
\subsection{Lights}
\index{lighting}
\index{lighting!point lights}
The most frequently used type of lights provided by Tachyon are positional 
point light sources.  The lights are actually small spheres, which are 
visible.  A point light is composed of three pieces of
information, a center, a radius (since its a sphere), and a color.
To define a light, simply write the {\bf LIGHT} keyword, followed by 
its {\bf CENTER} (a X, Y, Z coordinate), its {\bf RAD} (radius, a scalar),
and its {\bf COLOR} (a Red Green Blue triple).  The radius parameter will
accept any value of 0.0 or greater.  Lights of radius 0.0 will not be
directly visible in the rendered scene, but contribute light to the scene
normally. 
For a light, the color values
range from 0.0 to 1.0, any values outside this range may yield unpredictable
results.  A simple light definition looks like this:
\begin{verbatim}
  LIGHT CENTER 4.0 3.0 2.0 
        RAD    0.2 
        COLOR  0.5 0.5 0.5
\end{verbatim}
This light would be gray colored if seen directly, and would be 50\% 
intensity in each RGB color component. 


\index{lighting!directional lights}
Tachyon supports simple directional lighting, commonly used in 
CAD and scientific visualization programs for its performance
advantages over positional lights.  Directional lights cannot be
seen directly in scenes rendered by Tachyon, only their illumination 
contributes to the final image.  

\begin{verbatim}
DIRECTIONAL_LIGHT 
  DIRECTION 0.0 -1.0 0.0
  COLOR   1.0 0.0  0.0
\end{verbatim}

\index{lighting!spotlights}
Tachyon supports spotlights, which are described very similarly to a 
point light, but they are attenuated by angle from the direction vector,
based on a  ``falloff start'' angle and ``falloff end''angle.  Between
the starting and ending angles, the illumination is attenuated linearly.
The syntax for a spotlight description in a scene file is as follows.
\begin{verbatim}
SPOTLIGHT 
  CENTER  0.0 3.0  17.0 
  RAD     0.2
  DIRECTION 0.0 -1.0 0.0
    FALLOFF_START 20.0
    FALLOFF_END   45.0
  COLOR   1.0 0.0  0.0
\end{verbatim}

\index{lighting!attenuation}
The lighting system implemented by Tachyon provides various levels of
distance-based lighting attenuation.  By default, a light is not attenuated
by distance.  If the {\em attenuation} keywords is present immediately 
prior to the light's color, Tachyon will accept coefficients which are used 
to calculate distance-based attenuation, which is applied the light by 
multiplying with the resulting value.  The attenuation factor is calculated 
from the equation 
\begin{equation}
\label{eq:attenuation}
  \frac {1} {K_c + K_l d + k_q d^2}
\end{equation}

This attenuation equation should be familiar to some as it 
is the same lighting attenuation equation used by OpenGL.
The constant, linear, and quadratic terms are specified in a scene file
as shown in the following example.
\begin{verbatim}
LIGHT  
  CENTER  -5.0 0.0 10.0   
  RAD     1.0
  ATTENUATION CONSTANT 1.0 LINEAR 0.2 QUADRATIC 0.05
  COLOR   1.0 0.0 0.0  
\end{verbatim}


