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<h3><a name="s02_02_04">2.2.4 </a>The Light Source</h3>
<a name="s02_02_04_i1"><a name="s02_02_04_i2">
<dl class="famousquote">
 
 <dt>
   <em>You know you have been raytracing too long when ...<br> ... You take a photo course just to learn how to get 
  the lighting right.</em> 
 <dd>
   <em>-- Christoph Rieder</em> 
</dl>

<p>
  In any ray-traced scene, the light needed to illuminate our objects and their surfaces must come from a light 
 source. There are many kinds of light sources available in POV-Ray and careful use of the correct kind can yield very 
 impressive results. Let's take a moment to explore some of the different kinds of light sources and their various 
 parameters. 
</p>

<h4><a name="s02_02_04_01">2.2.4.1 </a>The Pointlight Source</h4>
<a name="s02_02_04_01_i1">
<p>
  Pointlights are exactly what the name indicates. A pointlight has no size, is invisible and illuminates everything 
 in the scene equally no matter how far away from the light source it may be (this behavior can be changed). This is 
 the simplest and most basic light source. There are only two important parameters, location and color. Let's design a 
 simple scene and place a pointlight source in it. 
</p>

<p>
  We create a new file and name it <code>litedemo.pov</code>. We edit it as follows: 
</p>

<pre>
  #include &quot;colors.inc&quot;
  #include &quot;textures.inc&quot;
  camera {
    location  &lt;-4, 3, -9&gt;
    look_at   &lt;0, 0, 0&gt;
    angle 48
  }
</pre>

<p>
  We add the following simple objects: 
</p>

<pre>
  plane {
    y, -1
    texture {
      pigment {
        checker
        color rgb&lt;0.5, 0, 0&gt;
        color rgb&lt;0, 0.5, 0.5&gt;
      }
      finish {
        diffuse 0.4
        ambient 0.2
        phong 1
        phong_size 100
        reflection 0.25
      }
    }
  }
  torus {
    1.5, 0.5
    texture { Brown_Agate }
    rotate &lt;90, 160, 0&gt;
    translate &lt;-1, 1, 3&gt;
  }
  box {
    &lt;-1, -1, -1&gt;, &lt;1, 1, 1&gt;
    texture { DMFLightOak }
    translate &lt;2, 0, 2.3&gt;
  }
  cone {
    &lt;0,1,0&gt;, 0, &lt;0,0,0&gt;, 1
    texture { PinkAlabaster }
    scale &lt;1, 3, 1&gt;
    translate &lt;-2, -1, -1&gt;
  }
  sphere {
    &lt;0,0,0&gt;,1
    texture { Sapphire_Agate }
    translate &lt;1.5, 0, -2&gt;
  }
</pre>

<p>
  Now we add a pointlight: 
</p>

<pre>
  light_source {
    &lt;2, 10, -3&gt;
    color White
  }
</pre>

<p>
  We render this at 200x150 <code>-A</code> and see that the objects are clearly visible with sharp shadows. The 
 sides of curved objects nearest the light source are brightest in color with the areas that are facing away from the 
 light source being darkest. We also note that the checkered plane is illuminated evenly all the way to the horizon. 
 This allows us to see the plane, but it is not very realistic. 
</p>

<h4><a name="s02_02_04_02">2.2.4.2 </a>The Spotlight Source</h4>
<a name="s02_02_04_02_i1"><a name="s02_02_04_02_i2">
<p>
  Spotlights are a very useful type of light source. They can be used to add highlights and illuminate features much 
 as a photographer uses spots to do the same thing. To create a spotlight simply add the <code><a href="s_111.html#s03_04_07_02">spotlight</a></code> 
 keyword to a regular point light. There are a few more parameters with spotlights than with pointlights. These are <code><a href="s_111.html#s03_04_07_02">radius</a></code>, 
 <code><a href="s_111.html#s03_04_07_02">falloff</a></code>, <code><a href="s_111.html#s03_04_07_02">tightness</a></code> 
 and <code><a href="s_111.html#s03_04_07_02">point_at</a></code>. The <code>radius</code> parameter is the angle of the 
 fully illuminated cone. The <code>falloff</code> parameter is the angle of the <em>umbra</em> cone where the light 
 falls off to darkness. The <code>tightness</code> is a parameter that determines the rate of the light falloff. The <code>point_at</code> 
 parameter is just what it says, the location where the spotlight is pointing to. Let's change the light in our scene 
 as follows: 
</p>

