Light Source Specials

### 3.7.6 Light Source Specials

#### 3.7.6.1 Using Shadowless Lights

Light sources can be assigned the `shadowless` keyword and no shadows will be cast due to its presence in a scene. Sometimes, scenes are difficult to illuminate properly using the lights we have chosen to illuminate our objects. It is impractical and unrealistic to apply a higher ambient value to the texture of every object in the scene. So instead, we would place a couple of fill lights around the scene. Fill lights are simply dimmer lights with the `shadowless` keyword that act to boost the illumination of other areas of the scene that may not be lit well. Let's try using one in our scene.

Remember the three colored area spotlights? We go back and un-comment them and comment out any other lights we have made. Now we add the following:

      light_source {
        <0, 20, 0>
        color Gray50
        shadowless
      }

This is a fairly dim light 20 units over the center of the scene. It will give a dim illumination to all objects including the plane in the background. We render it and see.

#### 3.7.6.2 Assigning an Object to a Light Source

Light sources are invisible. They are just a location where the light appears to be coming from. They have no true size or shape. If we want our light source to be a visible shape, we can use the `looks_like` keyword. We can specify that our light source can look like any object we choose. When we use `looks_like`, then `no_shadow` is applied to the object automatically. This is done so that the object will not block any illumination from the light source. If we want some blocking to occur (as in a lampshade), it is better to simply use a union to do the same thing. Let's add such an object to our scene. Here is a light bulb we have made just for this purpose:

      #declare Lightbulb = union {
        merge {
          sphere { <0,0,0>,1 }
          cylinder {
            <0,0,1>, <0,0,0>, 1
            scale <0.35, 0.35, 1.0>
            translate  0.5*z
          }
          texture {
            pigment {color rgb <1, 1, 1>}
            finish {ambient .8 diffuse .6}
          }
        }
        cylinder {
          <0,0,1>, <0,0,0>, 1
          scale <0.4, 0.4, 0.5>
          texture { Brass_Texture }
          translate  1.5*z
        }
        rotate -90*x
        scale .5
      }

Now we add the light source:

      light_source {
        <0, 2, 0>
        color White
        looks_like { Lightbulb }
      }

Rendering this we see that a fairly believable light bulb now illuminates the scene. However, if we do not specify a high ambient value, the light bulb is not lit by the light source. On the plus side, all of the shadows fall away from the light bulb, just as they would in a real situation. The shadows are sharp, so let's make our bulb an area light:

      light_source {
        <0, 2, 0>
        color White
        area_light <1, 0, 0>, <0, 1, 0>, 2, 2
        adaptive 1
        jitter
        looks_like { Lightbulb }
      }

We note that we have placed this area light in the x-y-plane instead of the x-z-plane. We also note that the actual appearance of the light bulb is not affected in any way by the light source. The bulb must be illuminated by some other light source or by, as in this case, a high ambient value.

#### 3.7.6.3 Using Light Fading

If it is realism we want, it is not realistic for the plane to be evenly illuminated off into the distance. In real life, light gets scattered as it travels so it diminishes its ability to illuminate objects the farther it gets from its source. To simulate this, POV-Ray allows us to use two keywords: `fade_distance`, which specifies the distance at which full illumination is achieved, and `fade_power`, an exponential value which determines the actual rate of attenuation. Let's apply these keywords to our fill light.

First, we make the fill light a little brighter by changing `Gray50` to `Gray75`. Now we change that fill light as follows:

      light_source {
        <0, 20, 0>
        color Gray75
        fade_distance 5
        fade_power 1
        shadowless
      }

This means that the full value of the fill light will be achieved at a distance of 5 units away from the light source. The fade power of 1 means that the falloff will be linear (the light falls off at a constant rate). We render this to see the result.

That definitely worked! Now let's try a fade power of 2 and a fade distance of 10. Again, this works well. The falloff is much faster with a fade power of 2 so we had to raise the fade distance to 10.