I have a perspective projection. When the user clicks on the screen, I want to compute the ray between the near and far planes that projects from the mouse point, so I can do some ray intersection code with my world.

I am using my own matrix and vector and ray classes and they all work as expected.

However, when I try and convert the ray to world coordinates my far always ends up as 0,0,0 and so my ray goes from the mouse click to the centre of the object space, rather than through it. (The x and y coordinates of near and far are identical, they differ only in the z coordinates where they are negatives of each other)

GLint vp[4];
glGetIntegerv(GL_VIEWPORT,vp);
matrix_t mv, p;
glGetFloatv(GL_MODELVIEW_MATRIX,mv.f);
glGetFloatv(GL_PROJECTION_MATRIX,p.f);
const matrix_t inv = (mv*p).inverse();
const float
    unit_x = (2.0f*((float)(x-vp[0])/(vp[2]-vp[0])))-1.0f,
    unit_y = 1.0f-(2.0f*((float)(y-vp[1])/(vp[3]-vp[1])));
const vec_t near(vec_t(unit_x,unit_y,-1)*inv);
const vec_t far(vec_t(unit_x,unit_y,1)*inv);
ray = ray_t(near,far-near);

What have I got wrong? (How do you unproject the mouse-point?)