BSP Tree Frequently Asked Questions (FAQ)
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This document is still under construction
This document should not be considered to be in final form by any
stretch of the imagination. All comments are welcome. I expect to go
through several iterations of writing and improving, so don't give up
on me for a few months yet.
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Questions
1. About this document
2. Acknowledgements
3. How can you contribute?
4. About the pseudo C++ code
5. What is a BSP Tree?
6. How do you build a BSP Tree?
7. How do you partition a polygon with a plane?
8. How do you remove hidden surfaces with a BSP Tree?
9. How do you remove hidden lines with a BSP Tree?
10. How do you accelerate ray tracing with a BSP Tree?
11. How do you perform boolean operations on polytopes with a BSP Tree?
12. How do you perform collision detection with a BSP Tree?
13. How do you handle dynamic scenes with a BSP Tree?
14. How do you compute shadows with a BSP Tree?
15. How do you extract connectivity information from BSP Trees?
16. How are BSP Trees useful for robot motion planning?
17. How are BSP Trees used in DOOM?
18. How can you make a BSP Tree more robust?
19. How efficient is a BSP Tree?
20. How can you make a BSP Tree more efficient?
21. How can you avoid recursion?
22. What is the history of BSP Trees?
23. Where can you find sample code and related resources?
24. References
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Answers
1. About this document
The purpose of this document is to provide answers to Frequently Asked
Questions about Binary Space Partitioning (BSP) Trees. This document
will be posted monthly to comp.graphics.algorithms. It is also
available via WWW at the URL:
http://www.graphics.cornell.edu/bspfaq/
You can also request that the FAQ be mailed to you in plain text and
HTML formats by sending e-mail to bsp-faq@graphics.cornell.edu with a
subject line of "SEND BSP TREE [what]". The "[what]" should be
replaced with any combination of "TEXT" and "HTML". Respectively,
these will return to you a plain text version of the FAQ, and an HTML
formatted version of the FAQ viewable with Mosaic or Netscape.
This document is maintained by Bretton Wade, a graduate student at the
Cornell University Program of Computer Graphics.
This document, and all its associated parts, are Copyright © 1995,
Bretton Wade. All rights reserved. Permisson to distribute this
collection, in part or full, via electronic means (emailed, posted or
archived) or printed copy are granted providing that no charges are
involved, reasonable attempt is made to use the most current version,
and all credits and copyright notices are retained. Requests for other
distribution rights, including incorporation in commercial products,
such as books, magazine articles, CD-ROMs, and binary applications
should be made to bsp-faq@graphics.cornell.edu.
This article is provided as is without any express or implied
warranties. While every effort has been taken to ensure the accuracy
of the information contained in this article, the
author/maintainer/contributors assume(s) no responsibility for errors
or omissions, or for damages resulting from the use of the information
contained herein.
The contents of this article do not necessarily represent the opinions
of Cornell University or the Program of Computer Graphics.
--
Last Update: 04/21/95 15:50:23
2. Acknowledgements
This document would not have been possible without the selfless
contributions and efforts of many individuals. I would like to take
the opportunity to thank each one of them. Please be aware that these
people may not be amenable to recieving e-mail on a random basis. If
you have any special questions, please contact Bretton Wade
(bwade@graphics.cornell.edu or bsp-faq@graphics.cornell.edu) before
trying to contact anyone else on this list. In no particular order:
* Bruce Naylor (naylor@research.att.com)
* Richard Lobb (richard@cs.auckland.ac.nz)
* Dani Lischinski (danix@cs.washington.edu)
* Chris Schoeneman (crs@lightscape.com)
* Philip Hubbard (pmh@graphics.cornell.edu)
* Jim Arvo (arvo@graphics.cornell.edu)
* Kevin Ryan (kryan@access.digex.net)
* Joseph Fiore (fiore@cs.buffalo.edu)
* Lukas Rosenthaler (rosenth@foto.chemie.unibas.ch)
* Anson Tsao (ansont@hookup.net)
* Robert Zawarski (zawarski@chaph.usc.edu)
* Ron Capelli (capelli@vnet.ibm.com)
* Eric A. Haines (erich@eye.com)
* Ian CR Mapleson (mapleson@cee.hw.ac.uk)
* Richard Dorman (richard@cs.wits.ac.za)
* Steve Larsen (larsen@sunset.cs.utah.edu)
* Timothy Miller (tsm@cs.brown.edu)
* Ben Trumbore (wbt@graphics.cornell.edu)
* Richard Matthias (richardm@cogs.susx.ac.uk)
* Ken Shoemake (shoemake@graphics.cis.upenn.edu)
* Seth Teller (seth@theory.lcs.mit.edu)
* Peter Shirley (shirley@graphics.cornell.edu)
If I have neglected to mention your name, and you contributed, please
let me know immediately!
