User's Guide

to the SOLID Interference Detection Library

last updated January 8, 1999

for version 2.0

Gino van den Bergen


Version 2, June 1991

Copyright © 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

[This is the first released version of the library GPL.  It is
 numbered 2 because it goes with version 2 of the ordinary GPL.]


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That's all there is to it!


SOLID is a library for collision detection of three-dimensional objects undergoing rigid motion and deformation. SOLID is designed to be used in interactive 3D graphics applications, and is especially suited for collision detection of objects and worlds described in VRML. Some of its features are:

Installing SOLID


The library is written in standard C++ and relies heavily on the STL. Currently, it compiles under GNU g++ version 2.8.1 and up. The library has a standard C API and can be linked to both C and C++ applications. In order to link it to C applications you need the libstdc++ library.


In case you have a recent GNU developers environment installed, simply typing `make' in the root of the distribution directory will build the library, a sample client and documentation.

If you want to use another compiler, you should make the necessary changes to the `Make-config' file in the root directory. See the comments in this file for details.

The auxiliary C++ classes in `include/3D' for performing affine transformation in 3D have all their code inlined, so you do not need to link a library in order to use them.

Trouble in Installation

Q: My compiler cannot figure out the templates used in the SOLID source! What's the matter?

A: Your compiler does not comply with the ISO standard for C++. Install the GNU g++ compiler version 2.8.1 or higher for your platform.


Building Shapes

The commands for creating and destroying shapes are

DtShapeRef dtBox(DtScalar x, DtScalar y, DtScalar z);
DtShapeRef dtCone(DtScalar radius, DtScalar height);
DtShapeRef dtCylinder(DtScalar radius, DtScalar height);
DtShapeRef dtSphere(DtScalar radius);
DtShapeRef dtNewComplexShape();

void dtDeleteShape(DtShapeRef shape);

Shapes are referred to by values of DtShapeRef. Currently, the type DtScalar is defined as double. The command dtBox creates a rectangular parallelepiped centered at the origin and aligned with the axes of the shape's local coordinate system. The parameters specify its extent along the respective coordinate axes. The commands dtCone and dtCylinder create respectively a cone and a cylinder centered at the origin and whose central axis is aligned with the Y-axis of the local coordinate system. The cone's apex is at y = height / 2. The command dtSphere creates a sphere centered at the origin of the local coordinate system.

Other shape types based on point data, such as polygon soups, simplicial complexes, (compositions of) convex polyhedra, are created by the dtNewComplexShape command. For constructing complex shapes the following commands are used:

DtShapeRef dtNewComplexShape();
void dtEndComplexShape();

void dtBegin(DtPolyType type);
void dtEnd();
void dtVertex(DtScalar x, DtScalar y, DtScalar z);

void dtVertexBase(const void *base);
void dtVertexIndex(DtIndex index);
void dtVertexIndices(DtPolyType type, DtCount count, const DtIndex *indices);
void dtVertexRange(DtPolyType type, DtIndex first, DtCount count); 

A complex shape is composed of d-dimensional polytopes, where d is at most 3. A d-polytope can be a simplex (point, line segment, triangle, tetrahedron), a convex polygon, or a convex polyhedron. The type of d-polytope is specified by a value of DtPolyType, defined as

typedef enum DtPolyType {
} DtPolyType;

A d-polytope is specified by enumerating its vertices. This can be done in two ways. In the first way, the vertices are specified by value, using the dtVertex command. The following example shows how the faces of a pyramid are specified.

DtShapeRef pyramid = dtNewComplexShape();

dtVertex(1.0, 0.0, 1.0); 
dtVertex(1.0, 0.0, -1.0); 
dtVertex(-1.0, 0.0, -1.0); 
dtVertex(-1.0, 0.0, 1.0);

dtVertex(1.0, 0.0, 1.0); 
dtVertex(1.0, 0.0, -1.0); 
dtVertex(0.0, 1.27, 0.0); 



In the second method, the vertices are stored in an array, and are referred to by indices. For each complex shape, we can specify a single array. A vertex array is specified by the command dtVertexBase, which takes the address of the first element in the array, referred to as the base of the array, as argument. The command dtVertexIndex is used for specifying vertices. See the following example:

DtScalar vertices[5 * 3] = { 
  1.0, 0.0, 1.0, 
  1.0, 0.0, -1.0, 
  -1.0, 0.0, -1.0, 
  -1.0, 0.0, 1.0, 
  0.0, 1.27, 0.0 

DtShapeRef pyramid = dtNewComplexShape();





Alternatively, the indices can be placed into an array and specified using the command dtVertexIndices, as in the following example:

DtScalar vertices[5 * 3] = { 
  1.0, 0.0, 1.0, 
  1.0, 0.0, -1.0, 
  -1.0, 0.0, -1.0, 
  -1.0, 0.0, 1.0, 
  0.0, 1.27, 0.0 

DtIndex face0[4] = { 0, 1, 2, 3 };
DtIndex face1[3] = { 0, 1, 4 };


DtShapeRef pyramid = dtNewComplexShape();

dtVertexIndices(DT_SIMPLEX, 4, face0);
dtVertexIndices(DT_SIMPLEX, 3, face1);



Finally, a polytope can be specified from a range of vertices using the command dtVertexRange. The range is specified by the first index and the number of vertices. In the following example a pyramid is constructed as a convex polyhedron, which is the convex hull of the vertices in the array.

