### 3.11.3 The Fog

You can use the `fog` feature to add fog of two different types to your scene: constant fog and ground fog. The constant fog has a constant density everywhere while the ground fog's density decreases as you move upwards.

The usage of both fog types will be described in the next sections in detail.

#### 3.11.3.1 A Constant Fog

The simplest fog type is the constant fog that has a constant density in all locations. It is specified by a `distance` keyword which actually describes the fog's density and a fog `color`.

The distance value determines the distance at which 36.8% of the background is still visible (for a more detailed explanation of how the fog is calculated read the reference section "Fog").

The fog color can be used to create anything from a pure white to a red, blood-colored fog. You can also use a black fog to simulate the effect of a limited range of vision.

The following example will show you how to add fog to a simple scene (`fog1.pov`).

      #include "colors.inc"
      camera {
        location  <0, 20, -100>
      }
      background { color SkyBlue }
      plane {
        y, -10
        pigment {
          checker color Yellow color Green
          scale 20
        }
      }
      sphere {
        <0, 25, 0>, 40
        pigment { Red }
        finish { phong 1.0 phong_size 20 }
      }
      sphere {
        <-100, 150, 200>,  20
        pigment { Green }
        finish { phong 1.0 phong_size 20 }
      }
      sphere {
        <100, 25, 100>, 30
        pigment { Blue }
        finish { phong 1.0 phong_size 20 }
      }
      light_source { <100, 120, 40> color White }
      fog {
        distance 150
        color rgb<0.3, 0.5, 0.2>
      }

A foggy scene

According to their distance the spheres in this scene more or less vanish in the greenish fog we used, as does the checkerboard plane.

#### 3.11.3.2 Setting a Minimum Translucency

If you want to make sure that the background does not completely vanish in the fog you can set the transmittance channel of the fog's color to the amount of background you always want to be visible.

Using as transmittance value of 0.2 as in

      fog {
        distance 150
        color rgbt<0.3, 0.5, 0.2, 0.2>
      }

the fog's translucency never drops below 20% as you can see in the resulting image (`fog2.pov`).

Fog with translucency threshold added

#### 3.11.3.3 Creating a Filtering Fog

The greenish fog we have used so far doesn't filter the light passing through it. All it does is to diminish the light's intensity. We can change this by using a non-zero filter channel in the fog's color (`fog3.pov`).

      fog {
        distance 150
        color rgbf<0.3, 0.5, 0.2, 1.0>
      }

The filter value determines the amount of light that is filtered by the fog. In our example 100% of the light passing through the fog will be filtered by the fog. If we had used a value of 0.7 only 70% of the light would have been filtered. The remaining 30% would have passed unfiltered.

A filtering fog

You'll notice that the intensity of the objects in the fog is not only diminished due to the fog's color but that the colors are actually influenced by the fog. The red and especially the blue sphere got a green hue.

#### 3.11.3.4 Adding Some Turbulence to the Fog

In order to make our somewhat boring fog a little bit more interesting we can add some turbulence, making it look like it had a non-constant density (`fog4.pov`).

      fog {
        distance 150
        color rgbf<0.3, 0.5, 0.2, 1.0>
        turbulence 0.2
        turb_depth 0.3
      }

Fog made more interesting with turbulence

The `turbulence` keyword is used to specify the amount of turbulence used while the `turb_depth` value is used to move the point at which the turbulence value is calculated along the viewing ray. Values near zero move the point to the viewer while values near one move it to the intersection point (the default value is 0.5). This parameter can be used to avoid noise that may appear in the fog due to the turbulence (this normally happens at very far away intersection points, especially if no intersection occurs, i. e. the background is hit). If this happens just lower the `turb_depth` value until the noise vanishes.

You should keep in mind that the actual density of the fog does not change. Only the distance-based attenuation value of the fog is modified by the turbulence value at a point along the viewing ray.

#### 3.11.3.5 Using Ground Fog

The much more interesting and flexible fog type is the ground fog, which is selected with the `fog_type` statement. It's appearance is described with the `fog_offset` and `fog_alt` keywords. The `fog_offset` specifies the height, i. e. y value, below which the fog has a constant density of one. The `fog_alt` keyword determines how fast the density of the fog will approach zero as one moves along the y axis. At a height of fog_offset+fog_alt the fog will have a density of 25%.

The following example (`fog5.pov`) uses a ground fog which has a constant density below y=25 (the center of the red sphere) and quickly falls off for increasing altitudes.

      fog {
        distance 150
        color rgbf<0.3, 0.5, 0.2, 1.0>
        fog_type 2
        fog_offset 25
        fog_alt 1
      }

An example of ground fog

#### 3.11.3.6 Using Multiple Layers of Fog

It is possible to use several layers of fog by using more than one fog statement in your scene file. This is quite useful if you want to get nice effects using turbulent ground fogs. You could add up several, differently colored fogs to create an eerie scene for example.

Just try the following example (`fog6.pov`).

      fog {
        distance 150
        color rgb<0.3, 0.5, 0.2>
        fog_type 2
        fog_offset 25
        fog_alt 1
        turbulence 0.1
        turb_depth 0.2
      }
      fog {
        distance 150
        color rgb<0.5, 0.1, 0.1>
        fog_type 2
        fog_offset 15
        fog_alt 4
        turbulence 0.2
        turb_depth 0.2
      }
      fog {
        distance 150
        color rgb<0.1, 0.1, 0.6>
        fog_type 2
        fog_offset 10
        fog_alt 2
      }

Using multiple layers of fog

You can combine constant density fogs, ground fogs, filtering fogs, non-filtering fogs, fogs with a translucency threshold, etc.

#### 3.11.3.7 Fog and Hollow Objects

Whenever you use the fog feature and the camera is inside a non-hollow object you won't get any fog effects. For a detailed explanation why this happens see "Empty and Solid Objects".

In order to avoid this problem you have to make all those objects hollow by either making sure the camera is outside these objects (using the `inverse` keyword) or by adding the `hollow` to them (which is much easier).