--
Last Update: 03/29/95 14:12:10
3. How can you contribute?
Please send all new questions, corrections, suggestions, and
contributions to bsp-faq@graphics.cornell.edu.
--
Last Update: 03/29/95 14:12:10
4. About the pseudo C++ code
Overview
The general efficiency of C++ makes it a well suited language for
programming computer graphics. Furthermore, the abstract nature of the
language allows it to be used effectively as a psuedo code for
demonstrative purposes. I will use C++ notation for all the examples
in this document.
In order to provide effective examples, it is necessary to assume that
certain classes already exist, and can be used without presenting
excessive details of their operation. Basic classes such as lists and
arrays fall into this category.
Other classes which will be very useful for examples need to be
presented here, but the definitions will be generic to allow for
freedom of interpretation. I assume points and vectors to each be an
array of 3 real numbers (X, Y, Z).
Planes are represented as an array of 4 real numbers (A, B, C, D). The
vector (A, B, C) is the normal vector to the plane. Polygons are
structures composited from an array of points, which are the vertices,
and a plane.
The overloaded operator for a dot product (inner product, scalar
product, etc.) of two vectors is the '|' symbol. This has two
advantages, the first of which is that it can't be confused with the
scalar multiplication operator. The second is that precedence of C++
operators will usually require that dot product operations be
parenthesized, which is consistent with the linear algebra notation
for an inner product.
The code for BSP trees presented here is intended to be educational,
and may or may not be very efficient. For the sake of clarity, the BSP
tree itself will not be defined as a class.
--
Last Update: 04/30/95 15:45:19
5. What is a BSP Tree?
Overview
A Binary Space Partitioning (BSP) tree represents a recursive,
hierarchical partitioning, or subdivision, of n-dimensional space. BSP
tree construction is a process which takes a subspace and partitions
it by any hyperplane that intersects the interior of that subspace.
The result is two new subspaces that can be further partitioned by
recursive application of the method.
A "hyperplane" in n-dimensional space is an n-1 dimensional object
which can be used to divide the space into two half-spaces. For
example, in three dimensional space, the "hyperplane" is a plane. In
two dimensional space, a line is used.
BSP trees are extremely versatile, because they are powerful sorting
and classification structures. They have uses ranging from hidden
surface removal and ray tracing hierarchies to solid modeling and
robot motion planning.
Example
An easy way to think about BSP trees is to limit the discussion to two
dimensions. To simplify the situation, let's say that we will use only
lines parallel to the X or Y axis, and that we will divide the space
equally at each node. For example, given a square somewhere in the XY
plane, we select the first split, and thus the root of the BSP Tree,
to cut the square in half in the X direction. At each slice, we will
choose a line of the opposite orientation from the last one, so the
second slice will divide each of the new pieces in the Y direction.
This process will continue recursively until we reach a stopping
point, and looks like this:
+-----------+ +-----+-----+ +-----+-----+
| | | | | | | |
| | | | | | d | |
| | | | | | | |
| a | -> | b X c | -> +--Y--+ f | -> ...
| | | | | | | |
| | | | | | e | |
| | | | | | | |
+-----------+ +-----+-----+ +-----+-----+
The resulting BSP tree looks like this at each step:
a X X ...
-/ \+ -/ \+
/ \ / \
b c Y f
-/ \+
/ \
e d
Other space partitioning structures
BSP trees are closely related to Quadtrees and Octrees. Quadtrees and
Octrees are space partitioning trees which recursively divide
subspaces into four and eight new subspaces, respectively. A BSP Tree
can be used to simulate both of these structures.
--
Last Update: 04/30/95 15:45:19
6. How do you build a BSP Tree?
Overview
Given a set of polygons in three dimensional space, we want to build a
BSP tree which contains all of the polygons. For now, we will ignore
the question of how the resulting tree is going to be used.
The algorithm to build a BSP tree is very simple:
1. Select a partition plane.
2. Partition the set of polygons with the plane.
3. Recurse with each of the two new sets.
Choosing the partition plane
The choice of partition plane depends on how the tree will be used,
and what sort of efficiency criteria you have for the construction.
For some purposes, it is appropriate to choose the partition plane
from the input set of polygons. Other applications may benefit more
from axis aligned orthogonal partitions.
