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| Since | 18.0 |
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. |
Editing lights in the viewport ¶
| To... | Do this |
|---|---|
|
Enter the Light state |
Select the Light node in the network, then in the viewport click the |
Handles tool, or press Mode |
Key |
Description |
|---|---|---|
Default |
⇧ Shift + A |
Position the light using handles. |
Diffuse |
⇧ Shift + D |
Click or drag on a surface to place the light normal to that point. |
Specular |
⇧ Shift + S |
Click or drag on a surface to place the light so there’s a reflected highlight at that point. You should look through the render camera when using this mode, since it only sets the highlight to show up a certain location from the current point of view. |
Shadow |
⇧ Shift + F |
Click on a lit surface to place the pivot point. This should be on the part of the model you want to control the shadow for (for example, the edge of a window frame, or a prominent feature of the model). ⇧ Shift-click or drag a point on the shadowed surface to move the point on the shadow corresponding to the pivot point. |
Look at |
⇧ Shift + T |
Position the look-at handle to set a look-at constraint on the light. Use the handles to orbit or elevate the light relative to the target point. This turns on the Enable look at parameter in the Light’s Constraints section. To constrain the light to look at another USD prim, fill in the Look at primitive parameter. |
All |
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Point Light shelf tool ¶
Adds a point light to the scene. A point light radiates light equally in all directions from a point in space. It is the most efficient type of light to calculate.
Spot Light shelf tool ¶
Adds a spot light to the scene. A spot light casts light from a point in space, in a certain direction. The beam of light is cone shaped, with a radius and falloff at the edges.
Area Light shelf tool ¶
Simulates a light in the shape of a line, rectangle, disk, or sphere.
Geometry Light shelf tool ¶
Simulates a light with the shape and color of a geometry object you select.
Geometry lights can be expensive to render. For simple cases you may want to try to simulate the effect you want using point lights and/or area lights. However, the geometry light is very useful since it’s more straightforward than using tricks with simple lights, and it uses the object’s surface shader to color the emitted light.
-
Select the object you want to emit light, then click the
Geometry Light tool on the shelf.or
-
With nothing selected, click the
Geometry Light tool on the shelf, then select the object(s) you want to act as portals for the environment light and press Enter.
Note
To get geometry lights to emit color based on their assigned surface shader, you need to define an export variable named Ce. This holds the emitted light. You can also use “emission” in the USD Preview Shader.
The object also has to have the “Treat As Light Source” property enabled.
Volume Light shelf tool ¶
Simulates illumination from an emissive volume.
-
Select the volume geometry you want to emit light.
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On the Lights and Cameras shelf tab, click the
Volume Light tool.
The volume light takes over the emission (Ce) component, which is treated as direct lighting for the new light source - making it possible to separate it into a distinct per-light deep raster plane separate from the combined direct_emission and indirect_emission planes.
Emission is disabled on the original object for all secondary rays (such as reflections) by disabling the vm_emitillum property on the source object. This ensures only the new light object directly illuminates other objects. (This property is on the Shading tab of the source object if you want to re-enable it.)
Parameters ¶
Sampling Behavior
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.
Start/End/Inc
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 @fstart, @fend, and @finc. 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).
Subframe Sampling
For each primary sample generated by this node, these parameters can cause additional samples to be generated around 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.
Shutter
Controls the method used to specify the shutter open and close times relative to the primary sample times.
Specify Manually
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.
Shutter Open/Close
When Shutter is set to 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.
Camera Prim
When Shutter is set to Use Camera Prim, this is the 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.
Samples
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
When turned on, forces a sample to be created at exactly the primary sample time. If the Samples value, together with the shutter open and close times, already place a sample at the primary sample time, turning on this option has no effect. Otherwise, this option causes 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.
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
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/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.
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.
Type
Cylinder Light
Light emitted outward from a cylinder. The cylinder is centered at the origin and has its major axis on the X axis. The cylinder does not emit light from the flat end-caps.
Distant Light
Light emitted from a distant source along the -Z axis. Also known as a directional light.
Disk Light
Light emitted from one side of a circular disk. The disk is centered in the XY plane and emits light along the -Z axis.
Geometry Light
Light emitted outward from a geometric prim (UsdGeomGprim), which is typically a mesh.
