int xpos = 0;   // current x position
int ypos = 0;   // current y position
int orig_xpos = 0; //where the user first clicked x
int orig_ypos = 0; //where the use first clicked y
GLdouble newx, newy, newz; //the new object space coordinates
GLdouble mx, my, mz; // move with this vector

GLdouble model_matrix[16];
GLdouble proj_matrix[16];
GLint view_matrix[4];

void setMatrixes()
{
    glGetDoublev(GL_MODELVIEW_MATRIX, model_matrix);
    glGetDoublev(GL_PROJECTION_MATRIX, proj_matrix);
    glGetIntegerv(GL_VIEWPORT, view_matrix);
}

void GLFWCALL onMouseButton( int button, int action )
{
    if( button == GLFW_MOUSE_BUTTON_LEFT )
    {
        dragging = action == GLFW_PRESS;
        glfwGetMousePos(&xpos, &ypos);

        if(orig_xpos == 0 && orig_ypos == 0)
        {
            orig_xpos = xpos;
            orig_ypos = ypos;
        }
        gluUnProject(xpos, ypos, 0, model_matrix, proj_matrix, view_matrix, &newx, &newy, &newz);
    }
}

void moveObject(int dx, int dy)
{
    GLdouble nx, ny, nz;
    gluUnProject(dx, dy, 0, model_matrix, proj_matrix, view_matrix, &nx, &ny, &nz);

    mx = newx - nx;
    my = newy - ny;
    mz = newz - nz;
}

void GLFWCALL onMouseMove(int x, int y)
{
    if(dragging)
    {
        moveObject(orig_xpos - x, orig_ypos - y);
    }
}

int main()
{
    init();

    glfwSetMouseButtonCallback(onMouseButton);
    glfwSetMousePosCallback(onMouseMove);
    glEnable(GL_DEPTH_TEST);

    glClearColor( 1,1,1,1 );

    while(running())
    {
        glClear(GL_COLOR_BUFFER_BIT + GL_DEPTH_BUFFER_BIT);
        glLoadIdentity();
        gluLookAt(15, 0, 5, 0, 0, 0, 0, 0, 1 );
        setMatrixes();

        /* A simple sphere, but should work with more simple objects */
        glColor3ub(0, 0, 0);
        glPushMatrix();
        glTranslated(mx, my, mz);
        drawSmoothUnitySphere();
        glPopMatrix();

        glfwSwapBuffers();
    }

    glfwTerminate();
    return 0;
}