Section 4.6
The Light Source

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.

Section 4.6.1
The Ambient Light Source

The ambient light source is used to simulate the effect of inter-diffuse reflection. If there wasn't inter-diffuse reflection all areas not directly lit by a light source would be completely dark. POV-Ray uses the ambient keyword to determine how much light coming from the ambient light source is reflected by a surface.

By default the ambient light source, which emits its light everywhere and in all directions, is pure white (rgb <1,1,1>). 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 "Ambient Light".

Below is an example of a red ambient light source.

global_settings { ambient_light rgb<1, 0, 0> }

Section 4.6.2
The Pointlight Source

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.

We create a new file and name it litedemo.pov. We edit it as follows:

#include "colors.inc" #include "textures.inc" camera { location <-4, 3, -9> look_at <0, 0, 0> angle 48 }

We add the following simple objects:

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

Now we add a pointlight:

light_source { <2, 10, -3> color White }

We render this at 200x150 -A 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.

Section 4.6.3
The Spotlight Source

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. There are a few more parameters with spotlights than with pointlights. These are radius, falloff, tightness and point_at. The radius parameter is the angle of the fully illuminated cone. The falloff parameter is the angle of the umbra cone where the light falls off to darkness. The tightness is a parameter that determines the rate of the light falloff. The point_at parameter is just what it says, the location where the spotlight is pointing to. Let's change the light in our scene as follows:

light_source { <0, 10, -3> color White spotlight radius 15 falloff 20 tightness 10 point_at <0, 0, 0> }

We render this at 200x150 -A 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.

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:

light_source { <10, 10, -1> color Red spotlight radius 12 falloff 14 tightness 10 point_at <2, 0, 0> } light_source { <-12, 10, -1> color Blue spotlight radius 12 falloff 14 tightness 10 point_at <-2, 0, 0> }

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.

Section 4.6.4
The Cylindrical Light Source

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 spotlight keyword with the cylinder 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!

Section 4.6.5
The Area Light Source

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 umbra, or an area of fuzziness where there is neither total light or shade. In order to simulate these soft shadows, a ray-tracer must give its light sources dimension. POV-Ray accomplishes this with a feature known as an area light.

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 adaptive 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 jitter 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.

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

light_source { <2, 10, -3> color White area_light <5, 0, 0>, <0, 0, 5>, 5, 5 adaptive 1 jitter }

This is a white area light centered at <2,10,-3>. 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 -A.

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.

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:

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

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 -A. 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.

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