Clipping a convex polygon against a plane
The following code clips an arbitrary convex polygon against a given plane.
The vertex parameter points to an array containing the number of vertices specified by the
vertexCount parameter, wound in counterclockwise order. The plane parameter
specifies the plane against which the polygon is clipped. The location array is used for
internal processing and must be large enough to hold vertexCount values. The
result array is where the clipped polygon is returned and must generally be large enough
to hold vertexCount + 1 vertices. The return value of the
function is the number of vertices in the clipped polygon.
This code uses the Vector3D and Point3D classes.
This code uses the Vector4D class.
#include "Vector4D.h"
enum
{
polygonInterior = 1,
polygonBoundary = 0,
polygonExterior = -1
};
const float boundaryEpsilon = 1.0e-3F;
long ClipPolygonAgainstPlane(long vertexCount, const Point3D *vertex,
const Vector4D& plane, signed char *location, Point3D *result)
{
long positive = 0;
long negative = 0;
for (long a = 0; a < vertexCount; a++)
{
float d = plane * vertex[a];
if (d > boundaryEpsilon)
{
location[a] = polygonInterior;
positive++;
}
else
{
if (d < -boundaryEpsilon)
{
location[a] = polygonExterior;
negative++;
}
else
{
location[a] = polygonBoundary;
}
}
}
if (negative == 0)
{
for (long a = 0; a < vertexCount; a++) result[a] = vertex[a];
return (vertexCount);
}
else if (positive == 0)
{
return (0);
}
long count = 0;
long previous = vertexCount - 1;
for (long index = 0; index < vertexCount; index++)
{
long loc = location[index];
if (loc == polygonExterior)
{
if (location[previous] == polygonInterior)
{
const Point3D& v1 = vertex[previous];
const Point3D& v2 = vertex[index];
Vector3D dv = v2 - v1;
float t = plane * v2 / (plane * dv);
result[count++] = v2 - dv * t;
}
}
else
{
const Point3D& v1 = vertex[index];
if ((loc == polygonInterior) && (location[previous] == polygonExterior))
{
const Point3D& v2 = vertex[previous];
Vector3D dv = v2 - v1;
float t = plane * v2 / (plane * dv);
result[count++] = v2 - dv * t;
}
result[count++] = v1;
}
previous = index;
}
return (count);
}
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