ok, the code below correctly draws the sphere where I want it after a few translations have been done previously: ======================== glTranslatef(4, 0.0, 6); glRotatef(90.0, 0.0, 1.0, 0.0); glScalef(5.0, 5.0, 5.0); //below sphere works fine ! gluSphere(rocket, 1.0f/2.0f, 7, 7); ======================== I need that precise position of the sphere and so directly after drawing the sphere I do the following to get the vector CVector vec1(0.0f, 0.0f, 0.0f); CVector tailfin; //vector with new position TransformVector4WithModel(vec1, tailfin); Then I draw the sphere like this and it should be exactly where it was above: glPushMatrix(); glLoadIdentity(); glTranslatef(tailfin.x, tailfin.y, tailfin.z); gluSphere(rocket, 1.0f/2.0f, 7, 7); glPopMatrix(); except it's not there and I can't find it anywhere !! This has got to be dead easy and so you're going to have to talk to me like a 3 year old. ASIDE TransformVector4WithModel function is like this: ============================================= void TransformVector4WithModel(CVector &vec, CVector &outvec) { //used to get new position float modelViewMatrix[16]; glGetFloatv(GL_MODELVIEW_MATRIX, modelViewMatrix); outvec.x = modelViewMatrix[0]*vec.x + modelViewMatrix[4]*vec.y + modelViewMatrix[8]*vec.z + modelViewMatrix[12]; outvec.y = modelViewMatrix[1]*vec.x + modelViewMatrix[5]*vec.y + modelViewMatrix[9]*vec.z + modelViewMatrix[13]; outvec.z = modelViewMatrix[2]*vec.x + modelViewMatrix[6]*vec.y + modelViewMatrix[10]*vec.z + modelViewMatrix[14]; }

I'm loathe to change it because it works everywhere else !
Is there something in the way I've done the translations ?

cheers

Ok so your code looks like this?

glTranslatef(4, 0.0, 6);
glRotatef(90.0, 0.0, 1.0, 0.0);
glScalef(5.0, 5.0, 5.0);
//below sphere works fine !
gluSphere(rocket, 1.0f/2.0f, 7, 7);

CVector vec1(0.0f, 0.0f, 0.0f);
CVector tailfin; //vector with new position
TransformVector4WithModel(vec1, tailfin);

glPushMatrix();
glLoadIdentity();
glTranslatef(tailfin.x, tailfin.y, tailfin.z);
gluSphere(rocket, 1.0f/2.0f, 7, 7);
glPopMatrix();

Let's go step by step:

glTranslatef(4, 0.0, 6);
glRotatef(90.0, 0.0, 1.0, 0.0);
glScalef(5.0, 5.0, 5.0);

So, in reverse order we scale, rotate, and translate.

gluSphere(rocket, 1.0f/2.0f, 7, 7);

now we draw a sphere centered at 4,0,6 with some rotation and scale.

CVector vec1(0.0f, 0.0f, 0.0f);
CVector tailfin; //vector with new position
TransformVector4WithModel(vec1, tailfin);

Assuming your matrix math works:
we take (0,0,0) and scale -> (0,0,0)
we take (0,0,0) and rotate -> (0,0,0)
we take (0,0,0) and translate -> (4,0,6)

glPushMatrix();
glLoadIdentity();

ok matrix cleared

glTranslatef(tailfin.x, tailfin.y, tailfin.z);

tailfin = (4,0,6), so now we have a translation by that vector

gluSphere(rocket, 1.0f/2.0f, 7, 7);

ok, we draw a sphere centered at (4,0,6) without rotation or scale. We already drew a sphere at that same location, but the first sphere was 5x the size of this one. So this sphere is inside the first one.