// Gets the three coefficients of a plane equation given three points on the plane.

 void gltGetPlaneEquation(GLTVector3 vPoint1, GLTVector3 vPoint2, GLTVector3 vPoint3, GLTVector3 vPlane)

 {

     // Get normal vector from three points. The normal vector is the first three coefficients

     // to the plane equation...

     gltGetNormalVector(vPoint1, vPoint2, vPoint3, vPlane);

     // Final coefficient found by back substitution

     vPlane[] = -(vPlane[] * vPoint3[] + vPlane[] * vPoint3[] + vPlane[] * vPoint3[]);

 }

 void gltMakeShadowMatrix(GLTVector3 vPoints[], GLTVector4 vLightPos, GLTMatrix destMat)

 {

     GLTVector4 vPlaneEquation;

     GLfloat dot;

     gltGetPlaneEquation(vPoints[], vPoints[], vPoints[], vPlaneEquation);

     // Dot product of plane and light position

     dot =   vPlaneEquation[]*vLightPos[] +

             vPlaneEquation[]*vLightPos[] +

             vPlaneEquation[]*vLightPos[] +

             vPlaneEquation[]*vLightPos[];

     // Now do the projection

     // First column

     destMat[] = dot - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     // Second column

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = dot - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     // Third Column

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = dot - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     // Fourth Column

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = 0.0f - vLightPos[] * vPlaneEquation[];

     destMat[] = dot - vLightPos[] * vPlaneEquation[];

 }

 // Given three points on a plane in counter clockwise order, calculate the unit normal

 void gltGetNormalVector(const GLTVector3 vP1, const GLTVector3 vP2, const GLTVector3 vP3, GLTVector3 vNormal)

     {

     GLTVector3 vV1, vV2;

     gltSubtractVectors(vP2, vP1, vV1);

     gltSubtractVectors(vP3, vP1, vV2);

     gltVectorCrossProduct(vV1, vV2, vNormal);

     gltNormalizeVector(vNormal);

     }

 第二种

 /** 创建投射矩阵 */

 void CPlanarShadow::CreateShadowMatrix(float m[], Vector3 point, Vector3 normal, float lp[])

 {

     /** 计算顶点到平面的距离 */

     float d = - ((normal.x * point.x) + (normal.y * point.y) + (normal.z * point.z));

     /** 计算光源向量和法向量的点积 */

     float dot = normal.x*lp[]  + normal.y*lp[] + normal.z*lp[] + d*lp[];

     /** 设置矩阵元素值 */

     m[]  = dot - lp[]*normal.x;  m[]  =       -lp[]*normal.x;     m[]  =      -lp[]*normal.x;  m[]  =        -lp[]*normal.x;

     m[]  =         -lp[]*normal.y;  m[]  = dot -lp[]*normal.y;  m[]  =      -lp[]*normal.y;  m[]  =        -lp[]*normal.y;

     m[]  =         -lp[]*normal.z;  m[]  =       -lp[]*normal.z;     m[] = dot  - lp[]*normal.z; m[] =        -lp[]*normal.z;

     m[] =         -lp[]*d;           m[] =       -lp[]*d;         m[] =      -lp[]*d;            m[] = dot -lp[]*d;

 }