In any case, you want to evaluate how your choice will affect the
results. It is desirable to have a balanced tree, where each leaf
contains roughly the same number of polygons. However, there is some
cost in achieving this. If a polygon happens to span the partition
plane, it will be split into two or more pieces. A poor choice of the
partition plane can result in many such splits, and a marked increase
in the number of polygons. Usually there will be some trade off
between a well balanced tree and a large number of splits.
Partitioning polygons
Partitioning a set of polygons with a plane is done by classifying
each member of the set with respect to the plane. If a polygon lies
entirely to one side or the other of the plane, then it is not
modified, and is added to the partition set for the side that it is
on. If a polygon spans the plane, it is split into two or more pieces
and the resulting parts are added to the sets associated with either
side as appropriate.
When to stop
The decision to terminate tree construction is, again, a matter of the
specific application. Some methods terminate when the number of
polygons in a leaf node is below a maximum value. Other methods
continue until every polygon is placed in an internal node. Another
criteria is a maximum tree depth.
Pseudo C++ code example
Here is an example of how you might code a BSP tree:
struct BSP_tree
{
plane partition;
list polygons;
BSP_tree *front,
*back;
};
This structure definition will be used for all subsequent example
code. It stores pointers to its children, the partitioning plane for
the node, and a list of polygons coincident with the partition plane.
For this example, there will always be at least one polygon in the
coincident list: the polygon used to determine the partition plane. A
constructor method for this structure should initialize the child
pointers to NULL.
void Build_BSP_Tree (BSP_tree *tree, list polygons)
{
polygon *root = polygons.Get_From_List ();
tree->partition = root->Get_Plane ();
tree->polygons.Add_To_List (root);
list front_list,
back_list;
polygon *poly;
while ((poly = polygons.Get_From_List ()) != 0)
{
int result = tree->partition.Classify_Polygon (poly);
switch (result)
{
case COINCIDENT:
tree->polygons.Add_To_List (poly);
break;
case IN_BACK_OF:
backlist.Add_To_List (poly);
break;
case IN_FRONT_OF:
frontlist.Add_To_List (poly);
break;
case SPANNING:
polygon *front_piece, *back_piece;
Split_Polygon (poly, tree->partition, front_piece, back_piece);
backlist.Add_To_List (back_piece);
frontlist.Add_To_List (front_piece);
break;
}
}
if ( ! front_list.Is_Empty_List ())
{
tree->front = new BSP_tree;
Build_BSP_Tree (tree->front, front_list);
}
if ( ! back_list.Is_Empty_List ())
{
tree->back = new BSP_tree;
Build_BSP_Tree (tree->back, back_list);
}
}
This routine recursively constructs a BSP tree using the above
definition. It takes the first polygon from the input list and uses it
to partition the remainder of the set. The routine then calls itself
recursively with each of the two partitions. This implementation
assumes that all of the input polygons are convex.
One obvious improvement to this example is to choose the partitioning
plane more intelligently. This issue is addressed separately in the
section, "How can you make a BSP Tree more efficient?".
--
Last Update: 05/08/95 13:10:25
7. How do you partition a polygon with a plane?
Overview
Partitioning a polygon with a plane is a matter of determining which
side of the plane the polygon is on. This is referred to as a
front/back test, and is performed by testing each point in the polygon
against the plane. If all of the points lie to one side of the plane,
then the entire polygon is on that side and does not need to be split.
If some points lie on both sides of the plane, then the polygon is
split into two or more pieces.
The basic algorithm is to loop across all the edges of the polygon and
find those for which one vertex is on each side of the partition
plane. The intersection points of these edges and the plane are
computed, and those points are used as new vertices for the resulting
pieces.
Implementation notes
Classifying a point with respect to a plane is done by passing the (x,
y, z) values of the point into the plane equation, Ax + By + Cz + D =
0. The result of this operation is the distance from the plane to the
point along the plane's normal vector. It will be positive if the
point is on the side of the plane pointed to by the normal vector,
negative otherwise. If the result is 0, the point is on the plane.
For those not familiar with the plane equation, The values A, B, and C
are the coordinate values of the normal vector. D can be calculated by
substituting a point known to be on the plane for x, y, and z.
Convex polygons are generally easier to deal with in BSP tree
construction than concave ones, because splitting them with a plane
always results in exactly two convex pieces. Furthermore, the
algorithm for splitting convex polygons is straightforward and robust.
Splitting of concave polygons, especially self intersecting ones, is a
significant problem in its own right.
