source: trunk/VUT/doc/SciReport/sampling.tex @ 283

\chapter{Global Visibility Sampling}

\label{chap:sampling}

The proposed visibility preprocessing framework consists of two major
steps.

\begin{itemize}
\item The first step is an aggressive visibility sampling which gives
initial estimate about global visibility in the scene. The sampling
itself involves several strategies which will be described bellow.
The important property of the aggressive sampling step is that it
provides a fast progressive solution to global visibility and thus it
can be easily integrated into the game development cycle. The
aggressive sampling will terminate when the average contribution of new
ray samples falls below a predefined threshold.

\item The second step is mutual visibility verification. This step
turns the previous aggressive visibility solution into either exact,
conservative or error bound aggressive solution. The choice of the
particular verifier is left on the user in order to select the best
one for a particular scene, application context and time
constrains. For example, in scenes like a forest an error bound
aggressive visibility can be the best compromise between the resulting
size of the PVS (and frame rate) and the visual quality. The exact or
conservative algorithm can however be chosen for urban scenes where
omission of even small objects can be more distracting for the
user. The mutual visibility verification will be described in the next
chapter.

\end{itemize}

In traditional visibility preprocessing the view space is
subdivided into view cells and for each view cell the set of visible
objects --- potentially visible set (PVS) is computed. This framework
has been used for conservative, aggressive and exact algorithms.

We propose a different strategy which has several advantages for
sampling based aggressive visibility preprocessing. The strategy is
based on the following fundamental ideas:
\begin{itemize}
\item Compute progressive global visibility instead of sequential from-region visibility
\item Replace the roles of view cells and objects for some parts of the computation
\end{itemize}

Both these points will be addressed in this chapter in more detail.

\section{Related work}
\label{VFR3D_RELATED_WORK}

Below we briefly discuss the related work on visibility preprocessing
in several application areas. In particular we focus on computing
from-region which has been a core of most previous visibility
preprocessing techniques.


\subsection{Aspect graph}

The first algorithms dealing with from-region visibility belong to the
area of computer vision. The {\em aspect
  graph}~\cite{Gigus90,Plantinga:1990:RTH, Sojka:1995:AGT} partitions
the view space into cells that group viewpoints from which the
projection of the scene is qualitatively equivalent. The aspect graph
is a graph describing the view of the scene (aspect) for each cell of
the partitioning. The major drawback of this approach is that for
polygonal scenes with $n$ polygons there can be $\Theta(n^9)$ cells in
the partitioning for unrestricted view space. A {\em scale space}
aspect graph~\cite{bb12595,bb12590} improves robustness of the method
by merging similar features according to the given scale.


\subsection{Potentially visible sets}


 In the computer graphics community Airey~\cite{Airey90} introduced
the concept of {\em potentially visible sets} (PVS).  Airey assumes
the existence of a natural subdivision of the environment into
cells. For models of building interiors these cells roughly correspond
to rooms and corridors.  For each cell the PVS is formed by cells
visible from any point of that cell.  Airey uses ray shooting to
approximate visibility between cells of the subdivision and so the
computed PVS is not conservative.

This concept was further elaborated by Teller et
al.~\cite{Teller92phd,Teller:1991:VPI} to establish a conservative
PVS.  The PVS is constructed by testing the existence of a stabbing
line through a sequence of polygonal portals between cells. Teller
proposed an exact solution to this problem using \plucker
coordinates~\cite{Teller:1992:CAA} and a simpler and more robust
conservative solution~\cite{Teller92phd}.  The portal based methods
are well suited to static densely occluded environments with a
particular structure.  For less structured models they can face a
combinatorial explosion of complexity~\cite{Teller92phd}. Yagel and
Ray~\cite{Yagel95a} present an algorithm, that uses a regular spatial
subdivision. Their approach is not sensitive to the structure of the
model in terms of complexity, but its efficiency is altered by the
discrete representation of the scene.

Plantinga proposed a PVS algorithm based on a conservative viewspace
partitioning by evaluating visual
events~\cite{Plantinga:1993:CVP}. The construction of viewspace
partitioning was further studied by Chrysanthou et
al.~\cite{Chrysanthou:1998:VP}, Cohen-Or et al.~\cite{cohen-egc-98}
and Sadagic~\cite{Sadagic}.  Sudarsky and
Gotsman~\cite{Sudarsky:1996:OVA} proposed an output-sensitive
visibility algorithm for dynamic scenes.  Cohen-Or et
al.~\cite{COZ-gi98} developed a conservative algorithm determining
visibility of an $\epsilon$-neighborhood of a given viewpoint that was
used for network based walkthroughs.

