OpenGL inverse projection matrix

inverseProjection.cpp

/*
Based off http://bookofhook.com/mousepick.pdf

OpenGL matrix order
m0 m4 m8 m12
m1 m5 m9 m13
m2 m6 m10 m14
m3 m7 m11 m15

Projection matrix
a 0 0 0
0 b 0 0
0 0 c d
0 0 e 0

Inverse projection
1/a, 0, 0, 0,
0, 1/b, 0, 0,
0, 0, 0, 1/e,
0, 0, 1/d, -c/(d*e)
*/

double op[16]; // original projection matrix
glGetDoublev(GL_PROJECTION_MATRIX, op);

double a = op[0];
double b = op[5];
double c = op[10];
double d = op[14];
double e = op[11];

double inverseProject[16] = { 0.0 };
inverseProject[0] = 1.0 / a;
inverseProject[5] = 1.0 / b;
inverseProject[11] = 1.0 / d;
inverseProject[14] = 1.0 / e;
inverseProject[15] = -c / (d * e);

//glLoadMatrixd(inverseProject);