<pre>
  light_source {
    &lt;0, 10, -3&gt;
    color White
    spotlight
    radius 15
    falloff 20
    tightness 10
    point_at &lt;0, 0, 0&gt;
  }
</pre>

<p>
  We render this at 200x150 <code>-A</code> and see that only the objects are illuminated. The rest of the plane and 
 the outer portions of the objects are now unlit. There is a broad falloff area but the shadows are still razor sharp. 
 Let's try fiddling with some of these parameters to see what they do. We change the falloff value to 16 (it must 
 always be larger than the radius value) and render again. Now the falloff is very narrow and the objects are either 
 brightly lit or in total darkness. Now we change falloff back to 20 and change the tightness value to 100 (higher is 
 tighter) and render again. The spotlight appears to have gotten much smaller but what has really happened is that the 
 falloff has become so steep that the radius actually appears smaller. 
</p>

<p>
  We decide that a tightness value of 10 (the default) and a falloff value of 18 are best for this spotlight and we 
 now want to put a few spots around the scene for effect. Let's place a slightly narrower blue and a red one in 
 addition to the white one we already have: 
</p>

<pre>
  light_source {
    &lt;10, 10, -1&gt;
    color Red
    spotlight
    radius 12
    falloff 14
    tightness 10
    point_at &lt;2, 0, 0&gt;
  }
  light_source {
    &lt;-12, 10, -1&gt;
    color Blue
    spotlight
    radius 12
    falloff 14
    tightness 10
    point_at &lt;-2, 0, 0&gt;
  }
</pre>

<p>
  Rendering this we see that the scene now has a wonderfully mysterious air to it. The three spotlights all converge 
 on the objects making them blue on one side and red on the other with enough white in the middle to provide a balance. 
</p>

<h4><a name="s02_02_04_03">2.2.4.3 </a>The Cylindrical Light Source</h4>
<a name="s02_02_04_03_i1"><a name="s02_02_04_03_i2">
<p>
  Spotlights are cone shaped, meaning that their effect will change with distance. The farther away from the 
 spotlight an object is, the larger the apparent radius will be. But we may want the radius and falloff to be a 
 particular size no matter how far away the spotlight is. For this reason, cylindrical light sources are needed. A 
 cylindrical light source is just like a spotlight, except that the radius and falloff regions are the same no matter 
 how far from the light source our object is. The shape is therefore a cylinder rather than a cone. We can specify a 
 cylindrical light source by replacing the <code>spotlight</code> keyword with the <code>cylinder</code> keyword. We 
 try this now with our scene by replacing all three spotlights with cylinder lights and rendering again. We see that 
 the scene is much dimmer. This is because the cylindrical constraints do not let the light spread out like in a 
 spotlight. Larger radius and falloff values are needed to do the job. We try a radius of 20 and a falloff of 30 for 
 all three lights. That's the ticket! 
</p>

<h4><a name="s02_02_04_04">2.2.4.4 </a>The Area Light Source</h4>
<a name="s02_02_04_04_i1"><a name="s02_02_04_04_i2">
<dl class="famousquote">
 
 <dt>
   <em>You know you have been raytracing too long when ...<br> ... You wear fuzzy clothing to soften your shadow.</em> 
  
