View Full Version : What's wrong please ?

09-26-2001, 05:14 AM
I want to place 4 rotating cubes ( rotation over themself )
they are placed at
10, 0, 0
-10, 0, 0
0, 0, 10
0, 0, -10
so there's one in each direction
but I can't get this to work when I want to have a camera using gluLookAt
I don't know what's wrong

I call a DrawScene function and then Camera.SetCamera() which is my own function that calculates the direction and position of the camera and call gluLookAt(...)
In my DrawScene I call the rendering function of the four cubes

static float fRotation = 0.0f;
glTranslatef(GetPos().x, GetPos().y, GetPos().z);
glRotatef( fRotation, 0.0f, 1.0f, 0.0f);


//Well I draw the cube here...

fRotation += 18.0f;

and I guess this works
and my camera call is this

gluLookAt( m_vPosition.x, m_vPosition.y, m_vPosition.z, //Position vector
m_vPosition.x + cosf(GetRadian(m_vAngle.y + 90)), m_vPosition.y + cosf(GetRadian(m_vAngle.x + 90)), m_vPosition.z + sinf(GetRadian(m_vAngle.y + 90)), //Point At Vector
0.0f, 1.0f, 0.0f ); //Up vector

Do you see what's wrong ?
Am I using the glPushMatrix() or glLoadIdentity() in a wrong way ?
Should I set the camera before drawing the geometries ? call glLoadIdentity before the gluLookAt ?
PLEASE HELP !!! http://www.opengl.org/discussion_boards/ubb/smile.gif

*Sleep is a waste of time*

09-26-2001, 05:42 AM
Try it the other way round. First set your camera then do all object transformations. (But please don't forget to remove the glLoadIdentity() in your object transformation).
Long explantion:
You have to read all transformations backwards. If your current modelview matrix is M and you insert a new matrix N (via translate, rotate or multiply) you'll get M*N. Your vertices will be multiplied from the right side yielding M*N*v where v is the vertex position.

09-26-2001, 05:55 AM
Thanks alot stefan it works perfectly
But I totally DON'T get it...
I tought I understand how openGL works it seems I don't...
How come I don't need to set the modelview to identity for each cube
doesn't the transformation for the second cube affect the first one ?
Or the transformation only affect the next glBegin/End ????
is it because of my glPushMatrix and glPopMatrix ? which I'm not sure what they do and why I should use them ?
Please enlighten me i'm confused hehehe

*Sleep is a waste of time*

09-26-2001, 09:43 AM
Take a look in the "redbook". The first release of this book is available online (e.g. http://fly.cc.fer.hr/~unreal/theredbook/)
In chapter 3 you'll find good explanations of how opengl does its transformations.