Sky Box geometry node

Creates a volume filled with volumetric clouds.

On this page	Overview Inputs Parameters Sky Setup Billowy Noise Value Correction Warping Alligator Noise Wispy Noise Value Correction Animation Output Bindings
Since	20.0

Overview ¶

This node can create a volume filled with clouds generated from noise.

You can also connect in a Sky Field volume, and the Sky Box will generate 3D clouds from it. Various parameters have a pop-up menu controlling how the parameter is set. The default, Set Uniform, uses the Sky Box’s parameter values. You can change this to Use Sky Layer to use the related parameter from the connected Sky Field node. See How to create a sky for how to use the Sky Box node with Sky Fields.

Inputs ¶

Sky Volume

When you connect a volume to this node’s first input, this is the name of the source volume for the skybox. You can connect multiple Sky Box nodes in sequence to combine multiple cloud types.

Sky Field

If you connect a Sky Field to this node’s second input, this is the name of the volume to use as the source of the cloud pattern. If you don’t connect the second input, this node generates cloud-like noise itself. The Sky Field node has parameters, such as Thickness, that influence how Sky Box generates the 3D clouds.

Bounding Box Geometry

If you connect geometry to the third input, this node uses its bounding box as the size of the sky box. You can turn this on or off using the Use Input If Wired checkbox on the Sky Setup tab. If the third input is not connected, or Use Input If Wired is off, the node uses the Size and Center parameters.

Parameters ¶

General

Sky Mask

If a mask volume named skymask is wired into the node’s second input, this blends between the original input and the Sky Field. If you set the popup menu to Use Sky Layer, this slider is disabled and the node uses the Sky Field node’s mask. If you set the popup menu to Scale Sky Layer, this slider multiplies the value in the skymask layer.

Voxel Size

The size of a voxel in the Pyro simulation. Cutting your voxel size in half will require eight times the memory and time, so a careful trade off between detail and pragmatism is required.

Seed

Random seed for this node’s noise. Changing this number generates different cloud patterns.

Weather

Coverage

Controls how much of the sky is obscured by clouds. Lower values will create smaller cloud patches with lesser density separated by larger spaces, while large values will create one continuous denser, large cloud volume.

When a Sky Field named coverage is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Precipitation

Additional control over how much of the sky is obscured by clouds.

When a Sky Field named precipitation is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Anvil

Controls how much the cloud spreads out at the top of the cloud.

When a Sky Field named anvil is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Override Look

Shading

Controls how to shade the volume. When generating cloud patterns, use Occlusion to see all the finer details and Reversed Occlusion to display clouds in a more realistic manner.

Ambient Occlusion

Uses ambient shadowing to display the volume.

Reversed Occlusion

Reverses the ambient shadowing for volumes which can be used to give a more realistic cloud look with internal scattering.

Density Scale

Fixed multiplier on the density field to control how opaque the volume is. If you are using a larger scene scale, you may need a smaller density scale.

Note

Fog density is density per unit length, and is independent of the resolution. So the same sized box, at 100 or 10 divisions, should be the same opacity. This means very small boxes will become transparent, especially if HDR rendering isn’t on. You can enable HDR rendering on the Scene tab of the Display Options dialog.

Setting the HDR Textures format to at least 16-bit for 3D textures can also help resolve low-density volumes. These settings can be found under the Texture tab of the Display Options.

Ambient Shadow Scale

Determines the intensity of self shadows from ambient light sources. The final amount of self-shadowing from these lights is controlled by the product of Density Scale, Shadow Scale, and Ambient Shadow Scale. The video demonstrates the value increasing from 0 to 1.

Note

Turning off this checkbox does not disable ambient shadowing. Instead, it keeps this property of the geometry unchanged. If no Ambient Shadow Scale is set, Ambient Occlusion on the Geometry tab of the Display Options controls the default amount of self-shadowing from ambient lights.

Note

The default headlight is treated as an ambient source by fog volumes.

Exposed Color

Color of smoke that’s receiving direct light from an ambient light source. The default white setting indicates that areas exposed to external lights are bright. You can set this darker than Occluded Color to achieve a more cloud-like look, where areas deeper within the cloud experience more light scattering and thus appear brighter. Set Ambient Colors must be enabled to control this parameter.

Occluded Color

Color of smoke that’s not receiving any light from an ambient light source. The default black setting indicates that areas occluded from light are dark. You can set this brighter than Exposed Color to achieve a more cloud-like appearance, where areas deeper within the cloud (that is, those occluded from outside light) are more luminous due to a greater degree of internal scattering. Set Ambient Colors must be enabled to control this parameter.