 <dd>
   <em>-- Mark Kadela</em> 
</dl>

<p>
  So far all of our light sources have one thing in common. They produce sharp shadows. This is because the actual 
 light source is a point that is infinitely small. Objects are either in direct sight of the light, in which case they 
 are fully illuminated, or they are not, in which case they are fully shaded. In real life, this kind of stark light 
 and shadow situation exists only in outer space where the direct light of the sun pierces the total blackness of 
 space. But here on Earth, light bends around objects, bounces off objects, and usually the source has some dimension, 
 meaning that it can be partially hidden from sight (shadows are not sharp anymore). They have what is known as an <em>umbra</em>, 
 or an area of fuzziness where there is neither total light or shade. In order to simulate these <em> soft</em> 
 shadows, a ray-tracer must give its light sources dimension. POV-Ray accomplishes this with a feature known as an area 
 light. 
</p>

<p>
  Area lights have dimension in two axis'. These are specified by the first two vectors in the area light syntax. We 
 must also specify how many lights are to be in the array. More will give us cleaner soft shadows but will take longer 
 to render. Usually a 3*3 or a 5*5 array will suffice. We also have the option of specifying an adaptive value. The <code> 
 adaptive</code> keyword tells the ray-tracer that it can adapt to the situation and send only the needed rays to 
 determine the value of the pixel. If adaptive is not used, a separate ray will be sent for every light in the area 
 light. This can really slow things down. The higher the adaptive value the cleaner the umbra will be but the longer 
 the trace will take. Usually an adaptive value of 1 is sufficient. Finally, we probably should use the <code> jitter</code> 
 keyword. This tells the ray-tracer to slightly move the position of each light in the area light so that the shadows 
 appear truly soft instead of giving us an umbra consisting of closely banded shadows. 
</p>

<p>
  OK, let's try one. We comment out the cylinder lights and add the following: 
</p>

<pre>
  light_source {
    &lt;2, 10, -3&gt;
    color White
    area_light &lt;5, 0, 0&gt;, &lt;0, 0, 5&gt;, 5, 5
    adaptive 1
    jitter
  }
</pre>

<p>
  This is a white area light centered at &lt;2,10,-3&gt;. It is 5 units (along the x-axis) by 5 units (along the 
 z-axis) in size and has 25 (5*5) lights in it. We have specified adaptive 1 and jitter. We render this at 200x150 <code>-A</code>. 
 
</p>

<p>
  Right away we notice two things. The trace takes quite a bit longer than it did with a point or a spotlight and the 
 shadows are no longer sharp! They all have nice soft umbrae around them. Wait, it gets better. 
</p>

<p>
  Spotlights and cylinder lights can be area lights too! Remember those sharp shadows from the spotlights in our 
 scene? It would not make much sense to use a 5*5 array for a spotlight, but a smaller array might do a good job of 
 giving us just the right amount of umbra for a spotlight. Let's try it. We comment out the area light and change the 
 cylinder lights so that they read as follows: 
</p>

<pre>
  light_source {
    &lt;2, 10, -3&gt;
    color White
    spotlight
    radius 15
    falloff 18
    tightness 10
    area_light &lt;1, 0, 0&gt;, &lt;0, 0, 1&gt;, 2, 2
    adaptive 1
    jitter
    point_at &lt;0, 0, 0&gt;
  }
  light_source {
    &lt;10, 10, -1&gt;
    color Red
    spotlight
    radius 12
    falloff 14
    tightness 10
    area_light &lt;1, 0, 0&gt;, &lt;0, 0, 1&gt;, 2, 2
    adaptive 1
    jitter
    point_at &lt;2, 0, 0&gt;
  }
  light_source {
    &lt;-12, 10, -1&gt;
    color Blue
    spotlight
    radius 12
    falloff 14
    tightness 10
    area_light &lt;1, 0, 0&gt;, &lt;0, 0, 1&gt;, 2, 2
    adaptive 1
    jitter
    point_at &lt;-2, 0, 0&gt;
  }
</pre>