Conservative algorithms for computing PVS developed by Durand et
al.~\cite{EVL-2000-60} and Schaufler et al.~\cite{EVL-2000-59}  make
use of several simplifying assumptions to avoid the usage of 4D data
structures.  Wang et al.~\cite{Wang98} proposed an algorithm that
precomputes visibility within beams originating from the restricted
viewpoint region. The approach is very similar to the 5D subdivision
for ray tracing~\cite{Simiakakis:1994:FAS} and so it exhibits similar
problems, namely inadequate memory and preprocessing complexities.
Specialized algorithms for computing PVS in \m25d scenes were proposed
by Wonka et al.~\cite{wonka00}, Koltun et al.~\cite{koltun01}, and
Bittner et al.~\cite{bittner:2001:PG}.

The exact mutual visibility method presented later in the report is
based on method exploting \plucker coordinates of
lines~\cite{bittner:02:phd,nirenstein:02:egwr,haumont2005:egsr}. This
algorithm uses \plucker coordinates to compute visibility in shafts
defined by each polygon in the scene.


\subsection{Rendering of shadows}


The from-region visibility problems include the computation of soft
shadows due to an areal light source. Continuous algorithms for
real-time soft shadow generation were studied by Chin and
Feiner~\cite{Chin:1992:FOP}, Loscos and
Drettakis~\cite{Loscos:1997:IHS}, and
Chrysanthou~\cite{Chrysantho1996a} and Chrysanthou and
Slater~\cite{Chrysanthou:1997:IUS}. Discrete solutions have been
proposed by Nishita~\cite{Nishita85}, Brotman and
Badler~\cite{Brotman:1984:GSS}, and Soler and Sillion~\cite{SS98}. An
exact algorithm computing an antipenumbra of an areal light source was
developed by Teller~\cite{Teller:1992:CAA}.


\subsection{Discontinuity meshing}


Discontinuity meshing is used in the context of the radiosity global
illumination algorithm or computing soft shadows due to areal light
sources.  First approximate discontinuity meshing algorithms were
studied by Campbell~\cite{Campbell:1990:AMG, Campbell91},
Lischinski~\cite{lischinski92a}, and Heckbert~\cite{Heckbert92discon}.
More elaborate methods were developed by
Drettakis~\cite{Drettakis94-SSRII, Drettakis94-FSAAL}, and Stewart and
Ghali~\cite{Stewart93-OSACS, Stewart:1994:FCSb}. These methods are
capable of creating a complete discontinuity mesh that encodes all
visual events involving the light source.

The classical radiosity is based on an evaluation of form factors
between two patches~\cite{Schroder:1993:FFB}. The visibility
computation is a crucial step in the form factor
evaluation~\cite{Teller:1993:GVA,Haines94,Teller:1994:POL,
Nechvile:1996:FFE,Teichmann:WV}. Similar visibility computation takes
place in the scope of hierarchical radiosity
algorithms~\cite{Soler:1996:AEB, Drettakis:1997:IUG, Daubert:1997:HLS}.



\subsection{Global visibility}

 The aim of {\em global visibility} computations is to capture and
describe visibility in the whole scene~\cite{Durand:1996:VCN}. The
global visibility algorithms are typically based on some form of {\em
line space subdivision} that partitions lines or rays into equivalence
classes according to their visibility classification. Each class
corresponds to a continuous set of rays with a common visibility
classification. The techniques differ mainly in the way how the line
space subdivision is computed and maintained. A practical application
of most of the proposed global visibility structures for 3D scenes is
still an open problem.  Prospectively these techniques provide an
elegant method for ray shooting acceleration --- the ray shooting
problem can be reduced to a point location in the line space
subdivision.


Pocchiola and Vegter introduced the visibility complex~\cite{pv-vc-93}
that describes global visibility in 2D scenes. The visibility complex
has been applied to solve various 2D visibility
problems~\cite{r-tsvcp-95,r-wvcav-97, r-dvpsv-97,Orti96-UVCRC}.  The
approach was generalized to 3D by Durand et
al.~\cite{Durand:1996:VCN}. Nevertheless, no implementation of the 3D
visibility complex is currently known. Durand et
al.~\cite{Durand:1997:VSP} introduced the {\em visibility skeleton}
that is a graph describing a skeleton of the 3D visibility
complex. The visibility skeleton was verified experimentally and  the
results indicate that its $O(n^4\log n)$ worst case complexity is much
better in practice. Pu~\cite{Pu98-DSGIV} developed a similar method to
the one presented in this chapter. He uses a BSP tree in \plucker
coordinates to represent a global visibility map for a given set of
polygons. The computation is performed considering all rays piercing
the scene and so the method exhibits unacceptable memory complexity
even for scenes of moderate size. Recently, Duguet and
Drettakis~\cite{duguet:02:sig} developed a robust variant of the
visibility skeleton algorithm that uses robust epsilon-visibility
predicates.

