OpenGL中两种计算投影矩阵的函数

OpenGL无意间同时看到两种创建投影矩阵的写法,可以说它们完成的是同样的功能,但写法完全不同,可以观摩一下什么叫做异曲同工之妙...

第一种:

gltMakeShadowMatrix函数是重点

 // 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:

CreateShadowMatrix函数是重点

 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;
 }