<p>
  We now have three area-spotlights, one unit square consisting of an array of four (2*2) lights, three different 
 colors, all shining on our scene. We render this at 200x150 <code>-A</code>. It appears to work perfectly. All our 
 shadows have small, tight umbrae, just the sort we would expect to find on an object under a real spotlight. 
</p>

<h4><a name="s02_02_04_05">2.2.4.5 </a>The Ambient Light Source</h4>
<a name="s02_02_04_05_i1">
<p>
  The <em>ambient light source</em> is used to simulate the effect of inter-diffuse reflection. If there was not 
 inter-diffuse reflection all areas not directly lit by a light source would be completely dark. POV-Ray uses the <code>ambient</code> 
 keyword to determine how much light coming from the ambient light source is reflected by a surface. 
</p>

<p>
  By default the ambient light source, which emits its light everywhere and in all directions, is pure white (<code>rgb 
 &lt;1,1,1&gt;</code>). Changing its color can be used to create interesting effects. First of all the overall light 
 level of the scene can be adjusted easily. Instead of changing all ambient values in every finish only the ambient 
 light source is modified. By assigning different colors we can create nice effects like a moody reddish ambient 
 lighting. For more details about the ambient light source see &quot;Ambient Light&quot;. 
</p>

<p>
  Below is an example of a red ambient light source. 
</p>

<pre>
  global_settings { ambient_light rgb&lt;1, 0, 0&gt; }
</pre>

<h4><a name="s02_02_04_06">2.2.4.6 </a>Light Source Specials</h4>
<a name="s02_02_04_06_i1">
<h5><a name="s02_02_04_06_01">2.2.4.6.1 </a>Using Shadowless Lights</h5>
<a name="s02_02_04_06_01_i1"><a name="s02_02_04_06_01_i2">
<p>
  Light sources can be assigned the <code>shadowless</code> 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 <em>fill lights</em> around the scene. Fill lights are simply dimmer 
 lights with the <code>shadowless</code> 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. 
</p>

<p>
  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: 
</p>

<pre>
  light_source {
    &lt;0, 20, 0&gt;
    color Gray50
    shadowless
  }
</pre>

<p>
  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. 
</p>

<h5><a name="s02_02_04_06_02">2.2.4.6.2 </a>Assigning an Object to a Light Source</h5>
<a name="s02_02_04_06_02_i1"><a name="s02_02_04_06_02_i2"><a name="s02_02_04_06_02_i3">
<p>
  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 <code>looks_like</code> keyword. We 
 can specify that our light source can look like any object we choose. When we use <code>looks_like</code>, then <code>no_shadow</code> 
 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 lamp shade), 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: 
</p>

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

<p>
  Now we add the light source: 
</p>

<pre>
  light_source {
    &lt;0, 2, 0&gt;
    color White
    looks_like { Lightbulb }
  }
</pre>

<p>
  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: 
</p>

<pre>
  light_source {
    &lt;0, 2, 0&gt;
    color White
    area_light &lt;1, 0, 0&gt;, &lt;0, 1, 0&gt;, 2, 2
    adaptive 1
    jitter
    looks_like { Lightbulb }
  }
</pre>

<p>
  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. 
</p>

<h5><a name="s02_02_04_06_03">2.2.4.6.3 </a>Using Light Fading</h5>
<a name="s02_02_04_06_03_i1"><a name="s02_02_04_06_03_i2"><a name="s02_02_04_06_03_i3">
<p>
  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: <code>fade_distance</code>, which specifies the 
 distance at which full illumination is achieved, and <code>fade_power</code>, an exponential value which determines 
 the actual rate of attenuation. Let's apply these keywords to our fill light. 
</p>

<p>
  First, we make the fill light a little brighter by changing <code> Gray50</code> to <code>Gray75</code>. Now we 
 change that fill light as follows: 
</p>

<pre>
  light_source {
    &lt;0, 20, 0&gt;
    color Gray75
    fade_distance 5
    fade_power 1
    shadowless
  }
</pre>

<p>
  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. 
</p>

<p>
  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. 
</p>
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