Sky Setup ¶

Bounds

Size

Controls the overall size of the sky region that is filled with clouds.

Center

Controls the overall position of the sky region that is filled with clouds.

Use Input If Wired

When this is on and geometry is connected to the third input, this node uses the bounding box of the third input instead of the Size and Center parameters. Turn this off to ignore anything connected to the third input and use Size and Center instead.

Use Convex Hull

Use the convex hull of the points for activation.

Automatic Y Bounds

Ignores the Y components of Size and Center , and calculates them using the Altitude and Thickness parameters.

Sky Layer

Altitude

The position on the Y-axis where the bottom of the cloud sits. Increase the number to position to cloud higher up in the atmosphere. Make sure this altitude falls into the bounding box given by Size and Center.

When a Sky Field named altitude is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Thickness

The vertical extent of the cloud. Smaller wispy clouds might require a thin layer, while large billowy clouds usually need a larger thickness. Make sure this altitude falls into the bounding box given by Size and Center.

When a Sky Field named thickness is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Curve Sky

Allows vertical deformation of the generated sky to create a sky dome. This is useful in shifting distant clouds to meet horizon lines.

Horizon Altitude

Sets the position on the Y-axis where the bottom of the sky dome sits.

Horizon Distance

The radius of the sky dome. Increase this value to create a larger sky dome.

Horizon Offset

Controls the center position of the sky dome. Use this control when the sky dome is not located at the origin.

Curvature Ramp

This ramps controls the curvature of the sky dome based on the distance from the center of the sky dome.

Sky Fields

Sample With Noise

When you convert a Sky Field (connected to the second input) to 3D clouds, the results might be visually displeasing. You can try turning on this option, which adds noise to the sampling points across the 3D Sky Box.

Amplitude

When Sample With Noise is on, this is the strength of the noise used to vary the sampling position. Larger values create more broken-up shapes, but also could distort the look of the Sky Field.

Element Size

Uniform scale of elements in the noise.

Offset

Offset within the evaluated noise field (added to each axis). If you have the general noise effect you want but want to get a different set of values for a different look, try changing the offset.

Roughness

The scale increment between iterations of fractal noise added to the basic output. The higher the value the larger the “jaggies” added to the output. You can use a negative value for roughness.

Vertical Profile

Top Profile

Scales the falloff from cloud to emptiness at the top of the cloud, from 0 to 1 relative to the Thickness of the Sky Box. Raising the first number gives the top of the clouds more density before they fall off. Lowering the second number makes the density at the top of the clouds fall off faster.

When a Sky Field named topprofile is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Flatten Top

Controls how flat the top of the clouds should appear.

Bottom Profile

Scales the falloff from cloud to emptiness at the bottom of the cloud, from 0 to 1 relative to the Thickness of the Sky Box. Raising the first number gives the bottom of the clouds more density before they fall off. Lowering the second number makes the density at the bottom of the clouds fall off faster.

When a Sky Field named bottomprofile is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Flatten Bottom

Controls how flat the bottom of the clouds should appear.

Vertical Extrusion

Raises the vertical thickness of the cloud relative to its given thickness (given by Thickness). If the thickness is set to 100 and this parameter is 1, the overall vertical thickness of the cloud will be 200.

When a Sky Field named verticalextrusion is wired into this node’s second input, change the menu on the right side of this parameter from Set Uniform to Use Sky Layer to overwrite the value of the parameter using the Sky Field.

Erode Boundary

The generated sky region is always clipped according to the given bounds. When the skybox is large enough, this clipping might not be visible. However, when only a small patch of clouds is needed, turn on this control to create a much more natural boundary based on a spacial noise. The boundary is always eroded in an oval shape.

Erosion

Controls how much cloud area erodes away from the boundary. Larger values will leave a smaller sky region with clouds, but their boundary will be more broken up.

Element Size

Uniform scale of elements in the spacial noise that is used for erosion.

Offset

Offset within the evaluated noise field (added to each axis). If you have the general noise effect you want, but want to get a different set of values for a different look, try changing the offset.

Contrast

This value expands or shrinks the overall range of tonal values. Particularly, each noise value is pushed towards (if Contrast is less than 1) or away from (if Contrast is greater than 1) medium grey (noise values at 0.5).

Camera Frustum

Use Camera

When this is turned on, sizes and positions the voxels of the VDB based on the frustum of a camera object.

Camera

The path to the camera object to align the VDB to. For example, /obj/cam1.

Z Far

The distance, in Houdini units, from the camera to the near side of the frustum used to calculate voxel size.