Pseudo C++ code example
Here is a very basic function to split a convex polygon with a plane:
void Split_Polygon (polygon *poly, plane *part, polygon *&front, polygon *&back)
{
int count = poly->NumVertices (),
out_c = 0, in_c = 0;
point ptA, ptB,
outpts[MAXPTS],
inpts[MAXPTS];
real sideA, sideB;
ptA = poly->Vertex (count - 1);
sideA = part->Classify_Point (ptA);
for (short i = -1; ++i < count;)
{
ptB = poly->Vertex (i);
sideB = part->Classify_Point (ptB);
if (sideB > 0)
{
if (sideA < 0)
{
// compute the intersection point of the line
// from point A to point B with the partition
// plane. This is a simple ray-plane intersection.
vector v = ptB - ptA;
real sect = part->Classify_Point (-ptA) / (part->Normal () | v);
outpts[out_c++] = inpts[in_c++] = ptA + (v * sect);
}
outpts[out_c++] = ptB;
}
else if (sideB < 0)
{
if (sideA > 0)
{
// compute the intersection point of the line
// from point A to point B with the partition
// plane. This is a simple ray-plane intersection.
vector v = ptB - ptA;
real sect = part->Classify_Point (-ptA) / (part->Normal () | v);
outpts[out_c++] = inpts[in_c++] = ptA + (v * sect);
}
inpts[in_c++] = ptB;
}
else
outpts[out_c++] = inpts[in_c++] = ptB;
ptA = ptB;
sideA = sideB;
}
front = new polygon (outpts, out_c);
back = new polygon (inpts, in_c);
}
A simple extension to this code that is good for BSP trees is to
combine its functionality with the routine to classify a polygon with
respect to a plane.
Note that this code is not robust, since numerical stability may cause
errors in the classification of a point. The standard solution is to
make the plane "thick" by use of an epsilon value.
--
Last Update: 05/08/95 13:10:25
8. How do you remove hidden surfaces with a BSP Tree?
Overview
Probably the most common application of BSP trees is hidden surface
removal in three dimensions. BSP trees provide an elegant, efficient
method for sorting polygons via a depth first tree walk. This fact can
be exploited in a back to front "painter's algorithm" approach to the
visible surface problem, or a front to back scanline approach.
BSP trees are well suited to interactive display of static (not
moving) geometry because the tree can be constructed as a preprocess.
Then the display from any arbitrary viewpoint can be done in linear
time. Adding dynamic (moving) objects to the scene is discussed in
another section of this document.
Painter's algorithm
The idea behind the painter's algorithm is to draw polygons far away
from the eye first, followed by drawing those that are close to the
eye. Hidden surfaces will be written over in the image as the surfaces
that obscure them are drawn. One condition for a successful painter's
algorithm is that there be a single plane which separates any two
objects. This means that it might be necessary to split polygons in
certain configurations. For example, this case can not be drawn
correctly with a painter's algorithm:
+------+
| |
+---------------| |--+
| | | |
| | | |
| | | |
| +--------| |--+
| | | |
+--| |--------+ |
| | | |
| | | |
| | | |
+--| |---------------+
| |
+------+
One reason that BSP trees are so elegant for the painter's algorithm
is that the splitting of difficult polygons is an automatic part of
tree construction. Note that only one of these two polygons needs to
be split in order to resolve the problem.
To draw the contents of the tree, perform a back to front tree
traversal. Begin at the root node and classify the eye point with
respect to its partition plane. Draw the subtree at the far child from
the eye, then draw the polygons in this node, then draw the near
subtree. Repeat this procedure recursively for each subtree.
Scanline hidden surface removal
It is just as easy to traverse the BSP tree in front to back order as
it is for back to front. We can use this to our advantage in a
scanline method method by using a write mask which will prevent pixels
from being written more than once. This will represent significant
speedups if a complex lighting model is evaluated for each pixel,
because the painter's algorithm will blindly evaluate the same pixel
many times.
The trick to making a scanline approach successful is to have an
efficient method for masking pixels. One way to do this is to maintain
a list of pixel spans which have not yet been written to for each scan
line. For each polygon scan converted, only pixels in the available
spans are written, and the spans are updated accordingly.
The scan line spans can be represented as binary trees, which are just
one dimensional BSP trees. This technique can be expanded to a two
dimensional screen coverage algorithm using a two dimensional BSP tree
to represent the masked regions. Any convex partitioning scheme, such
as a quadtree, can be used with similar effect.
Implementation notes
When building a BSP tree specifically for hidden surface removal, the
partition planes are usually chosen from the input polygon set.
However, any arbitrary plane can be used if there are no intersecting
or concave polygons, as in the example above.