 Discrete methods aiming to describe visibility in a 4D data structure
were presented by Chrysanthou et al.~\cite{chrysanthou:cgi:98} and
Blais and Poulin~\cite{blais98a}.  These data structures are closely
related to the {\em lumigraph}~\cite{Gortler:1996:L,buehler2001} or
{\em light field}~\cite{Levoy:1996:LFR}. An interesting discrete
hierarchical visibility algorithm for two-dimensional scenes was
developed by Hinkenjann and M\"uller~\cite{EVL-1996-10}. One of the
biggest problems of the discrete solution space data structures is
their memory consumption required to achieve a reasonable
accuracy. Prospectively, the scene complexity
measures~\cite{Cazals:3204:1997} provide a useful estimate on the
required sampling density and the size of the solution space data
structure.


\subsection{Other applications}

 Certain from-point visibility problems determining visibility over a
period of time can be transformed to a static from-region visibility
problem. Such a transformation is particularly useful for antialiasing
purposes~\cite{grant85a}. The from-region visibility can also be used
in the context of simulation of the sound
propagation~\cite{Funkhouser98}. The sound propagation algorithms
typically require lower resolution than the algorithms simulating the
propagation of light, but they need to account for simulation of
attenuation, reflection and time delays.

\section{Algorithm Description}

This section first describes the setup of the global visibility
sampling algorithm. In particular we describe the view cell
representation and the novel concept of from-object based
visibility. The we outline the different visibility sampling
strategies.

\subsection{View Space Partitioning}




Before the visibility computation itself, we subdivide the space of
all possible viewpoints and viewing directions into view cells. A good
partition of the scene into view cells is an essential part of every
visibility system. If they are chosen too large, the PVS (Potentially
Visible Set)  of a view cells is too large for efficient culling. If
they are chosen too small or without consideration, then neighbouring
view cells contain redundant PVS information, hence boosting the PVS
computation and storage costs for the scene. In the left image of
figure~\ref{fig:vienna_viewcells} we see view cells of the Vienna
model, generated by triangulation of the streets.  In the closeup
(right image) we can see that each triangle is extruded to a given
height to form a view cell prism.

\begin{figure}[htb]
  \centerline{
    \includegraphics[height=0.35\textwidth,draft=\DRAFTFIGS]{images/vienna_viewcells_01}
     \includegraphics[height=0.35\textwidth,draft=\DRAFTFIGS]{images/vienna_viewcells_07}
    }

  \caption{(left) Vienna view cells. (right) The view cells are prisms with a triangular base. }
  \label{fig:vienna_viewcells}
\end{figure}

In order to efficiently use view cells with our sampling method, we
require a view cell representation which is

\begin{itemize}
\item optimized for view cell - ray intersection.
\item flexible, i.e., it can represent arbitrary geometry.
\item naturally suited for a hierarchical approach. %(i.e., there is a root view cell containing all others)
\end{itemize}

We meet these requirements by employing spatial subdivisions (i.e.,
KD trees and BSP trees), to store the view cells. The initial view
cells are associated with the leaves. The reason why we chose BSP
trees as view cell representation  is that they are very
flexible. View cells forming arbitrary closed meshes can be closely
matched.  Therefore we are able to find a view cells with only a few
view ray-plane intersections.  Furthermore, the hierarchical
structures can be exploited as hierarchy of view cells. Interior nodes
form larger view cells containing the children. If necessary, a leaf
can be easily subdivided into smaller view cells.

