Obtaining model coordinates giving wrong values back in OpenGl - c++

Question

So i've got a little program that draws a few spheres, then i attempt to right click on them. Upon right clicking, it draws a line between the near and far planes, under the mouse when i click. However, its giving strange results, such as the line being out by quite a long way. The direction of the line is right however, just the line is maybe about 10 to the left along X, or 5 to right right along Y (Those are random examples).
Here's my code :
Positioning the camera
gluLookAt(mouse.getScrollY(), 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 1.0f);
glRotated(mouse.getAngleV(), 0.0f, -1.0f, 0.0f);
glRotated(mouse.getAngleH(), 0.0f, 0.0f, -1.0f);
mouse.getScrollY is simply a value based on how far the camera is scrolled back form the origin.
Obtaining the coordinates
void Mouse::GetGLPos(int x, int y)
{
//init vars:
GLint viewport[4];
GLdouble modelview[16];
GLdouble projection[16];
GLfloat winX, winY;
GLdouble posX, posY, posZ;
GLdouble FposX, FposY, FposZ;
//get gl specs
glGetDoublev( GL_MODELVIEW_MATRIX, modelview ); //get Modelmatrix
glGetDoublev( GL_PROJECTION_MATRIX, projection ); //get projection matrix
glGetIntegerv( GL_VIEWPORT, viewport ); //get viewport values
//calculate the gl mouseposition
winX = (float)x;
winY = (float)viewport[3] - (float)y;
std::cout << "X "<< winX << " Y " << winY << endl;
gluUnProject( winX, winY, 0.0, modelview, projection, viewport, &posX, &posY, &posZ);
gluUnProject( winX, winY, 1.0, modelview, projection, viewport, &FposX, &FposY, &FposZ);
std::cout << "Near positions:" << posX << " | " << posY << " | " << posZ << endl;
std::cout << " Far positions:" << FposX << " | " << FposY << " | " << FposZ << endl << endl;
for (int i = 0; i <= 15; i++)
{
cout << modelview[i] << endl;
}
if (counter == 5)
{
counter = 0;
}
LinestoreNear[counter + 1] = Vec3(posX, posY, posZ);
LinestoreFar[counter + 1] = Vec3(FposX, FposY, FposZ);
counter ++;
mouseOgl[0] = posX;
mouseOgl[1] = posY;
mouseOgl[2] = posZ;
}
x and y that are passed to it are simply the mouse's x and y coordinates on the screen. ((328,657) for example)
And finally, just in case, here is the drawing the line
glDisable(GL_LIGHTING);
for (int i = 0; i <= 4 - 1 ; i++)
{
glColor3f(1.0f, 1.0f, 1.0f);
glBegin(GL_LINES);
glVertex3f(mouse.LinestoreNear[i].a, mouse.LinestoreNear[i].b, mouse.LinestoreNear[i].c);
glVertex3f(mouse.LinestoreFar[i].a, mouse.LinestoreFar[i].b, mouse.LinestoreFar[i].c);
glEnd();
}
glEnable(GL_LIGHTING);

Answer 1

heres a very brief overview of the process you need to use
to get world coordinates that are correct you need to have a 4x4 model matrix for each object in your scene. The model matrix contains all the transformations convert manipulate model coordinates to/from world coordinates.
ie every time you call glTranslate, you have to transform the model matrix as follows:
T = [ 1 0 0 x ]
[ 0 1 0 y ]
[ 0 0 1 z ]
[ 0 0 0 1 ]
M' = M * T
You can then use the transformed model matrix to obtain world space coordinates
Homogenise the coords first however (or check they are already) ie. just set w = 1, if they are not already homogenous coordinates.
If you do not convert the coordinates between object space and world space then you will get the behaviour you are experiencing, and it is easy to do it at the wrong stage as well, so plan out the required steps and you should be fine:)
Hope this helps.

Answer 2

Finally got time to verify your results. It works exactly as it should.
With LookAt you specify (GLU will calculate these matrices for you) camera position and rotation matrices (indirectly, via 'up' and 'view point' vectors, but still it will result into these transformations). Just right after you specify two additional rotation matrices - which means, you rotating a world around a zero point (which is already shifted by LookAt). For you, this kind of rotation will look like camera rotating around certain point at space, maintaining constant distance. Lines keeping their place, with some distortion introduced by perspective transformation (if you have it), and being clipped by far clipping plane (this will look like lines becoming shorter and longer during rotation).