Pseudo C++ code example
Using the BSP_tree structure defined in the section, "How do you build
a BSP Tree?", here is a simple example of a back to front tree
traversal:
void Draw_BSP_Tree (BSP_tree *tree, point eye)
{
real result = tree->partition.Classify_Point (eye);
if (result > 0)
{
Draw_BSP_Tree (tree->back, eye);
tree->polygons.Draw_Polygon_List ();
Draw_BSP_Tree (tree->front, eye);
}
else if (result < 0)
{
Draw_BSP_Tree (tree->front, eye);
tree->polygons.Draw_Polygon_List ();
Draw_BSP_Tree (tree->back, eye);
}
else // result is 0
{
// the eye point is on the partition plane...
Draw_BSP_Tree (tree->front, eye);
Draw_BSP_Tree (tree->back, eye);
}
}
If the eye point is classified as being on the partition plane, the
drawing order is unclear. This is not a problem if the
Draw_Polygon_List routine is smart enough to not draw polygons that
are not within the viewing frustum. The coincident polygon list does
not need to be drawn in this case, because those polygons will not be
visible to the user.
It is possible to substantially improve the quality of this example by
including the viewing direction vector in the computation. You can
determine that entire subtrees are behind the viewer by comparing the
view vector to the partition plane normal vector. This test can also
make a better decision about tree drawing when the eye point lies on
the partition plane. It is worth noting that this improvement
resembles the method for tracing a ray through a BSP tree, which is
discussed in another section of this document.
Front to back tree traversal is accomplished in exactly the same
manner, except that the recursive calls to Draw_BSP_Tree occur in
reverse order.
--
Last Update: 05/08/95 13:10:25
9. How do you remove hidden lines with a BSP Tree?
Overview
--
Last Update: 04/30/95 15:45:19
10. How do you accelerate ray tracing with a BSP Tree?
Overview
Ray tracing a BSP tree is very similar to hidden surface removal with
a BSP tree. The algorithm is a simple forward tree walk, with a few
additions that apply to ray casting.
MORE TO COME
--
Last Update: 04/30/95 15:45:19
11. How do you perform boolean operations on polytopes with a BSP Tree?
Overview
There are two major classes of solid modeling methods with BSP trees.
For both methods, it is useful to introduce the notion of an in/out
test.
An in/out test is a different way of talking about the front/back test
we have been using to classify points with respect to planes. The
necessity for this shift in thought is evident when considering
polytopes instead of just polygons. A point can not be merely in front
or back of a polytope, but inside or outside. Somewhat formally, a
point is inside of a polytope if it is inside of, or in back of, each
hyperplane which composes the polytope, otherwise it is outside.
Incremental construction
Incremental construction of a BSP Tree is the process of inserting
convex polytopes into the tree one by one. Each polytope has to be
processed according to the operation desired.
It is useful to examine the construction process in two dimensions.
Consider the following figure:
A B
+-------------+
| |
| |
| E | F
| +-----+-------+
| | | |
| | | |
| | | |
+-------+-----+ |
D | C |
| |
| |
+-------------+
H G
Two polygons, ABCD, and EFGH, are to be inserted into the tree. We
wish to find the union of these two polygons. Start by inserting
polygon ABCD into the tree, choosing the splitting hyperplanes to be
coincident with the edges. The tree looks like this after insertion of
ABCD:
AB
-/ \+
/ \
/ *
BC
-/ \+
/ \
/ *
CD
-/ \+
/ \
/ *
DA
-/ \+
/ \
* *
Now, polygon EFGH is inserted into the tree, one polygon at a time.
The result looks like this:
A B
+-------------+
| |
| |
| E |J F
| +-----+-------+
| | | |
| | | |
| | | |
+-------+-----+ |
D |L :C |
| : |
| : |
+-----+-------+
H K G
AB
-/ \+
/ \
/ *
BC
-/ \+
/ \
/ \
CD \
-/ \+ \
/ \ \
/ \ \
DA \ \
-/ \+ \ \
/ \ \ \
/ * \ \
EJ KH \
-/ \+ -/ \+ \
/ \ / \ \
/ * / * \
LE HL JF
-/ \+ -/ \+ -/ \+
/ \ / \ / \
* * * * FG *
-/ \+
/ \
/ *
GK
-/ \+
/ \
* *
Notice that when we insert EFGH, we split edges EF and HE along the
edges of ABCD. this has the effect of dividing these segments into
pieces which are inside ABCD, and outside ABCD. Segments EJ and LE
will not be part of the boundary of the union. We could have saved our
selves some work by not inserting them into the tree at all. For a
union operation, you can always throw away segments that land in
inside nodes. You must be careful about this though. What I mean is
that any segments which land in inside nodes of side the pre-existing
tree, not the tree as it is being constructed. EJ and LE landed in an
inside node of the tree for polygon ABCD, and so can be discarded.