Currently we consider three different approaches to generate the
initial view cell BSP tree.  The third method is not restricted to BSP
trees, but BSP trees are preferred  because of their greater
flexibility.


\begin{table}
\centering \footnotesize
\begin{tabular}{|l|c|c|}
  \hline\hline
  View cells & Vienna selection & Vienna full \\\hline\hline
  \#view cells & 105 & 16447 \\\hline\hline
  \#input polygons & 525 & 82235 \\\hline\hline
  BSP tree generation time & 0.016s & 10.328s \\\hline\hline
  %%view cell insertion time & 0.016s & 7.984s \\\hline\hline
  \#nodes & 1137 & 597933 \\\hline\hline
  \#interior nodes & 568 & 298966\\\hline\hline
  \#leaf nodes  & 569 & 298967\\\hline\hline
  \#splits & 25 & 188936\\\hline\hline
  max tree depth & 13 & 27\\\hline\hline
  avg tree depth & 9.747 & 21.11\\\hline\hline
  
 \end{tabular}
 \caption{Statistics for view cell BSP tree on the Vienna view cells and a selection of the Vienna view cells.}\label{tab:viewcell_bsp}
\end{table}


\begin{itemize}

\item We use a number of input view cells given in advance. As input
view cell  any closed mesh can be applied. The only requirement is
that the any two view cells do not overlap.  The view cell
polygons are extracted, storing a pointer to the parent view cell
with the polygon. The BSP is build from these polygons using
some global optimizations like tree balancing or least splits. The 
polygons guide the split process as they are filtered down the tree. 
The subdivision terminates when there is only one polygon left, which is coincident 
to the last split plane. Then two leaves are created and the view cell pointer 
(stored with the polygon) is inserted into the leaf representing the inside of the view cell. 
One input view cell can be associated with many leaves in case
a view cell was split during the traversal. On the other hand, each leafs corresponds
to exactly one or no view cell.  Some statistics about using this
method on the Vienna view cells set are given in
table~\ref{tab:viewcell_bsp}.

However, sometimes a good set of view
cells is not available. Or the scene is changed frequently, and the
designer does not want to create new view cells on each change.  In
such a case one of the following two methods should rather be chosen,
which generate view cells automatically.


\item We apply a BSP tree subdivision to the scene geometry. Whenever
the subdivision terminates in a leaf, a view cell is associated with
the leaf node.  This simple approach is justified because it places
the view cell borders  along some discontinuities in the visibility
function.

\begin{figure}[htb]
  \centerline{
    \includegraphics[height=0.35\textwidth,draft=\DRAFTFIGS]{figs/viewcell_part}
      }
  \caption{A good view cell partition with respect to the sample rays piercing the scene objects
  and the view cell minimizes the number of rays
  piercing more than one view cell. During subdivision, this can be achieved by aligning
  the split plane with one of the long sides of occluder $O$. }
  \label{fig:viewcell_part}
\end{figure}

\item  The view cell generation can be guided by the sampling
process. We start with with a single initial view cell representing
the whole space. If a given threshold is reached during the
preprocessing (e.g., the view cell is pierced by too many rays
resulting in a large PVS), the view cell is subdivided into smaller
cells using some criteria.

In order to evaluate the best split plane, we first have to define the
characteristics of a good view cell partition:  The view cells should
be quite large, while their PVS stays rather small.  The PVS of each
two view cells should be as distinct as possible, otherwise they could
be merged into a larger view cell if the PVSs are too similar.  E.g.,
for a building, the perfect view cells are usually the single rooms
connected by portals.

Hence we can define some useful criteria for the split: 1) the number
of rays should be roughly equal among the new view cells. 2) The split
plane should be chosen in a way that the ray sets are disjoint, i.e.,
the number of rays contributing to more than one cell should be
minimized. 3) For BSP trees, the split plane should be aligned with
some scene geometry which is large enough to contribute a lot of
occlusion power. This criterion can be naturally combined with the
second one.  As termination criterion we can choose the minimum PVS /
piercing ray size or the maximal tree depth. An illustration of
a good and a bad choice of a split plane is given in figure~\ref{fig:viewcell_part}.
\end{itemize}


% In the future we aim to extend the view cell construction by using
% feedback from the PVS computation: the view cells which contain many
% visibility changes will be subdivided further and neighboring view
% cells with similar PVSs will be merged. We want to gain a more precise
% information about visibility by selectively storing rays with the
% view cells and computing visibility statistics for subsets of rays
% which intersect subregions of the given view cell.

\subsection{From-Object Based Visibility}

 Our framework is based on the idea of sampling visibility by casting
casting rays through the scene and collecting their contributions. A
visibility sample is computed by casting a ray from an object towards
the view cells and computing the nearest intersection with the scene
objects. All view cells pierced by the ray segment can the object and
thus the object can be added to their PVS. If the ray is terminated at
another scene object the PVS of the pierced view cells can also be
extended by this terminating object. Thus a single ray can make a
number of contributions to the progressively computed PVSs. A ray
sample piercing $n$ view cells which is bound by two distinct objects
contributes by at most $2*n$ entries to the current PVSs. Apart from
this performance benefit there is also a benefit in terms of the
sampling density: Assuming that the view cells are usually much larger
than the objects (which is typically the case) starting the sampling
deterministically from the objects increases the probability of small
objects being captured in the PVS.

