89 lines
3.2 KiB
TeX
89 lines
3.2 KiB
TeX
\newpage
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\subsection{Integrators}
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\label{sec:integrators}
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In Mitsuba, the different rendering techniques are collectively referred to as
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\emph{integrators}, since they perform integration over a high-dimensional
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space. Each integrator represents a specific approach for solving
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the light transport equation---usually favored in certain scenarios, but
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at the same time affected by its own set of intrinsic limitations.
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Therefore, it is important to carefully select an integrator based on
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user-specified accuracy requirements and properties of the scene to be
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rendered.
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In Mitsuba's XML description language, a single integrator
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is usually instantiated by declaring it at the top level within the
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scene, e.g.
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\begin{xml}
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<scene version=$\MtsVer$>
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<!-- Instantiate a unidirectional path tracer that
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renders paths up to a depth of 5 -->
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<integrator type="path">
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<integer name="maxDepth" value="5"/>
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</integrator>
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<!-- Some geometry to be rendered -->
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<shape type="sphere">
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<bsdf type="lambertian"/>
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</shape>
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</scene>
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\end{xml}
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This section gives a brief overview of the available choices
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along with their parameters.
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\subsubsection*{Path length}
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\begin{figure}[htb!]
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\centering
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\hfill
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\smallrendering{Max. length = 1}{pathlength-1}
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\smallrendering{Max. length = 2}{pathlength-2}
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\smallrendering{Max. length = 3}{pathlength-3}
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\smallrendering{Max. length = $\infty$}{pathlength-all}
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\caption{
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\label{fig:pathlengths}
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These Cornell box renderings demonstrate the visual
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effect of a maximum path length. As the paths
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are allowed to grow longer, the color saturation
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increases due to multiple scattering interactions
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with the colored surfaces. At the same time, the
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computation time increases.
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}
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\end{figure}
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Almost all integrators use the concept of \emph{path length}.
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Here, a path refers to a chain of scattering events that
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starts at the light source and ends at the eye or camera.
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It is often useful to limit the path length (\figref{pathlengths})
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when rendering scenes for preview purposes, since this reduces the amount
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of computation that is necessary per pixel. Furthermore, such renderings
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usually converge faster and therefore need fewer samples per pixel.
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When reference-quality is desired, one should always leave the path
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length unlimited.
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\begin{figure}[h!]
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\centering
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\vspace{-5mm}
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\includegraphics[width=10cm]{images/path_explanation.pdf}
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\vspace{-5mm}
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\caption{
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\label{fig:path-explanation}
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A ray of emitted light is scattered by an object and subsequently
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reaches the eye/camera.
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In Mitsuba, this is a \emph{length-2} path, since it has two edges.
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}
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\end{figure}
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Mitsuba counts lengths starting at $1$, which correspond to
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visible light sources (i.e. a path that starts at the light
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source and ends at the eye or camera without any scattering
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interaction in between).
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A length-$2$ path (also known as ``direct illumination'') includes
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a single scattering event (\figref{path-explanation}).
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\subsubsection*{Progressive versus non-progressive}
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Some of the rendering techniques in Mitsuba are \emph{progressive}.
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What this means is that they display a rough preview, which
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improves over time. Leaving them running indefinitely will
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continually reduce noise (e.g. in Metropolis Light Transport)
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or both noise and bias (e.g. in Progressive Photon Mapping).
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