Our tree now looks like this:
A B
+-------------+
| |
| |
| |J F
| +-------+
| | |
| | |
| | |
+-------+-----+ |
D |L :C |
| : |
| : |
+-----+-------+
H K G
AB
-/ \+
/ \
/ *
BC
-/ \+
/ \
/ \
CD \
-/ \+ \
/ \ \
/ \ \
DA \ \
-/ \+ \ \
/ \ \ \
* * \ \
KH \
-/ \+ \
/ \ \
/ * \
HL JF
-/ \+ -/ \+
/ \ / \
* * FG *
-/ \+
/ \
/ *
GK
-/ \+
/ \
* *
Now, we would like some way to eliminate the segments JC and CL, so
that we will be left with the boundary segments of the union. Examine
the segment BC in the tree. What we would like to do is split BC with
the hyperplane JF. Conveniently, we can do this by pushing the BC
segment through the node for JF. The resulting segments can be
classified with the rest of the JF subtree. Notice that the segment BJ
lands in an out node, and that JC lands in an in node. Remembering
that we can discard interior nodes, we can eliminate JC. The segment
BJ replaces BC in the original tree. This process is repeated for
segment CD, yielding the segments CL and LD. CL is discarded as
landing in an interior node, and LD replaces CD in the original tree.
The result looks like this:
A B
+-------------+
| |
| |
| |J F
| +-------+
| |
| |
| L |
+-------+ |
D | |
| |
| |
+-----+-------+
H K G
AB
-/ \+
/ \
/ *
BJ
-/ \+
/ \
/ \
LD \
-/ \+ \
/ \ \
/ \ \
DA \ \
-/ \+ \ \
/ \ \ \
* * \ \
KH \
-/ \+ \
/ \ \
/ * \
HL JF
-/ \+ -/ \+
/ \ / \
* * FG *
-/ \+
/ \
/ *
GK
-/ \+
/ \
* *
As you can see, the result is the union of the polygons ABCD and EFGH.
To perform other boolean operations, the process is similar. For
intersection, you discard segments which land in exterior nodes
instead of internal ones. The difference operation is special. It
requires that you invert the polytope before insertion. For simple
objects, this can be achieved by scaling with a factor of -1. The
insertion process is then cinducted as an intersection operation,
where segments landing in external nodes are discarded.
Tree merging
--
Last Update: 04/30/95 15:45:20
12. How do you perform collision detection with a BSP Tree?
Overview
Detecting whether or not a point moving along a line intersects some
object in space is essentially a ray tracing problem. Detecting
whether or not two complex objects intersect is something of a tree
merging problem.
Typically, motion is computed in a series of Euler steps. This just
means that the motion is computed at discrete time intervals using
some description of the speed of motion. For any given point P moving
from point A with a velocity V, it's location can be computed at time
T as P = A + (T * V).
Consider the case where T = 1, and we are computing the motion in one
second steps. To find out if the point P has collided with any part of
the scene, we will first compute the endpoints of the motion for this
time step. P1 = A + V, and P2 = A + (2 * V). These two endpoints will
be classified with respect to the BSP tree. If P1 is outside of all
objects, and P2 is inside some object, then an intersection has
clearly occurred. However, if P2 is also outside, we still have to
check for a collision in between.
Two approaches are possible. The first is commonly used in
applications like games, where speed is critical, and accuracy is not.
This approach is to recursively divide the motion segment in half, and
check the midpoint for containment by some object. Typically, it is
good enough to say that an intersection occurred, and not be very
accurate about where it occurred.
The second approach, which is more accurate, but also more time
consuming, is to treat the motion segment as a ray, and intersect the
ray with the BSP Tree. This also has the advantage that the motion
resulting from the impact can be computed more accurately.
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13. How do you handle dynamic scenes with a BSP Tree?
Overview
So far the discussion of BSP tree structures has been limited to
handling objects that don't move. However, because the hidden surface
removal algorithm is so simple and efficient, it would be nice if it
could be used with dynamic scenes too. Faster animation is the goal
for many applications, most especially games.
The BSP tree hidden surface removal algorithm can easily be extended
to allow for dynamic objects. For each frame, start with a BSP tree
containing all the static objects in the scene, and reinsert the
dynamic objects. While this is straightforward to implement, it can
involve substantial computation.
If a dynamic object is separated from each static object by a plane,
the dynamic object can be represented as a single point regardless of
its complexity. This can dramatically reduce the computation per frame
because only one node per dynamic object is inserted into the BSP
tree. Compare that to one node for every polygon in the object, and
the reason for the savings is obvious. During tree traversal, each
point is expanded into the original object.