Z Scale

The distance, in Houdini units, from the camera to the far side of the frustum used to calculate voxel size.

Billowy Noise ¶

The default noise generated inside the sky box to create billowy cumulus cloud like shapes.

Billowy Noise

Noise Type

The type of noise to generate. Different algorithms give noise with different characteristics.

Perlin Cloud

Generates a perlin noise-based cloud pattern influenced by the Distortion parameter.

Perlin noise with Distortion set to -1 (left) and +1 (right):

Simplex Cloud

Generates a simplex noise-based cloud pattern influenced by the Distortion parameter. Simple noise is similar to Perlin, but the noise lattice is on a tetrahedral mesh rather than a grid. This can avoid the grid patterns often visible in Perlin noise.

Simplex noise with Distortion set to -1 (left) and +1 (right):

Fast Simplex Cloud

Generates a fast simplex noise-based cloud pattern influenced by the Distortion parameter. This simplex noise function uses a different lattice structure and a cheaper accumulation method.

Simplex noise with Distortion set to -1 (left) and +1 (right):

Element Size

Uniform scale of elements in the noise.

Click the Per Component button for separate scales along each axis.

Element Scale

When you turn on the Per Component button, this lets you scale the Element Size separately across each axis.

Offset

Offset within the evaluated noise field (added to each axis). If you have the general noise effect you want, but want to get a different set of values for a different look, try changing the offset.

You can animate the noise using an expression such as $T * 0.25 here. This is faster to compute than Animate Noise, but gives the visual effect of “panning” across the noise field, which may or may not be acceptable.

Click the Per Component button for separate additional offsets along each axis.

Offset

When you turn on the Per Component button, this lets you add the a separate offset along each axis.

Worley Details

Add Worley Details

When turned on, a worley noise is combined with the noise pattern (specified by Noise Type) to give puffy cloud noise. Enabling this, will largely diminish the distortion effect (specified by Distortion).

Noise pattern with added worley noise:

Blend

Controls the amount of mixing between the noise pattern (specified by Noise Type) and the worley noise. A value of 0 means no worley noise is applied to the noise pattern.

Erosion

Controls how fast the edges of the noise pattern turn to zero. The larger the value, the more the edges are eroded creating smaller and smaller cloud patches.

Element Size Scale

Multiplier on the Element Size parameter that effects the worley noise pattern.

Value Correction ¶

Gamma

Controls the overall gamma of the generated noise. Values less than 1 will darken the noise, increasing the range of values in originally bright areas. Conversely, values greater than 1 will stretch out the range of values for originally dark areas, which will increase the overall brightness of the noise.

Contrast

Fractal

Max octaves

The number of iterations of distortion to add to the output of the basic noise. The more iterations you add, the more “detailed” the output. Note that the output may have fewer octaves than this parameter (that is, increasing the parameter will eventually stop adding detail), because the node eventually stops when there’s no more room to add more detail in the output.

Lacunarity

The frequency increment between iterations of fractal noise added to the basic output. Note that you can use a negative value.

Roughness

The scale increment between iterations of fractal noise added to the basic output. The higher the value the larger the “jaggies” added to the output. You can use a negative value for roughness.

Warping ¶

Distortion

Controls how much the noise is distorted in the direction of decreasing noise values if Distortion is greater than 1, and in the direction of increasing noise values if Distortion is less than 1. When worley details are added to the noise pattern, the effect of distortion are largely diminished.

Distortion with a value of 1:

Distortion with a value of -1:

Stretch

How much to stretch the noise in each direction. The advantage of using this instead of Element Size is that it preserves some details unstretched, making the stretched noise pattern more natural looking.

Comparing Element Scale of 3 (left) to Stretch value of 3 (right) along the X-axis:

Droop

Warps the generated noise such that the noise pattern is bending or hanging towards the given droop direction.

Noise pattern without (left) and with droop applied (right):

Droop Direction

The direction in which the noise is warped.

Alligator Noise ¶

Applies an additional alligator noise to the default billowy noise generated inside the Sky Box to create billowy cumulus cloud like shapes.

Alligator Noise

Add Alligator Noise

Generates an additional alligator noise that is combined with the default billowy noise. Use the Vertical Profile Mask in the Mask folder to control the vertical blending of this noise.

Amplitude Scale

Controls how much the alligator noise will affect the default billowy noise.

Element Size Scale

Uniform scale of elements in the noise.

Click the Per Component button for separate scales along each axis.

Element Scale

When you turn on the Per Component button, this lets you scale the Element Size separately across each axis.

Offset

Offset within the evaluated noise field (added to each axis). If you have the general noise effect you want, but want to get a different set of values for a different look, try changing the offset.

