The Rainbow

### 3.11.4 The Rainbow

The `rainbow` feature can be used to create rainbows and maybe other more strange effects. The rainbow is a fog like effect that is restricted to a cone-like volume.

#### 3.11.4.1 Starting With a Simple Rainbow

The rainbow is specified with a lot of parameters: the angle under which it is visible, the width of the color band, the direction of the incoming light, the fog-like distance based particle density and last but not least the color map to be used.

The size and shape of the rainbow are determined by the `angle` and `width` keywords. The `direction` keyword is used to set the direction of the incoming light, thus setting the rainbow's position. The rainbow is visible when the angle between the direction vector and the incident light direction is larger than angle-width/2 and smaller than angle+width/2.

The incoming light is the virtual light source that is responsible for the rainbow. There needn't be a real light source to create the rainbow effect.

The rainbow is a fog-like effect, i.e. the rainbow's color is mixed with the background color based on the distance to the intersection point. If you choose small distance values the rainbow will be visible on objects, not just in the background. You can avoid this by using a very large distance value.

The color map is the crucial part of the rainbow since it contains all the colors that normally can be seen in a rainbow. The color of the innermost color band is taken from the color map entry 0 while the outermost band is take from entry 1. You should note that due to the limited color range any monitor can display it is impossible to create a real rainbow. There are just some colors that you cannot display.

The filter channel of the rainbow's color map is used in the same way as with fogs. It determines how much of the light passing through the rainbow is filtered by the color.

The following example shows a simple scene with a ground plane, three spheres and a somewhat exaggerated rainbow (`rainbow1.pov`).

      #include "colors.inc"
      camera {
        location <0, 20, -100>
        look_at <0, 25, 0>
        angle 80
      }
      background { color SkyBlue }
      plane { y, -10 pigment { color Green } }
      light_source { <100, 120, 40> color White }
      // declare rainbow's colors
      #declare r_violet1 = color rgbf<1.0, 0.5, 1.0, 1.0>;
      #declare r_violet2 = color rgbf<1.0, 0.5, 1.0, 0.8>;
      #declare r_indigo  = color rgbf<0.5, 0.5, 1.0, 0.8>;
      #declare r_blue    = color rgbf<0.2, 0.2, 1.0, 0.8>;
      #declare r_cyan    = color rgbf<0.2, 1.0, 1.0, 0.8>;
      #declare r_green   = color rgbf<0.2, 1.0, 0.2, 0.8>;
      #declare r_yellow  = color rgbf<1.0, 1.0, 0.2, 0.8>;
      #declare r_orange  = color rgbf<1.0, 0.5, 0.2, 0.8>;
      #declare r_red1    = color rgbf<1.0, 0.2, 0.2, 0.8>;
      #declare r_red2    = color rgbf<1.0, 0.2, 0.2, 1.0>;
      // create the rainbow
      rainbow {
        angle 42.5
        width 5
        distance 1.0e7
        direction <-0.2, -0.2, 1>
        jitter 0.01
        color_map {
          [0.000  color r_violet1]
          [0.100  color r_violet2]
          [0.214  color r_indigo]
          [0.328  color r_blue]
          [0.442  color r_cyan]
          [0.556  color r_green]
          [0.670  color r_yellow]
          [0.784  color r_orange]
          [0.900  color r_red1]
        }
      }

Some irregularity is added to the color bands using the `jitter` keyword.

A colorful rainbow

The rainbow in our sample is much too bright. You'll never see a rainbow like this in reality. You can decrease the rainbow's colors by decreasing the RGB values in the color map.

#### 3.11.4.2 Increasing the Rainbow's Translucency

The result we have so far looks much too bright. Just reducing the rainbow's color helps but it's much better to increase the translucency of the rainbow because it is more realistic if the background is visible through the rainbow.

We can use the transmittance channel of the colors in the color map to specify a minimum translucency, just like we did with the fog. To get realistic results we have to use very large transmittance values as you can see in the following example (`rainbow2.pov`).

      rainbow {
        angle 42.5
        width 5
        distance 1.0e7
        direction <-0.2, -0.2, 1>
        jitter 0.01
        color_map {
          [0.000  color r_violet1 transmit 0.98]
          [0.100  color r_violet2 transmit 0.96]
          [0.214  color r_indigo  transmit 0.94]
          [0.328  color r_blue    transmit 0.92]
          [0.442  color r_cyan    transmit 0.90]
          [0.556  color r_green   transmit 0.92]
          [0.670  color r_yellow  transmit 0.94]
          [0.784  color r_orange  transmit 0.96]
          [0.900  color r_red1    transmit 0.98]
        }
      }

The transmittance values increase at the outer bands of the rainbow to make it softly blend into the background.

A much more realistic rainbow

The resulting image looks much more realistic than our first rainbow.

#### 3.11.4.3 Using a Rainbow Arc

Currently our rainbow has a circular shape, even though most of it is hidden below the ground plane. You can easily create a rainbow arc by using the `arc_angle` keyword with an angle below 360 degrees.

If you use `arc_angle 120` for example you'll get a rainbow arc that abruptly vanishes at the arc's ends. This does not look good. To avoid this the `falloff_angle` keyword can be used to specify a region where the arc smoothly blends into the background.

As explained in the rainbow's reference section (see "Rainbow") the arc extends from -arc_angle/2 to arc_angle/2 while the blending takes place from -arc_angle/2 to -falloff_angle/2 and falloff_angle/2 to arc_angle/2. This is the reason why the `falloff_angle` has to be smaller or equal to the `arc_angle`.

In the following examples we use an 120 degrees arc with a 45 degree falloff region on both sides of the arc (`rainbow3.pov`).

      rainbow {
        angle 42.5
        width 5
        arc_angle 120
        falloff_angle 30
        distance 1.0e7
        direction <-0.2, -0.2, 1>
        jitter 0.01
        color_map {
          [0.000  color r_violet1 transmit 0.98]
          [0.100  color r_violet2 transmit 0.96]
          [0.214  color r_indigo  transmit 0.94]
          [0.328  color r_blue    transmit 0.92]
          [0.442  color r_cyan    transmit 0.90]
          [0.556  color r_green   transmit 0.92]
          [0.670  color r_yellow  transmit 0.94]
          [0.784  color r_orange  transmit 0.96]
          [0.900  color r_red1    transmit 0.98]
        }
      }

The arc angles are measured against the rainbows up direction which can be specified using the `up` keyword. By default the up direction is the y-axis.

A rainbow arc

We finally have a realistic looking rainbow arc.