Implementation notes
Inserting a point into the BSP tree is very cheap, because there is
only one front/back test at each node. Points are never split, which
explains the requirement of separation by a plane. The dynamic object
will always be drawn completely in front of the static objects behind
it.
A dynamic object inserted into the tree as a point can become a child
of either a static or dynamic node. If the parent is a static node,
perform a front/back test and insert the new node appropriately. If it
is a dynamic node, a different front/back test is necessary, because a
point doesn't partition three dimesnional space. The correct
front/back test is to simply compare distances to the eye. Once
computed, this distance can be cached at the node until the frame is
drawn.
An alternative when inserting a dynamic node is to construct a plane
whose normal is the vector from the point to the eye. This plane is
used in front/back tests just like the partition plane in a static
node. The plane should be computed lazily and it is not necessary to
normalize the vector.
Cleanup at the end of each frame is easy. A static node can never be a
child of a dynamic node, since all dynamic nodes are inserted after
the static tree is completed. This implies that all subtrees of
dynamic nodes can be removed at the same time as the dynamic parent
node.
Advanced methods
Tree merging, "ghosts", real dynamic trees... MORE TO COME
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14. How do you compute shadows with a BSP Tree?
Overview
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15. How do you extract connectivity information from BSP Trees?
Overview
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16. How are BSP Trees useful for robot motion planning?
Overview
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17. How are BSP Trees used in DOOM?
Overview
Before you can understand how DOOM uses a BSP tree to accelerate its
rendering process, you have to understand how the world is represented
in DOOM. When someone creates a DOOM level in a level editor they draw
linedefs in a 2d space. Yes, that's right, DOOM is only 2d. These
linedefs (ignoring the special effects linedefs) must be arranged so
that they form closed polygons. One linedef may be used to form the
outline of two polygons (in which case it is known as a two-sided
linedef) and one polygon may be contained within another, but no
linedefs may cross. Each enclosed area of the world (i.e. polygon) is
assigned a floor height, ceiling height, floor and ceiling textures, a
lower texture and an upper texture. The lower texture is visible when
a linedef is viewed from a direction where the floor is lower in the
adjoining area. An equivalent thing is true for the upper texture. A
set of these enclosed areas that all have the same attributes is known
as a sector.
When the level is saved by the editor some new information is created
including the BSP tree for that level. Before the BSP tree can be
created, all the sectors have to be split into convex polygons known
as sub-sectors. If you had a sector that was a square area, then that
would translate exactly into a sub-sector. Whereas if that sector was
contained inside another larger square sector, the larger one would
have to be split into four, four sided sub-sectors to make all the
sub-sectors convex. When more complex sectors are split into
sub-sectors the linedefs that bound that sector may need to be broken
into smaller lengths. These linedef sections are called segs.
Given a point on the 2d map, the renderer (which isn't discussed here)
wants a list of all the segs that are visible from that viewpoint in
closest first order. Because of the restrictions placed on the DOOM
world, the renderer can easily tell when the screen has been filled so
it can stop looking for segs at this time. This is quicker than
rendering all the segs from back to front and using a method like
painters algorithm.
Each node in the BSP tree defines a partition line (this does not have
be a linedef in the world but usually is) which is the equivalent to
the partition plane of a 3d BSP tree. It then has left and right
pointers which are either another node for further sub-division or a
leaf, the leaf being a sub-sector in DOOM. The BSP tree in DOOM is
effectively being used to sort whole sub-sectors rather than
individual lines front to back. Each node also defines an orthogonal
bounding box for each side of the partition. All segs on a particular
side of the partition must be within that box. This speeds up the
searching process by allowing whole branches of the tree to be
discarded if that bounding box isn't visible. The test for visibility
is simply if the bounding box lies wholly or partly within the cone
defined by the left and right edges of the screen.
During the display update process the BSP tree is searched starting
from the node containing the sub-sector that the player is currently
in. The search moves outwards through the tree (searching the other
half of the current node before moving onto the other half of the
parents node). When a partition test is performed the branch chosen is
the one on the same side as the player. This facilitates the front to
back searching. Each time a leaf is encountered the segs in that
sub-sector are passed to the renderer. If the renderer has returned
that the screen is filled then the process stops, otherwise it
continues until the tree has been fully searched (in which case there
is an error in the level design).
In case you're thinking that it is inefficient to dump a whole
sub-sectors worth of segs into the renderer at once, the segs in a
sub-sector can be back-face culled very quickly. DOOM stores the angle
of linedefs (of which segs are part). When the angle of the players
view is calculated this allows segs to be culled in a single
instruction! Angles are stored as a 16 bit number where 0 is east an
65535 is 1/63336 south of east.
