# Volume2ΒΆ

The Volume2 material can be used to render any volumetric objects like, fogs, smokes, clouds, fires, explosions or liquids.

## Color Control

The color of the volumetric is controlled either using physically based parameters or using a mapping that converts the expected color into physical value. Set Color Control to Color to set the resulting color in the Color tab of the shader, or Physical to set the albedo and extinction values in the Physical tab of the shader.

Both sets of parameters are exclusive. Setting Color Control uses one set and ignores the other.

## Using Color controls

Use the color controls to render the volumetric with a given resulting color. The physical parameters of the volumetric, such as albedo and extinction are deduced from the color parameters.

Note that the actual resulting color is dependent on many other parameters, notably that maximum number of bounces and asymmetry of the scattering phase function, and is only an approximation to some degree.

### Color > Color

The expected resulting color of the volume.

Color > Color = white, turquoise and skin-like pink.

### Color > Scattering Range

The range of diffusion of each R, G and B channel. The higher the range, the farther the color scatters.

Color > Scattering Range = [1,1,1], [1,0.6,0.6] and[1,0.3,0.3], with a skin-pink Color.

### Color > Scale

A scaling factor for the whole Scattering Range. The smaller the scale, the shorter the scattering distance.

Color > Scale = 3, 1 and 0.3.

This value is the invert of the Density, and is provided to be consistent with length units instead so the parametrization is intuitive to artists.

## Using Physical controls

Use the physical controls to render the volumetric with known volumetric physical quantities. Note that the resulting color will not map with the albedo notably, as the albedo is a single scattering property, and the actual resulting color is the product of multiple scattering in the volumetric.

### Physical > Albedo

The single scattering albedo of the volumetric. It is the ratio of the scattering coefficient over the extinction coefficient, or in other words, how much photons will be scattered over how much photons are absorbed. A pure white albedo means the volumetric scatters all the light like a cloud, while a black albedo means the volumetric absorbs all the light.

Physical > Albedo = [1,1,1], [0.9,0.8,0.7] and [0.8,0.6,0.4].

White volumetric are notably difficult to render, and may require hundreds to thousands of bounces to render properly! Use the Multiple Scattering parameters to help simulate true multiple scattering.

### Physical > Extinction

The extinction coefficient of the volumetric. Higher values result in a more opaque volumetric.

Physical > Extinction = [1,1,1], [1,0.8,0.6] and [1,0.6,0.2].

Non physical extinction values can also have surprising results!

Physical > Extinction = [1,1,1], [1,0.2,0.6] and [1,0.6,0.2], with the same Physical > Albedo = [0.8,0.6,0.4].

## General Controls

### Density

The density of the volumetric. Higher values produce denser volumes.

Density = 1, 3 and 10.

### Incandescence

The incandescence color of the volumetric.

Incandescence = black, 0.5 green and full green.

### Asymmetry (A/B)

Controls how photons are scattered, backward for Asymmetry < 0, isotropically at 0, forward for Asymmetry < 0. Natural volumetric media usually have a positive asymmetry.

Asymmetry = -0.5, 0, 0.5 and 0.85 with a light source in front of the volume, with 0 bounce.

And with a light source at the back of the volume, with 0 bounce. Note how the highlight is narrower with higher Asymmetry.

### Asymmetry Mix

The Volume2 material is a dual Henyey-Greenstein (HG) and both HG lobes are smoothly blended. Asymmetric Mix = 0 results in only the B lobe used, Asymmetric Mix = 0.5 results in both A and B lobes equally used and Asymmetric Mix = 1 results in only the A lobe used. Each A and B lobes have their own assymetry, and this allows to better approximate real life scattering phase functions.

For instance, clouds phase function exhibits a strong forward lobe, but also a smaller backward lobe. Both A and B lobes can be used to model this behaviour.

The HG lobes are blended to preserve energy.

## Black Body

The Black Body parameters simulate a physical incandescence using the quantic black body theory, which states that a heated material emits a precise spectrum of light, and thus a precise incadescence color.

### Black Body > Temperature

The temperature of the volume, in Kelvin.

Black Body > Temperature = 1100K, 1300K and 1500K.

### Black Body > Intensity

The overall scaling factor of the blacvk body incandescence.

black Body > Intensity = 0.1, 0.5 and 1, with Black Body > Temperature = 1300K.

## Multi Scatter

Rendering highly forward scattering white media such as clouds or snow is difficult, require high number of bounces, and is usually very expensive. The Multi Scatter allows changing the properties of the volumetric bounce after bounce with a few parameters, to help render these kind of volumetric with a limited number of bounces.

This technique is based on the Art-Directable Multiple Volume Scattering paper proposed by Magnus Wrenninge.

This technique is not energy preserving and alters the accuracy of the physical scattering. More importantly, the result is quite dependent on the number of allowed bounces in the volumetric.

### Multi Scatter > Light Penetration

Decreases the volume extinction bounce after bounce. This results in the volumetric casting less shadows over itself and more direct lighting penetrating the volumetric.

Light Penetration = 0, 0.33, 0.66 and 0.95, with 1 volume bounce.

Light Penetration = 0, 0.33, 0.66 and 0.95, with 2 volume bounces.

Light Penetration = 0, 0.33, 0.66 and 0.95, with 4 volume bounces.

### Multi Scatter > Bounce Attenuation

Decreases the direct lighting intensity bounce after bounce. Increasing the Light Penetration can result in volume being visibly too bright although the diffuse feeling is pleasing, this will help compensate the correct illumination.

Bounce Attenuation = 0, 0.2, 0.4 and 0.6, with 4 volume bounces and Light Penetration = 0.66.

### Multi Scatter > Isotropy

Decreases the absolute value of the Asymmetry bounce after bounce. This results in a volumetric more and more isotropic, which simulates the fact that after a number bounces, photons tend to have a random direction.

Increasing the Isotropy usually results in brighter, more diffuse and less noisy renders, but less accurate.

Isotropy = 0, 0.33, 0.66 and 1, with 4 volume bounces, Light Penetration = 0.66 and Bounce Attenuation = 0.2.