Overview

Voxel-based Volumetric Fog is a new feature that uses volume textures as a view-frustum-shaped voxel buffer to store the incoming light and properties of participating media.

The older Volumetric Fog system can only handle sun with dynamic shadow and exponential height fog density, also in dense fog situations the fog's appearance is not consistent between opaque and transparent materials.

This new voxel-based Volumetric Fog system can handle sun with dynamic shadow, environment probes, ambient light, and regular light with dynamic shadows, as well as variations in fog density. It also supports the application of Volumetric Fog in a consistent way in respect to opaque and transparent materials.

Voxel-based Volumetric Fog consists of two parts; 1: ray-marching Volumetric Fog and 2: analytical Volumetric Fog.

Ray-marching Volumetric Fog handles all types of light and differing fog densities, while analytical Volumetric Fog handles sun light without dynamic shadow and exponential height fog density.

The area near to the camera is covered by ray-marching Volumetric Fog, while everything else is covered by analytical Volumetric Fog.

Usage

To enable Volumetric Fog in your level, set the CVar e_VolumetricFog=1 in the console window or by adding it to the system.cfg.

Volumetric Fog Appearance

Voxel-based Volumetric Fog control is handled via the parameters found in the "Volumetric Fog" group in the Time of Day dialog window. (See picture above).

Volumetric Fog appearance types are controlled through various parameters and as listed in the table below.

Global Fog - Time of Day

Global fog control parameters are found in the "Volumetric Fog" group. The "Volumetric Fog" group can be found in the Time of Day dialog window.

Sun Radial Scattering Setup

Anisotropy(atmosphere) and Anisotropy(radial) parameters essentially work in the same way. Anisotropy(atmosphere) should be used to produce an atmospheric fog that covers the whole sky - to achieve this set a low value (close to 0.0). Anisotropy(radial) should be used to produce a bloom effect around the sun - to achieve this set a high value (close to 1.0). The Radial blend factor blends these two fog components to create various fog effects.

Setting the Radial blend mode = 1 and the Radial blend factor = 0.0 allows you to see sun atmosphere scattering only, hence checking the results of adjusting the Color(atmospheric) and the Anisotropy(atmosphereic) on your fog effect can easily be achieved. The Anisotropy parameter allows you to control how much incoming light is scattered and in which direction, while setting the Anisotropy(atmosphere) parameter close to 0 makes the scattering more isotropic and therefore produces a uniform atmospheric fog.

Next, setting Radial blend mode = 1 and Radial blend factor = 1.0 allows you to see sun radial scattering only, hence checking the results of adjusting the Color(radial) and Anisotropy(radial) on your fog effect can easily be achieved. Adjusting the Anisotropy(radial) parameter to high (0.95) makes a scattering more anisotropic and therefore produces a bloom effect around the sun.

Finally, adjusting Radial blend factor parameter between 0 and 1 blends the sun atmosphere scattering and sun radial scattering, hence the final result becomes a more life-like atmospheric fog effect.

Fog Volume

To control the local fog density and color, put the FogVolume entity into a level and add a fog density and color into the area.

The following parameters have been added for voxel-based Volumetric Fog.

Fog Volume Entity Setup

With the introduction of control over the density noise, then some interesting shapes can be created within the fog volume entity. Below is a starting guide on how to configure the various parameters to achieve a fog bank effect.

1. To begin with, setup a large volume (50m in XYZ), add some color, increase the wind influence (5) and ramp up the Global Density to a high value (100) - this will make it easier to see what you are doing when you are changing the various parameters.
   
   
   
   
2. Using the DensityNoiseOffset slider, reduce the value down into the negative range (-1.2). This will add noise into the volume and break up its solid shape into patches.
   
   
   
   
3. Next adjust the DensityNoiseScale slider (default 1). This will define the thickness of the individual patches of fog (referred to here as clouds), or if you look at it from the other direction it will define how big the spaces are in-between each cloud. Reducing the value to (0.5) will thin out the cloud density and increase the spacing between each cloud.
   DensityNoiseOffset and DensityNoiseScale work closely together, so adjusting one value usually means that the other will require a tweak to achieve the desired result.
   
   
   
   
4. Adjusting the DensityNoiseFrequency allows you to define how many fog patches you want. In our example we have increased the Z value from 10 to 100. This creates a layered effect within the volume simulating different fog patches stacked on top of each other. Adding higher values in X & Y would mean there would be more individual fog patches within the volume.
   Having variations within the XYZ values will randomly breakup the fog patches into random sizes.
   
   
   
   
5. Modifying the DensityNoiseTimeFrequency allows the individual fog patches to morph into different shapes during the course of their lifetime. Keep this value low (0.2) otherwise they will morph too quickly and will look very unnatural. (This is hard to represent in a static picture, so for consistency we have used a higher value (1) for the screenshot below). .
   
   
   
   

6. Final result. Check the checkbox UseGlobalFogColor (doing so helps to achieve more natural blending in the scene and overrides the red colored fog - you should default the fog back to white). The screenshot below represents standing within the fog volume, looking horizontally which helps show how the additional values in DensityNoiseFrequency generate the various fog patches.

   Note: In order for the screenshot below to demonstrate the final result it was necessary for us to have a high fog density setting. During production you would not have the fog density set so high (normal levels used in production settings are in the range of 1-3), this ensures the fog effect you are trying to create is subtle/lifelike for e.g. like you would see around lakes or rivers - this does of course depend on how thick you want your fog volume.

   
   
   
   

Flowgraph

The fog volume can be controlled through Flowgraph to modify the parameters at run-time.

Particle