Click the Per Component button for separate additional offsets along each axis.

Offset

When you turn on the Per Component button, this lets you add the a separate offset along each axis.

Value Correction

Gamma

Contrast

Fractal

Max Octaves

Lacunarity

The frequency increment between iterations of fractal noise added to the basic output. Note that you can use a negative value.

Roughness

The scale increment between iterations of fractal noise added to the basic output. The higher the value the larger the “jaggies” added to the output. You can use a negative value for roughness.

Masking

Vertical Profile Mask

This ramp lets you control the strength of the alligator noise function across the vertical profile of the skybox. The horizontal axis represents the vertical height (minimum on the left, maximum on the right). The vertical axis represents the strength of output when the noise function generates that point within the range.

Wispy Noise ¶

Generates wispy noise inside the Sky Box to create more broken-up cloud shapes.

Wispy Noise

Add Wispy Noise

Generates a fine, feather like wispy noise that can be combined or overwritten by the default billowy noise. Use the controls inside the Mask folder to change the blending with the billowy noise.

Amplitude

Controls the strength of the wispy noise. Larger values create finer and more broken up shapes, while lower values create smoother wispy details.

When Attribute Type is Vector, click the Per Component button for separate amplitudes along each axis.

Amplitude Scale

When Attribute Type is Vector and you turn on the Per Component button, this lets you scale the Amplitude separately across each axis.

Element Size Scale

Uniform scale of elements in the noise.

Click the Per Component button for separate scales along each axis.

Element Scale

When you turn on the Per Component button, this lets you scale the Element Size separately across each axis.

Offset

Offset within the evaluated noise field (added to each axis). If you have the general noise effect you want, but want to get a different set of values for a different look, try changing the offset.

Click the Per Component button for separate additional offsets along each axis.

Offset

When you turn on the Per Component button, this lets you add the a separate offset along each axis.

Turbulence

Add Turbulence

When enabled, a turbulence noise is generated to displace the default wispy noise to create additional finer wispy details.

Noise Type

The type of noise to generate. Different algorithms give noise with different characteristics.

Fast

The default. A faster and more interesting variant of Perlin noise.

Frequency is scaled by 1.25.

Sparse Convolution

Sparse Convolution noise is similar to Worley noise. Does not have artifacts at grid points.

Frequency is scaled by 1.25.

Alligator

Produces a bumpy output. Named for its alleged resemblance to alligator skin.

Frequency is scaled by 1.64.

Perlin

A noise where the visual details are the same size. Wikipedia article

Perlin Flow

A noise that’s stable over time, like a rotated Perlin noise, useful to create noise that seems to swirl and flow smoothly across time. Use the Flow rotation parameter below to control the rotation.

Simplex

A noise similar to Perlin but the noise lattice is on a tetrahedral mesh rather than a grid. This can avoid the grid patterns often visible in Perlin noise.

Worley Cellular F1

Produces cellular features similar to plant cells, ocean waves, honeycombs, cratered landscapes, and so on. Wikipedia article

Worley Cellular F2-F1

A variant of Worley noise that produces blunted and cornered features.

Manhattan Cellular F1

A variant of Worley F1 noise that uses Manhattan distance calculation. Useful when you want unusual-looking noise.

Manhattan Cellular F2-F1

A variant of Worley F2-F1 noise that uses Manhattan distance calculation. Useful when you want unusual-looking noise.

Chebyshev Cellular F1

A variant of Worley F1 noise that uses Chebyshev distance calculation. Useful when you want unusual-looking noise.

Chebyshev Cellular F2-F1

A variant of Worley F2-F1 noise that uses Chebyshev distance calculation. Useful when you want unusual-looking noise.

Perlin Cloud

Generates perlin noise based cloud pattern influenced by the Distortion parameter.

Perlin noise with Distortion set to -1 (left) and +1 (right):

Simplex Cloud

Generates simplex noise based cloud pattern influenced by the Distortion parameter. Simplex noise similar to Perlin, but the noise lattice is on a tetrahedral mesh rather than a grid. This can avoid the grid patterns often visible in Perlin noise.

Simplex noise with Distortion set to -1 (left) and +1 (right):

Fast Simplex Cloud

Generates fast simplex noise based cloud pattern influenced by the Distortion parameter. This simplex noise function uses a different lattice structure and a cheaper accumulation method.

Simplex noise with Distortion set to -1 (left) and +1 (right):

Amplitude

Controls the strength of the turbulent noise. Larger values create finer and more broken up shapes, while lower values create smoother wispy details.

