// 矩阵按行优先存储

首先说一下opengl中的矩阵表示， 一般在c/c++中定义的矩阵和opengl中的矩阵分别是：
/*****************************************************************************
We define a 4x4 matrix array, OpenGL linear matrix format:
referenced as Row,Column as:

| 0,0   0,1   0,2   0,3 |             |a0 a4 a8 a12|

| 1,0   1,1   1,2   1,3 |             |a1 a5 a9 a13|

| 2,0   2,1   2,2   2,3 |             |a2 a6 a10 a14|

| 3,0   3,1   3,2   3,3 |             |a3 a7 a11 a15|

*/ 两者行和列正好相反，因此，对于一个4×4的矩阵GLdouble m[16], 那么在操作这个矩阵时必须小心

GLint gluProject(GLdouble objx, GLdouble objy, GLdouble objz,
const GLdouble model[16], const GLdouble proj[16],
const GLint viewport[4],
GLdouble * winx, GLdouble * winy, GLdouble * winz)
{
/* matrice de transformation */
GLdouble in[4], out[4];
/* initilise la matrice et le vecteur a transformer */
in[0] = objx;
in[1] = objy;
in[2] = objz;
in[3] = 1.0;
transform_point(out, model, in);
transform_point(in, proj, out);
/* d’ou le resultat normalise entre -1 et 1 */
if (in[3] == 0.0)
return GL_FALSE;
in[0] /= in[3];
in[1] /= in[3];
in[2] /= in[3];
/* en coordonnees ecran */
*winx = viewport[0] + (1 + in[0]) * viewport[2] / 2;
*winy = viewport[1] + (1 + in[1]) * viewport[3] / 2;
/* entre 0 et 1 suivant z */
*winz = (1 + in[2]) / 2;
return GL_TRUE;
}
/*
* Transform a point (column vector) by a 4x4 matrix. I.e. out = m * in
* Input: m - the 4x4 matrix
* in - the 4x1 vector
* Output: out - the resulting 4x1 vector.
*/

static void
transform_point(GLdouble out[4], const GLdouble m[16], const GLdouble in[4])
{
#define M(row,col) m[col*4+row]
out[0] =
M(0, 0) * in[0] + M(0, 1) * in[1] + M(0, 2) * in[2] + M(0, 3) * in[3];
out[1] =
M(1, 0) * in[0] + M(1, 1) * in[1] + M(1, 2) * in[2] + M(1, 3) * in[3];
out[2] =
M(2, 0) * in[0] + M(2, 1) * in[1] + M(2, 2) * in[2] + M(2, 3) * in[3];
out[3] =
M(3, 0) * in[0] + M(3, 1) * in[1] + M(3, 2) * in[2] + M(3, 3) * in[3];
#undef M
}

参考：

http://www.soidc.net/articles/1215485053486/20081129/1215945559526_1.html

http://sutiany.itpub.net/post/6261/33673

http://www.opengl.org/wiki/GluProject_and_gluUnProject_code