DtScalar vertices[5 * 3] = { 
  1.0, 0.0, 1.0, 
  1.0, 0.0, -1.0, 
  -1.0, 0.0, -1.0, 
  -1.0, 0.0, 1.0, 
  0.0, 1.27, 0.0 

DtShapeRef pyramid = dtNewComplexShape();
dtVertexRange(DT_POLYHEDRON, 0, 5);

Note that the vertices of a simplex need not be affinely independent, and the vertices specifying a convex polyhedron need not be extreme vertices of the convex hull. However, in order to construct a proper convex polygon, the vertices should be approximately coplanar and specified in the order as they appear on the boundary.

Creating and Moving Objects

An object is an instance of a shape. The commands for creating, moving and deleting objects are

void dtCreateObject(DtObjectRef object, DtShapeRef shape); 
void dtDeleteObject(DtObjectRef object);
void dtSelectObject(DtObjectRef object);

void dtLoadIdentity();

void dtLoadMatrixf(const float *m);
void dtLoadMatrixd(const double *m);

void dtMultMatrixf(const float *m);
void dtMultMatrixd(const double *m);

void dtTranslate(DtScalar x, DtScalar y, DtScalar z);
void dtRotate(DtScalar x, DtScalar y, DtScalar z, DtScalar w);
void dtScale(DtScalar x, DtScalar y, DtScalar z);

An object is referred to by a DtObjectRef, which is defined as void *. Any pointer type may be used to refer to an object. In general, a pointer to a structure in the client application associated with the collision object should be used.

An object's motion is specified by changing the placement of the local coordinate system of the shape. Initially, the local coordinate system of an object coincides with the world coordinate system.

The current object is the last created or selected object. The placement of the current object is changed, either by translations, rotations and nonuniform scalings, or by using an OpenGL 4x4 column-major matrix representing an affine transformation. Placements are specified absolute or relative to the previous placement. Rotations are specified using quaternions. Following example shows how a pair of objects are given absolute placements.

dtCreateObject(&object1, hammer);
dtCreateObject(&object2, nail);

dtTranslate(0, 1, 1);
dtRotate(0, 0, 1, 0);

dtTranslate(0, 1, 0);
dtRotate(0, 0, 0, 1);

Who's Afraid of Quaternions?

A quaternion is a four-dimensional vector. The set of quaternions of length one (points on a four-dimensional sphere) map to the set of orientations in three-dimensional space. Since in many applications an orientation defined by either a rotation axis and angle or by a triple of Euler angles is more convenient, the package includes code for quaternion operations. The code is found in a library of C++ classes for 3D affine transformations. The classes are found in the `include/3D' directory. All the code is inlined so you do not need to link a library in order to use the classes.

The quaternion class is found in the file `Quaternion.h'. The class has constructors and methods for setting a quaternion. For example

Quaternion q1(axis, angle);
Quaternion q2(yaw, pitch, roll);


q1.setRotation(axis, angle);
q2.setEuler(yaw, pitch, roll);


dtRotate(q1[X], q1[Y], q1[Z], q1[W]);

Also included is a static method Quaternion::random(), which returns a random orientation.

Response Handling

Collision response in SOLID is handled by means of callback functions. The callback functions have the type DtResponse defined by

 typedef void (*DtResponse)(
   void *client_data, 
   DtObjectRef object1, 
   DtObjectRef object2,
   const DtCollData *coll_data)

Here, client_data is a pointer to an arbitrary structure in the client application, object1 and object2 are colliding objects, and coll_data is the response data computed by SOLID.

Currently, there are three types of response: simple, smart and witnessed response. For simple response the value of coll_data is NULL. For smart and witnessed response coll_data points to the following structure

typedef struct DtCollData {
  DtVector point1;
  DtVector point2;
  DtVector normal;
} DtCollData;

An object of this type represents a pair of points of the respective objects. The points point1 and point2 are given relative to the local coordinate system of their respective objects object1 and object2. The normal field is used for smart response only.

For witnessed response, the points represent a witness of the collision. As a result of this the global coordinates of the witness points are equal. For smart response, the points represent the closest point pair of the objects at placements from the previous time frame. The normal field contains the difference of the global coordinates of the closest point pair, i.e., normal = Global(point1) - Global(point2). We will discuss this type of response more thoroughly further on.