Point Light
Like a sphere light, but with zero radius.
Rectangle Light
Light emitted from one side of a rectangle. The rectangle is centered in the XY plane and emits light along the -Z axis. The rectangle is 1 unit in length in the X and Y axis. In the default position, a texture file’s min coordinates should be at (+X, +Y) and max coordinates at (-X, -Y).
Sphere Light
Light emitted outward from a sphere.
Transform ¶
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.
Base properties ¶
Radius
When Type is Cylinder, Disk, or Sphere, the radius of the light.
Length
When Type is Cylinder, the length of the light from end to end.
Geometry
When Type is Geometry, the path to a geometry prim to use as the light shape.
Width
When Type is Rectangle, the width of the light area.
Height
When Type is Rectangle, the height of the light area.
Angle
When Type is Distant, the angular size of the light in degrees.
As an example, the Sun is approximately 0.53 degrees as seen from Earth. Higher values broaden the light and therefore soften shadow edges.
Clipping Range
Don’t show geometry closer than the near distance or farther away than the far distance (in world units).
Exposure
Scales the power output exponentially as a power of 2, similar to an F-stop control. 0 does not scale intensity, negative numbers scale intensity down by the square, positive numbers scale intensity up. Often you will set the Intensity to what looks like the right amount of light, and then use this parameter to “tweak” it.
Intensity
A linear scale on the power output of the light.
Color
The color of the light, in energy-linear terms.
Texture
Maps a texture to the area geometry of lights. This is currently only supported with rectangular lights.
Enable color temperature
Use Color temperature to control the light color. The color calculated by Color temperature is multiplied by the RGB color in Color, so if you want the “pure” temperature color, you should set Color to 1.0, 1.0, 1.0.
Color temperature
When Enable color temperature is on, the temperature (in degrees Kelvin) to simulate. The valid range is from 1000 to 10000. Lower values are warmer and higher values are cooler. The default is a common white point, D65.
If necessary, you can compute the same color value using the UsdLuxBlackbodyTemperatureAsRgb() function from the USD library.
Normalize power
Divides the set power output (intensity scaled by exposure) by the surface area or angular size of the light. This makes it easier to adjust and compare energy output without it being affected by the size of the light.
Diffuse multiplier
Scales the effect this light has on the diffuse response of shaders. This is a non-physical “tweak”.
Specular multiplier
Scales the effect this light has on the specular response of shaders. This is a non-physical “tweak”.
Scale Guide Geometry
Applies a uniform scaling factor the the guide geometry drawn for the light 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.
Shaping ¶
Spotlight
Add the necessary API to the light to turn it into a spotlight.
Angle
The spread angle of the spotlight beam, in degrees.
Softness
Blurs the edges of the spotlight beam, simulating light leakage and bounce.
Left barndoor
Slides in a light blocker from the left, covering part of the spotlight cone. 1.0 reaches all the way across the cone, blocking all light.
Left barndoor edge
Extends the solid light blocker above by an additional soft edge. 1.0 creates a gradient as wide as the spotlight cone.
Right barndoor
Slides in a light blocker from the right, covering part of the spotlight cone. 1.0 reaches all the way across the cone, blocking all light.
Right barndoor edge
Extends the solid light blocker above by an additional soft edge. 1.0 creates a gradient as wide as the spotlight cone.
Top barndoor
Slides in a light blocker from the top, covering part of the spotlight cone. 1.0 reaches all the way across the cone, blocking all light.
Top barndoor edge
Extends the solid light blocker above by an additional soft edge. 1.0 creates a gradient as wide as the spotlight cone.
Bottom barndoor
Slides in a light blocker from the bottom, covering part of the spotlight cone. 1.0 reaches all the way across the cone, blocking all light.
Bottom barndoor edge
Extends the solid light blocker above by an additional soft edge. 1.0 creates a gradient as wide as the spotlight cone.
Focus (checkbox)
Add the necessary API to the light to be able to focus the beam.
Focus
A control to shape the spread of light. Higher focus values pull light towards the center and narrow the spread. (This is implemented as an off-axis cosine power exponent.)
Focus tint
Off-axis color tint, tints the light in the falloff region. The default is black.
IES
Add the necessary API to the light to use an IES file describing angular distribution of light.
File
An IES (Illumination Engineering Society) format file to use to control angular distribution of light.