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18. How can you make a BSP Tree more robust?
Overview
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19. How efficient is a BSP Tree?
Space complexity
For hidden surface removal and ray tracing accelleration, the upper
bound is O(n ^ 2) for n polygons. The expected case is O(n) for most
models. MORE LATER
Time complexity
For hidden surface removal and ray tracing accelleration, the upper
bound is O(n ^ 2) for n polygons. The expected case is O(n) for most
models. MORE LATER
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20. How can you make a BSP Tree more efficient?
Bounding volumes
Bounding spheres are simple to implement, take only a single plane
comparison, using the center of the sphere.
Tree balancing
For hidden surface problem, doesn't affect runtime, assuming that no
splitting occurs...
Balancing vs. splitting
Reference Counting
Other Optimizations
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21. How can you avoid recursion?
standard binary tree search/sort techniques apply.
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22. What is the history of BSP Trees?
Overview
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23. Where can you find sample code and related resources?
The companion source code to this document is available via FTP at:
* file://ray.graphics.cornell.edu/pub/bsptree/
or, you can also request that the source be mailed to you by sending
e-mail to bsp-faq@graphics.cornell.edu with a subject line of "SEND
BSP TREE SOURCE". This will return to you a UU encoded copy of the
sample C++ source code.
Pat Fleckenstein and Rob Reay have put together a FAQ on 3D graphics,
which includes a blurb on BSP Trees, and an ftp site with some sample
code. They seem to have an unusual affinity for ftp sites, and
therefore won't link the BSP tree FAQ from their document:
* 3D FAQ
* file://ftp.csh.rit.edu/pub/3dfaq/
Implementing and Using BSP Trees
24. Accompanying C++ source
Other sources for sample BSP Tree code are:
* file://ftp.idsoftware.com/tonsmore/utils/level_edit/node_builders/
* file://ftp.princeton.edu/pub/Graphics/GraphicsGems/GemsIII/
A C code implementation of ray tracing with BSP trees.
* file://ftp.princeton.edu/pub/Graphics/GraphicsGems/GemsV/
Norman Chin has provided extensive C code for BSP trees. An excellent
resource.
* file://ftp.cs.brown.edu/pub/sphigs.tar.Z
If you are interested in game programming, check out the
rec.games.programmer FAQ.
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* References
* Dadoun, N., Kirkpatrick, D., and Walsh, J., The Geometry of Beam Tracing,
Proceedings of the ACM Symposium on Computational Geometry, 55--61, jun
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* Chin, N., and Feiner, S., Near Real-Time Shadow Generation Using BSP
Trees, Computer Graphics (SIGGRAPH '89 Proceedings), 23(3), 99--106, jul
1989.
* Chin, N., and Feiner, S., Fast object-precision shadow generation for
area light sources using BSP trees, Computer Graphics (1992 Symposium on
Interactive 3D Graphics), 25(2), 21--30, mar 1992.
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sep 1992.
* Naylor, B., Amanatides, J., and Thibault, W., Merging BSP Trees Yields
Polyhedral Set Operations, Computer Graphics (SIGGRAPH '90 Proceedings),
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* Chin, N., and Feiner, S., Fast object-precision shadow generation for
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1992.
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Computer Graphics and Applications, 11(5), 79--85, sep 1991.
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space curves with applications, Scientific Visualization of Physical
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Internat. J. Comput. Geom. Appl., 1(3), 243--261, 1991.
* Arvo, J., Linear Time Voxel Walking for Octrees, Ray Tracing News, feb
1988.
* Jansen, F., Data Structures for Ray Tracing, Data Structures for Raster
Graphics, 57--73, 1986.
* MacDonald, J., and Booth, K., Heuristics for Ray Tracing Using Space
Subdivision, Proceedings of Graphics Interface '89, 152--63, jun 1989.
* Naylor, B., and Thibault, W., Application of BSP Trees to Ray Tracing and
CSG Evaluation, Tech. Rep. GIT-ICS 86/03, feb 1986.
* Sung, K., and Shirley, P., Ray Tracing with the BSP Tree, Graphics Gems
III, 271--274, 1992.
* Fuchs, H., Kedem, Z., and Naylor, B., On Visible Surface Generation by A
Priori Tree Structures, Conf. Proc. of SIGGRAPH '80, 14(3), 124--133, jul
1980.
* Paterson, M., and Yao, F., Efficient Binary Space Partitions for
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Geometry, 5(5), 485--503, 1990.
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This document was last updated on Sunday, 26-Mar-95 19:22:54 EST
Bretton Wade (bwade@graphics.cornell.edu)