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.

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).

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.

Controls the method used to specify the shutter open and close times relative to the primary sample times.

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.

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.

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.

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.

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.

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).

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.

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. With Rebuild Render-Specific Parameters you reset all parameters from render-specific tabs to their default values for the specific node. If you want to reset the parameters for the entire scene, you can also open the top-level Render menu and choose Rebuild All Render-Specific LOP Parameters.

This parameter is multiplied by Scale and creates a Rayleigh scattering coefficient for each color channel representing the amount of light that is scattered by pure air.

Multiplier for Rayleigh Scatter.

Controls, how fast the air density falls off with increasing altitude. This parameter is measured in kilometers and multiplied by Unit Scale. For example, with the default of 8 km and a Unit Scale of 1000, air density is 37% at 8000 units above sea level.

Distribution uses Nishita’s exponential decay model (referred to as “Scale Height” in literature) for calculating air density.

This parameter is multiplied by Scale and creates a Mie scattering coefficient for each color channel representing the amount of light that is scattered by aerosols.

Multiplier for Mie Scatter.

This parameter is multiplied by Absorption Scale and creates a Mie absorption coefficient for each color channel representing the amount of light that is absorbed by aerosols.

Multiplier for Mie Absorption.

Distribution uses Nishita’s exponential decay model (referred to as “Scale Height” in literature) for calculating air density.

Phase ranges from -1 to 1 and covers full back scatter to full forward scatter. Scattering represents the atmosphere’s anisotropy. Aniostropy means that rays don’t propagate evenly in all directions.

This parameter is multiplied by Scale and creates an ozone absorption coefficient for each color channel representing the amount of light that is scattered by the ozone layer.

Multiplier for ozone Absorption.

The maximum ozone density/absorption is reached at the parameter’s value given in kilometers measured from the Earth’s surface. The value is multiplied by Unit Scale. For example, at an Altitude of 8 km and a Unit Scale of 1000, the final value will be 8000.

This value indicates the total height range in kilometers at which ozone absorbs (sun)light. Absorption reaches its maximum at the Altitude parameter’s value. Note, that Width is multiplied by Unit Scale. For example, when Width is 10 km and Unit Scale is 1000, the final value will be 10000.

The ground reflectivity that affects the sky color (when multiple scattering is turned on), as well as its appearance in primary and bounce rays.

Controls visibility of the ground depending on different types of rays. For instance, you may not want the ground to be visible in the camera, but still want the sky and indirect bounce rays from underside of clouds (and other geometry) to be affected by the ground color. You can use one of the following expressions

This parameter is multiplied by Unit Scale and represents the position where the planetary atmosphere is centered around. Planet Center is measured in kilometers.

This parameter is multiplied by Unit Scale and represents the distance from the Planet Center in which the atmosphere starts. Sea Level is measured in kilometers.

Multiplier to every parameter in Karma Sky Atmosphere that represents position or distance.

Custom label assigned to atmospheric volume for use with light path expression.

This parameter is multiplied by Unit Scale and represents the minimum distance between samples when marching through atmosphere. Reducing this parameter can help eliminate banding artifacts when modeling a particularly thick atmosphere, but at extra cost.

This parameter is multiplied by Unit Scale represents the maximum distance between samples when marching through atmosphere. We recommend leaving Maximum Stepsize at 5 to 10 times the value of Minimum Stepsize.

Specifies, how often an atmospheric ray can propagate through the atmosphere.

Scatter and absorption coefficients are interpreted in the OpenColorIO color space specified by this parameter, and transformed into scene_linear (i.e. working space) when computing transmittance. Leave it empty to apply no transformation. This can help in preserving the look if the lookdev for the atmosphere was done in a different color space than the project’s working space (however, due to the nature of RGB rendering, it can never be an exact match).

X and Y resolution for the transmittance table. The X axis corresponds to the light angle, and the Y axis corresponds to the viewer’s altitude. With appropriate settings, there is no need to deviate from the default settings. An exception is if there are huge differences between the Distribution or ozone Altitude parameters. In this case it can help to increase the Y resolution to avoid bias.

Distance between ground level and sea level. Ground is modeled as a sphere with radius calculated as Sea Level - Ground Offset. You can set this parameter to values greater than zero to lower the ground and avoid discontinuity artifacts from objects crossing ground level. As a consequence, you’ll see a vacuum between the atmosphere and the ground which may be noticeable near horizon. We recommend leaving this parameter at 0 and keeping the rest of the scene above Sea Level instead.

Allow ground elements to receive shadows (i.e. cast shadow rays).

Allows rendering of the atmosphere on the other side of the planet.

Set alpha to 1 in IPR mode. Turned on by default to avoid misleading appearance due to compositing against light viewport background.