Angle scale
A global scale on angles found in the IES file.
Shadow ¶
Enable
When this is on, objects hit by this light will cast shadows (subject to shadow masks).
Color
Tints the color of shadows cast by this light. Changing this from black (0, 0, 0) is a non-physical “tweak”.
Falloff
The near distance at which shadow falloff begins.
The default value (-1) indicates no falloff.
Falloff Gamma
A gamma (i.e., exponential) control over shadow strength with linear distance within the falloff zone (between Falloff and Distance.
Distance
The maximum distance to check for objects shadowing this light. The default is -1, meaning no limit. Setting this may speed up rendering if you know all objects you want shadowed are within a certain distance.
Karma ¶
Karma ¶
Light Shader
Defines a VEX shader to be used for light evaluation. If the string is empty, the value will be taken from the HOUDINI_VEX_DEFAULT_LIGHTSURFACE configuration variable.
Active Radius
Artificially clamp the illumination distance from the light. Outside this radius, the light will have no effect.
Point Light Radius
As point lights get closer to geometry, the attenuation equation causes the intensity of lights to go to infinity (a singularity caused by the division by the distance between the light and the surface). Increasing this value to something larger than zero creates an artificial buffer zone to prevent the light from going to infinity. If the radius is set to 1, the brightest value for the point light will be twice the intensity specified on the light. If the value is 1.4142 (sqrt(2)), the brightest value will be the intensity specified.
For distant geometry, there should be very little change in the intensity.
MIS Bias
Controls whether multiple importance sampling will sample from the BSDF, the light, or both the BSDF and light.
Sampling Quality
This is the per-light sampling quality, that acts as a multiplier on the global Light Sampling Quality. Increasing the quality will add additional samples for this light source, improving the sampling quality of this light relative to other light sources.
Sampling Mode
When using the light tree for rendering, Karma will try to put any light source that’s compatible into the tree. This option can force a light to be excluded from the tree and subject to uniform sampling, which can sometimes yield an improved sampling quality the light at the expense of speed.
Maximum HDRI Size
If an environment map is larger than this resolution, it will be scaled down when performing texture analysis. If the map has some very small, very bright values, this filtering may affect how sampling of the environment map is performed.
Single Sided
Does the light emit light from a single side (ie along the normal) or from both sides? This only has effect on planar area light types.
Render Light Geometry
Causes the light source geometry to be visible in the render.
Light Geometry Casts Shadow
Causes the light source geometry to cast shadows in the render.
LPE Tag
Custom label assigned to lights or objects for use with light path expression.
Portal MIS Bias
Controls whether the dome light is more likely to sample from the environment map or the portal geometry. For high-contrast HDRI with strong light sources, it may be beneficial to sample more from the environment map (set it less than 0), and for low-contrast/flat HDRI, the opposite may be true (set it greater than 0). Leave it at 0 for balanced approach.
Illuminate Background Holdout
If a light exists to add new light source instead of trying to replicate lighting in the live action plate, enable this parameter so that its effect on background holdout geometry shows up in indirect bounces and its contribution to shadow holdout AOVs is omitted.
Contributions
Provides detailed control over which BSDF component the light affects.
Contributes to Caustics
Default is true. Note that Karma only renders caustics if they are enabled on both the light and the object. This option has no effect unless you also turn on Enable Caustics on the geometry object.
Attenuation Type
Controls the type of attenuation on the light source in the default light
shader. This may be:
none
No attenuation
half
Half distance attenuation
physical
Physically correct attenuation
Attenuation Start
Minimum light distance to be used for attenuation.
Attenuation
The distance from the light at which Half Distance Attenuation produces half the light intensity.
Spread
Ranges between 0 to 1 and controls beam direction along the light’s normal. A value of 1 results in diffuse emission, and a value close to 0 gives parallel beams of light similar to directional light. Note that the Spread option is only supported for area lights with Type set to Rectangle.
Tips ¶
-
Lights do not generally support non-uniform scale values, so by default the Scale parameter of this node is hidden. In versions of Houdini before 19.0, this parameter was visible, so for backward compatibility, the Scale parameter will be visible if any of the scale components are not
1.0. Reverting this parameter to its default values will cause the parameter to disappear from the parameter dialog.
