around the -Z axis, clockwise, in degrees), applied after the lookout transform.
Whether the view through this camera is perspective or orthographic.
Don’t show geometry closer than the near distance or farther away than the far distance (in world units).
Perspective focal length in millimeters. This drives an expression on a hidden parameter to generate a value in the units expected by USD, which is tenths of a world unit, as defined by the stage’s meters per unit setting.
These parameters control the size and position of the virtual sensor of the camera primitive.
Similar to the standard control menus associated with each parameter on an Edit Properties LOP, this menu controls how both the horizontal and vertical aperture attributes should be handled.
All other choices in this menu behave like every other control menu, presenting separate horizontal and vertical aperture value parameters which can be set directly, or used to modify existing values.
Horizontal aperture in millimeters. This drives an expression on a hidden parameter to generate a value in the units expected by USD, which is tenths of a world unit, as defined by the stage’s meters per unit setting.
Vertical aperture in millimeters. This drives an expression on a hidden parameter to generate a value in the units expected by USD, which is tenths of a world unit, as defined by the stage’s meters per unit setting.
When Control aperture is set to
Set Horizontal Aperture and Aspect Ratio, this parameter specifies the desired aperture aspect ratio used to calculate the vertical aperture from the horizontal aperture. Render Settings and Render Product nodes can use the camera aperture aspect ratio to set the correct resolution automatically.
Horizontal Aperture Offset
Horizontal offset of the aperture from the center of the image defined by the view frustum expressed in millimeters. This drives an expression on a hidden parameter to generate a value in the units expected by USD, which is tenths of a world unit, as defined by the stage’s meters per unit setting.
Vertical Aperture Offset
Vertical offset of the aperture from the center of the image defined by the view frustum expressed in millimeters. This drives an expression on a hidden parameter to generate a value in the units expected by USD, which is tenths of a world unit, as defined by the stage’s meters per unit setting.
Scale Guide Geometry
Applies a uniform scaling factor the the guide geometry drawn for the camera in the viewport. The default value is an expression that inverts the “meters per unit” value of the USD stage to produce a consistent real-world size for the guide geometry.
Draw a background image behind the geometry in the viewer when looking through this camera. This can be used to replace background elements in the scene with a pre-rendered image, saving memory and processing time.
Draw a foreground image over the geometry in the viewer when looking through this camera. If the image contains a depth AOV, this is used to z-composite the image into the scene. This can be used to replace large portions of the scene with a pre-rendered image, saving memory and processing time.
Shutter-open time, relative to the start of frame, in
UsdTimeCode units. This is used to calculate motion blur. You can use a negative value to indicate that the shutter opens before each frame.
Shutter-close time, relative to the start of frame, in
UsdTimeCode units. If this is not greater than Shutter open there the shutter will never open and the renderer will produce a black image.
Distance from the camera to the focal plane, in world units.
The ratio of the focal length to the camera aperture size. Default is
0, which turns off focusing.
Exposure adjustment, as a log base-2 value. The default of 0.0 has no effect. A value of 1.0 will double the image-plane intensities in a rendered image; a value of -1.0 will halve them.