At this phase of the computation we not only start the samples from
the objects, but we also store the PVS information centered at the
objects. Instead of storing a PVS consisting of objects visible from
view cells, every object maintains a PVS consisting of potentially
visible view cells. While these representations contain exactly the
same information as we shall see later the object centered PVS is
better suited for the importance sampling phase as well as the
visibility verification phase.


\subsection{Naive Randomized Sampling}

The naive global visibility sampling works as follows: At every pass
of the algorithm visits scene objects sequentially. For every scene
object we randomly choose a point on its surface. Then a ray is cast
from the selected point according to the randomly chosen direction
(see Figure~\ref{fig:sampling}). We use a uniform distribution of the
ray directions with respect to the half space given by the surface
normal. Using this strategy the samples at deterministically placed at
every object, with a randomization of the location on the object
surface. The uniformly distributed direction is a simple and fast
strategy to gain initial visibility information.

\begin{figure}%[htb]
  \centerline{
    \includegraphics[width=0.4\textwidth, draft=\DRAFTFIGS]{figs/sampling}
  }
  
%\label{tab:online_culling_example}
  \caption{Three objects and a set of view cells corresponding to leaves
    of an axis aligned BSP tree. The figure depicts several random
    samples cast from a selected object (shown in red). Note that most
    samples contribute to more view cells.  }
  \label{fig:sampling}
\end{figure}

The described algorithm accounts for the irregular distribution of the
objects: more samples are placed at locations containing more
objects. Additionally every object is sampled many times depending on
the number of passes in which this sampling strategy is applied. This
increases the chance of even a small object being captured in the PVS
of the view cells from which it is visible.

Each ray sample can contribute by a associating a number of view cells
with the object from which the sample was cast. If the ray does not
leave the scene it also contributes by associating the pierced view
cells to the terminating object. Thus as the ray samples are cast we
can measure the average contribution of a certain number of most
recent samples.  If this contribution falls below a predefined
constant we move on to the next sampling strategy, which aim to
discover more complicated visibility relations.
 

\subsection{Accounting for View Cell Distribution}

 The first modification to the basic algorithm accounts for irregular
distribution of the view cells. Such a case is common for example in
urban scenes where the view cells are mostly distributed in a
horizontal direction and more view cells are placed at denser parts of
the city. The modification involves replacing the uniformly
distributed ray direction by directions distributed according to the
local view cell directional density. This means placing more samples at
directions where more view cells are located: We select a random
view cell which lies at the half space given by the surface normal at the
chosen point. We pick a random point inside the view cell and cast a
ray towards this point.


\subsection{Accounting for Visibility Events}

 Visibility events correspond to appearance and disappearance of
objects with respect to a moving view point. In polygonal scenes the
events defined by event surfaces defined by three distinct scene
edges. Depending on the edge configuration we distinguish between
vertex-edge events (VE) and triple edge (EEE) events. The VE surfaces
are planar planes whereas the EEE are in general quadratic surfaces.

 To account for these events we explicitly place samples passing by
the object edges which are directed to edges and/or vertices of other
objects. In this way we perform stochastic sampling at boundaries of
the visibility complex~\cite{Durand:1996:VCN}.

The first strategy starts similarly to the above described sampling
methods: we randomly select an object and a point on its surface. Then
we randomly pickup an object from its PVS. If we have mesh
connectivity information we select a random silhouette edge from this
object and cast a sample which is tangent to that object at the
selected edge.

The second strategy works as follows: we randomly pickup two objects
which are likely to see each other. Then we determine a ray which is
tangent to both objects. For simple meshes the determination of such
rays can be computed geometrically, for more complicated ones it is
based again on random sampling. The selection of the two objects works
as follows: first we randomly select the first object and a random
non-empty view cell for which we know that it can see the object. Then
we randomly select an object associated with that view cell as the
second object.


 
\section{Summary}

This chapter described the global visibility sampling algorithm which
forms a core of the visibility preprocessing framework. The global
visibility sampling computes aggressive visibility, i.e. it computes a
subset of the exact PVS for each view cell. The aggressive sampling
provides a fast progressive solution and thus it can be easily
integrated into the game development cycle. The sampling itself
involves several strategies which aim to progressively discover more
visibility relationships in the scene.

The methods presented in this chapter give a good initial estimate
about visibility in the scene, which can be verified by the mutual
visibility algorithms described in the next chapter.