When Attribute Type is Vector, click the Per Component button for separate amplitudes along each axis.

Amplitude Scale

When Attribute Type is Vector and you turn on the Per Component button, this lets you scale the Amplitude separately across each axis.

Attenuation

Flattens the noise to prevent extreme spikes by damping the values. Higher values create a smoother look.

Value Correction ¶

Gamma

Contrast

Fractal

Max Octaves

Lacunarity

The frequency increment between iterations of fractal noise added to the basic output. Note that you can use a negative value.

Roughness

The scale increment between iterations of fractal noise added to the basic output. The higher the value the larger the “jaggies” added to the output. You can use a negative value for roughness.

Warping

Distortion

Distortion with a value of 1:

Distortion with a value of -1:

Stretch

Comparing Element Scale of 3 (left) to Stretch value of 3 (right) along the X-axis:

Droop

Warps the generated noise such that the noise pattern is bending or hanging towards the given droop direction.

Noise pattern without (left) and with droop applied (right):

Droop Direction

The direction in which the noise is warped.

Masking

Mask Center

Masks the wispy noise based on the density of the default billowy noise using a threshold range. Mask Center and Mask Width control the masking range. By default, where the billowy noise density is at the minimum and maximum of this range, the wispy strength multiplier is 1 and 0, respectively. Wispy noise is reduced in areas where the mask value is smaller. Set this value to 1 and Mask Width to 0 to fill the skybox with wispy noise.

Mask Width

Controls the range of values used for masking the wispy noise in conjunction with Mask Center. The effective range of mask values is from Mask Center - Mask Width * 0.5 to Mask Center + Mask Width * 0.5.

Vertical Profile Mask

This ramp lets you control the strength of the wispy noise function across the vertical profile of the skybox. The horizontal axis represents the vertical height (minimum on the left, maximum on the right). The vertical axis represents the strength of output when the noise function generates that point within the range.

Animation ¶

Animation

Pulse Duration

Vary the generated noise with time. This controls the rate of change of the noise pattern.

Time

The time at which to evaluate the cloud noise.

Directional Offset

Enable Directional Offset

When turned on, offsets the evaluated noise patterns that gives the visual effect of “panning” across the noise field simulating a wind like effect.

Speed

Controls how fast the noise pattern movies in the direction given by Direction.

Direction

The direction that is used to offset the noise pattern.

Upward Bias

When turned on, this controls how much the direction (given by Direction) will blend towards the Y-axis. Use this control to add a small amount of upwards motion to the direction.

Cloud Top Offset

Controls the offset difference between the bottom and the top of the skybox.

Output ¶

Density

Normalize Density

With reduced coverage values (given by Coverage) the density output by the skybox is also reduced. When this is turned on, the density range will more closely match the 0 to 1 range.

Maximum Density

When the checkbox is on, clamp density values so they are never greater than this value.

Billowy Scale

Multiplies the density using this value in the regions where billowy noise is generated.

Wispy Scale

Multiplies the density using this value in the regions where the wispy noise is generated. Generally, wispy noise is more opaque than a filled billowy cloud.

Enable Remap Ramp

Turn this on to shape the output of the density with a ramp.

Density Remap Ramp

When Enable Remap Ramp is on, this ramp lets you control the density output. The horizontal axis represents the input density range (minimum on the left, maximum on the right). The vertical axis represents the number to output when the density value is at that point within the range.

Scale Along Vertical Profile

Turn this on to shape the output of the density along the vertical profile of the skybox.

Vertical Density Ramp

This ramp lets you control the output density across the vertical profile of the skybox. The horizontal axis represents the vertical height (minimum on the left, maximum on the right). The vertical axis represents the density scale to output when the noise function generates that point within the range.

Post-Process

Deactivate

Deactivates any voxels that have the background value. This is useful for cleaning up the result of an operation that may have speculatively activated a large band of voxels, but may not have placed non-background values in all of them.

For example, you may have a VDB Activate before a Volume VOP with Expand turned on to ensure you have room to displace the volume. Then you can use one with Deactivate to free up the voxels you didn’t need to use.

Compute Max Density

Computes the minimum and maximum values of the density volume given by Density Volume.

Bindings ¶

Bindings

Density Volume

The name of the volume to be used as density for the cloud box. Usually you should leave it as density.

Overview ¶

Inputs ¶

Parameters ¶

Sky Setup ¶

Billowy Noise ¶

Value Correction ¶

Warping ¶

Alligator Noise ¶

Wispy Noise ¶

Value Correction ¶

Animation ¶

Output ¶

Bindings ¶

Geometry nodes