[Algorithms] Dummie Matrix math questions

[Bruno S. <bcs...@gm...>]

Hello Folks,

I am reading this list emails (and learning a lot) from some time and I
would like to ask some help with basic 3d transformations that is giving a
lot of headaches for me on last days (and nights)...

On the framework that I am working (d3d9) I have a basic scene graph system,
each node has a transformation matrix.

On rendering, I have a code (pseudo code) like this:

Vec3 fwd;

fwd.x = cameraMatrix._11;
fwd.y = cameraMatrix._21;
fwd.z = cameraMatrix._31;

//My coordinate system, x is forward, y is right, Z is up (right hand with Z
pointing up)
Vec3 origin = cameraOrigin;
Vec3 at = origin + fwd;

Vec3 up;
up.x = cameraMatrix._13;
up.x = cameraMatrix._23;
up.x = cameraMatrix._33;

D3DXMATRIX viewMat;
D3DXMatrixLookAtRH(&viewMat, cameraOrigin, &at, &up);

screen->SetViewTransform(viewMat);

After setting up the camera transformation like above, I go rendering my
objects:

for each object
{
 D3DXMATRIX mat = object.matrix;
 screen->SetWorldTransform(mat);

 DrawObject();
}

And everything works fine.

But, now I am trying to move a object forward, and this is where my problems
start, for moving my objects, I do:

void moveObject(float dist)
{
 Vec3 fwd;

 fwd.x = objectMatrix._11;
 fwd.y = objectMatrix._21;
 fwd.z = objectMatrix._31;

 fwd *= dist;

 objectOrigin += fwd;
}

This appears to work, but I found out that the objects matrix appears to be
inverted, they will move on very strange directions, if on the above
function I use a invert matrix, everything works fine! I don't know what I
am missing here, because I was expecting the above function to work without
using the inverse matrix.

I have similar code on opengl few years ago and it worked. Does anyone has
any clue what is going on? Or at least, is this expected? :)

Please note for all math, I am using d3dx math functions.

Thank you for your help!

Regards
Bruno Sanches