Response is defined as default response for all pairs of objects, as object response for all pairs containing a given object, or as pair response for a particular pair of objects. The commands for defining and undefining response are

void dtSetDefaultResponse(DtResponse response, DtResponseType type, 
			  void *client_data)  
void dtClearDefaultResponse()

void dtSetObjectResponse(DtObjectRef obj, DtResponse response, 
			 DtResponseType type, void *client_data)
void dtClearObjectResponse(DtObjectRef obj)
void dtResetObjectResponse(DtObjectRef obj)

void dtSetPairResponse(DtObjectRef obj1, DtObjectRef obj2, 
		       DtResponse response, DtResponseType type, 
		       void *client_data)
void dtClearPairResponse(DtObjectRef obj1, DtObjectRef obj2)
void dtResetPairResponse(DtObjectRef obj1, DtObjectRef obj2)

A response is defined by either a Set or a Clear command. The Clear command defines the response to be nil (no response).

Initially, the default response is nil and all pairs of objects have a default response. If for an object pair, one of the objects has an object response defined, then this response overrules the default response. A pair response overrules any object or default response. If for both objects there is an object response defined, then one of the responses is chosen. In this case, one of the responses may be forced to be chosen by defining it as a pair response.

A response is undefined, i.e., reset to a more general setting, by the Reset commands. The command dtResetPairResponse resets the response of a pair of objects to an object response, if one is defined for an object in the pair, or otherwise to the default response. The command dtResetObjectResponse resets the responses of the object pairs, for which no other object response or a pair response is defined, to the default response. Note that whenever an object is deleted, the object response and all pair responses that are set for this object are reset automatically.

The DtResponseType is defined by

  typedef enum DtResponseType { 
  } DtResponseType

Setting the response type to DT_NO_RESPONSE is equivalent to clearing the response.

The response callback functions are executed for each colliding pair of objects by calling

int dtTest()

This function returns the number of callback functions that are executed.

Smart Response

For physics-based simulations it is often necessary to have a representation of the collision plane of a pair of colliding objects in order to compute the reaction forces. From a single configuration of two colliding objects it is hard to compute a collision plane, since there is no knowledge of how this configuration came about. Therefore, SOLID uses the configuration of the objects from the previous time frame for approximating the collision plane. If the objects were disjoint in the previous time frame, then the vector defined by the difference of the closest point pair of the objects is a good approximation of the collision plane's normal.

By selecting smart response for a pair of objects, the closest point pair and the normal from the previous time frame are computed. The points point1 and point2 are given in local coordinates and the normal relative to the global basis and pointing away from object2. In order to compute these values, the configuration of objects must be stored in each time frame. This is done by calling

void dtProceed();

Note that in order to guarantee that a non-zero normal can be found, the dtProceed command may only be called if all object pairs for which a smart response is defined, are disjoint! The common way of guarding this is by iteratively doing collision tests and changing the placements until the objects are disjoint. Note that it is possible and often necessary to call dtTest multiple times before calling dtProceed.

Deformable Models

SOLID handles deformations of complex shapes. In this context deformations are specified by changes of vertex positions. Complex shapes that are defined using a vertex array in the client application may be deformed by changing the array elements, or specifying a new array. SOLID is notified of a change of vertices by the command

void dtChangeVertexBase(DtShapeRef shape, const void *base);

When using convex polygons or convex polyhedra as shape components, the client should warrant that the vertex changes do not violate the convexity and topology (planar graph embedding) of a component!

Note that in order to use smart response for deformable shapes, the change of vertices should be done by specifying a new array. The vertex array of the previous time frame should be kept intact, otherwise SOLID can not determine the configuration of objects of the previous time frame. This is best handled by applying a `double buffering' technique. After a call to dtProceed, the new vertex positions are placed in the free buffer of a pair of vertex buffers, and dtChangeVertexBase is called using this buffer, after which the other buffer becomes the free buffer.


In computer animations there is usually a lot of frame coherence (objects move smoothly). In these cases, caching and reusing earlier computations will yield a considerable performance improvement. The caching option of SOLID enables an incremental sort on the set of objects, in order to reduce the number of pairwise intersection tests. Moreover, when the cashing option is on, data from earlier intersection tests is stored and used for faster determination of the intersection status of a pair of objects. Caching is enabled and disabled by

void dtEnableCaching()
void dtDisableCaching()

Caching may be enabled or disabled at any time during a simulation. This option is enabled by default.

Projects and other things left to do

Coming Attractions

Look out for my thesis describing the design and performance evaluation of SOLID, to appear in March, 1999.

Bug Reports

Please send remarks, questions and bug reports to, or write to Gino van den Bergen, Department of Mathematics and Computing Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

This document was generated on 24 January 1999 using the texi2html translator version 1.51a.