\subsection{Surface scattering models}
Surface scattering models describe the way in which light reflects
from surfaces in the scene. Even slight adjustments to such a description
can significantly change the appearance of a rendering.
To achieve realistic results, Mitsuba comes with a library of 
both general-purpose models (smooth or rough glass, metal, plastic, ..) and 
more specialized models (woven cloth, masks, ..).

Throughout the documentation and within the scene description language, 
the word \emph{BSDF} is used synonymously with the term ``surface
scattering model''. This is an abbreviation for 
\emph{Bidirectional Scattering Distribution Function}, a more 
precise technical description of the model's properties.
In Mitsuba, BSDFs are assigned to \emph{shapes}, which
describe the visible surfaces in the scene. In the scene 
description language, this assignment must
either be performed by nesting BSDFs within shapes, 
or they can be named and then later referenced 
by by name.

The following fragment shows an example of both kinds of usages:
\begin{xml}
<scene>
	<!-- Creating a named BSDF for later use -->
	<bsdf type=".. BSDF type .." id="myNamedMaterial">
		<!-- BSDF parameters go here -->
	</bsdf>

	<shape type="sphere">
		<!-- Example of referencing a named material -->

		<ref id="myNamedMaterial"/>
	</shape>

	<shape type="sphere">
		<!-- Example of using an unnamed material -->

		<bsdf type=".. BSDF type ..">
			<!-- BSDF parameters go here -->
		</bsdf>
	</shape>
</scene>
\end{xml}
It is generally more economical to use named BSDFs when they
are used in several places, since this reduces Mitsuba's internal
memory usage.
