Constructing a Shadow Matrix

void shadowMatrix(GLfloat shadowMat[4][4], GLfloat groundplane[4], GLfloat lightpos[4])

{

GLfloat dot;

/* Find dot product between light position vector and ground plane normal. */

dot = groundplane[X] * lightpos[X] +

groundplane[Y] * lightpos[Y] +

groundplane[Z] * lightpos[Z] +

groundplane[W] * lightpos[W];

shadowMat[0][0] = dot - lightpos[X] * groundplane[X];

shadowMat[1][0] = 0.f - lightpos[X] * groundplane[Y];

shadowMat[2][0] = 0.f - lightpos[X] * groundplane[Z];

shadowMat[3][0] = 0.f - lightpos[X] * groundplane[W];

shadowMat[X][1] = 0.f - lightpos[Y] * groundplane[X];

shadowMat[1][1] = dot - lightpos[Y] * groundplane[Y];

shadowMat[2][1] = 0.f - lightpos[Y] * groundplane[Z];

shadowMat[3][1] = 0.f - lightpos[Y] * groundplane[W];

shadowMat[X][2] = 0.f - lightpos[Z] * groundplane[X];

shadowMat[1][2] = 0.f - lightpos[Z] * groundplane[Y];

shadowMat[2][2] = dot - lightpos[Z] * groundplane[Z];

shadowMat[3][2] = 0.f - lightpos[Z] * groundplane[W];

shadowMat[X][3] = 0.f - lightpos[W] * groundplane[X];

shadowMat[1][3] = 0.f - lightpos[W] * groundplane[Y];

shadowMat[2][3] = 0.f - lightpos[W] * groundplane[Z];

shadowMat[3][3] = dot - lightpos[W] * groundplane[W];

}