Simple fog is the basic environment fog settings used when creating fog effects in Maya. As with any environment fog type, Maya creates a volumetric material. In the case of simple fog attributes, the clipping distance can be adjusted as well as the height, saturation point and color.

Above is a scene created primarily from polygonal surfaces with textures taken from a library. The environment fog has not been added yet so that you can reference changes with Simple Fog. To add the environment fog go the the Render Options within the Render Globals and click on the box to the right of the Environment Fog field.

When examining the fog layout in the Hypershade notice that the envFogMaterial is connected as a Volume Material. This is rendered as a volume within the scene and is not assigned to a surface. In addition a envFogLight node is created which is light linked to the envFogSE. The envFogLight does not illuminate the geometry within the scene, it is used to exclusively link the EnvFogSE to the light object. If the fog requires an illumination source, the light can be activated by turning on "Illuminates by Default" in the lights attributes.

The image above shows the basic results of Simple Fog. The Saturation Distance is the only thing modified to control where the fog is the most dense. The Saturation Distance is expressed in terms of grid units from the rendered camera view. For example this scene is approximately 26 cm from camera to the buildings at the end of the street and the Saturation Distance is 8 cm or ~1/3 the depth.

Using the same setting as the last image the Saturation Distance is set to 30 cm or just behind the buildings at the end of the street.

Clipping planes are also useful for giving fog a distance start and end point. In the envFog attributes there are two modes for clipping. Fog Near/Far adjusts the fog clip distance locally offset from the camera. The Camera Near/Far uses the cameras clipping plane values as set by the camera attribute editor. In the above example the Fog Near/Far is set just before the overpass for the Near and to 200 cm for the Far. The Saturation Distance is set to the same depth as the Near clip to create a sharp wall of fog.

Use Height -ON enables the fog to grade from a top to bottom height within a scene. The Min/Max value is set to the working grid units so that is determines exactly where the fog deminishes. The Blend Range controls how quickly the fog grades as it approaches the boundary of the Min/Max Height. In the above scene the fog Min is at the ground level 0 and the max is set just above the height of the camera. Additionally, a Fog Near/Far value clips the start of the fog to be a few units from the camera with the Saturation Distance also a few units from camera.

Once the camera moves above or below the Min/Max Height, a sharp boundary exists for the height clip. In this example the Fog Max Height is below the camera.

Color Based Transparency by default is ON which makes darker color values more transparent. This is the opposite of transparency in shaders which darker colors are more opaque. If you wanted to create a smoggy scene or have the appearance of light being blocked by thick smoke, turn OFF Color Based Transparency. The image above uses this idea to create a low smog bank. The Height Min/Max is set closely below and above the camera with a high Blend Range (6) to soften the outer limits.

This example uses a very dark color value with Color Based Transparency unpicked. The Use Height is OFF and the Saturation Distance is just before the bridge to create a dark gloom.

Use Layer is the last option in the Simple Fog Attributes. When writing the tutorial I was unable to get results claimed to be achieved in the manuals. Namely the ability to map the fog with a 3D volume texture to break up the density of fog. If I am able to solve this mystery it will be added to the tutorial.