Houdini 20.0 Nodes Geometry nodes

Volume Visualization geometry node

Adjusts attributes for multi-volume visualization.

Since 11.0

The Volume Visualization operator adds detail attributes to its input geometry that tell the viewport how to render sets of multiple volumes. For example, you may want to take a density volume and color it according to three separate Cd.x, Cd.y, and Cd.z volumes.

There are two major components to the viewport volume visualization.

First there is the opaque smoke. This smoke occludes geometry behind it and can also cast shadows on itself from light sources. Finally, a diffuse color can be specified for what light colors it reflects rather than absorbs.

Note

A density of 1 means a density of 1 unit of smoke per unit of length; this will not fully block light.

Second there is an emissive, glowing, component. This field is added directly to the final image, washing out but not occluding geometry behind it. This is useful for fire-style effects. It can also be useful for visualizing data because it allows interior detail to shine through the outer layers.

Parameters

Visualization Mode

Volume primitives have a built-in visualization mode. If a Density Field is specified, it will be overridden with this visualization mode.

Minimum

Values in the Density Field will be remapped from the given Minimum and Maximum values into the 0..1 range. This is the default range for all other volumes unless the override is specified.

Maximum

Values in the Density Field will be remapped from the given Minimum and Maximum values into the 0..1 range. This is the default range for all other volumes unless the override is specified.

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.

Shadow Scale

An additional multiplier on the density field, applied after the Density Scale, when the volume is used for lighting. Increase this parameter to get darker self-shadows on low-density volumes.

Set Ambient Colors

Enable this toggle to control the appearance of smoke volumes lit with ambient light sources (by specifying Exposed Color and Occluded Color).

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.

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.

Shadow Color

Controls the color of shadows cast by the smoke. The complements of these values act like additional density multipliers for light of each color (red, green, or blue).

Viewport Quality

Max Vis Res

By default, the viewport limits the visualization resolution of volumes (settings for this behavior are found under the 3D Textures section of the Display Options' Texture tab). You can enable this parameter to further restrict the visualization resolution. This has the additional benefit of enforcing the specified limit when loaded into a Houdini session with more generous texture limits.

Ambient Map Size

Controls size of the occlusion map that approximates self-shadowing from ambient light sources. Larger values produce higher quality results, but are slower to shade.

Ambient Step Rate

Specifies how often the density texture is sampled when calculating self-shadowing from ambient light sources. Smaller values are slower to shade, but produce higher quality results.

The rest of this operator’s parameters select the volumes and remap their values for viewport visualization. Each of the Smoke and Emission tabs has a selection set for the intensity field and a color field. The value of the field parameters will be matched against the name attribute of the volume primitives to find the desired volumes. * and ? can be used to allow for looser matches. With color fields, if one volume is specified, it is treated as a monochromatic field. If multiple volumes are specified (or a single vector volume), they will be tied to the red, green, and blue channels.

Each field can also have its own mapping range. The volume will be linearly mapped from this range into the 0..1 range. If an override range is not enabled, Minimum and Maximum parameters are used as the default range.

Finally, each field has a menu to select how the respective ramp is used. If set to No Ramp, the volume’s values are used unclamped and unaffected by the ramp. Clamped Ramp will clamp the volume’s values to the 0..1 range and then apply the ramp. Periodic Ramp will take the modulus of the volume’s value with 1 and send that to the ramp. This can be useful to create checkerboard and striped effects to better see large ranges of values. If it is a color ramp and there is a single volume bound, that volume’s value is looked up to get the color. If multiple volumes (or a vector VDB) are bound, then each component is looked up to get the red, green, and blue values independently.

Emission color also has the Physical Blackbody option that maps the values in the Emission Color Field using the blackbody radiation spectrum. This mode produces results similar to the Physical (Black-Body) Color Mode on the Pyro Shader VOP.

Smoke

Density Field

Name of the main volume. If Visualization Mode is set to Smoke, this field should contain the optical thickness of the smoke. For a Height Field, this field should store the elevation at each voxel.

Diffuse Field

Name of the color volume for the smoke or the height field.

Emission

Emission Scale

Intensity of the emissive component for smoke visualization. Higher values of this parameter increase brightness of the internal glow (for example, of the flames in a fire simulation).

Emission Field

Name of the field containing intensity of emission for smoke visualization. When Visualization Mode is set to Height Field, this field acts like a visualization stencil: the viewport will only draw parts of the height field where the Emission Field value is at least 0.5.

Emission Color Field

Name of the color volume for the emissive component of smoke. This field has no effect with height fields.

Temperature Scale

Scale applied to values in the Emission Color Field before mapping them to color using the blackbody spectrum.

Temperature at 0 (K)

This parameter sets the temperature corresponding to a value of 0.0 in the Emission Color Field. This value is in Kelvin.

Temperature at 1 (K)

This parameter sets the temperature corresponding to a value of 1.0 in the Emission Color Field. This value is in Kelvin.

Adaptation

Manipulates the low end of the generated intensity.

Burn

Manipulates the high end of the generated intensity.

Enable OpenGL Scattering

This parameter enables approximation of internal scattering in the viewport. This feature is experimental and may be changed or removed in future versions of Houdini.

Extinction Ratio

The proportion of light that is absorbed by the smoke (as opposed to being scattered out). Value of 0 suggests that all light that interacts with the smoke is scattered, making the smoke appear softer and brighter. At the other extreme, value of 1 signals that all light is absorbed.

Warning

Values close to 1 can introduce rendering artefacts in some cases. You can reduce the Extinction Ratio in those situations.

Scattering Iterations

Scattering is approximated by an iterative process, and this parameter controls the number of refinement iterations that are performed. Larger values allow more light to scatter further, but also take longer to shade in the viewport.

See also

Geometry nodes