Shadow matrix

   groundPlane = D4(0, 1, 0, -.001);
   double dot = groundPlane.a * lightPos[0] +
                groundPlane.b * lightPos[1] +
                groundPlane.c * lightPos[2] +
                groundPlane.d * lightPos[3];
   shadowMatrix[0]  = dot - lightPos[0] * groundPlane.a;
   shadowMatrix[4]  = 0.  - lightPos[0] * groundPlane.b;
   shadowMatrix[8]  = 0.  - lightPos[0] * groundPlane.c;
   shadowMatrix[12] = 0.  - lightPos[0] * groundPlane.d;

   shadowMatrix[1]  = 0.  - lightPos[1] * groundPlane.a;
   shadowMatrix[5]  = dot - lightPos[1] * groundPlane.b;
   shadowMatrix[9]  = 0.  - lightPos[1] * groundPlane.c;
   shadowMatrix[13] = 0.  - lightPos[1] * groundPlane.d;

   shadowMatrix[2]  = 0.  - lightPos[2] * groundPlane.a;
   shadowMatrix[6]  = 0.  - lightPos[2] * groundPlane.b;
   shadowMatrix[10] = dot - lightPos[2] * groundPlane.c;
   shadowMatrix[14] = 0.  - lightPos[2] * groundPlane.d;

   shadowMatrix[3]  = 0.  - lightPos[3] * groundPlane.a;
   shadowMatrix[7]  = 0.  - lightPos[3] * groundPlane.b;
   shadowMatrix[11] = 0.  - lightPos[3] * groundPlane.c;
   shadowMatrix[15] = dot - lightPos[3] * groundPlane.d;