Getting Viewing direction from World Matrix

OpenGL OpenGL: Advanced Coding
1

Hello?
Can someone tell me how can i get the viewing
vector from the World Matrix?

2

You can get the view vector by doing the following:

First you invert the View-Matrix and than you multipy the invers-View-Matrix with the vector (0,0,1)

3

is it necessary to invert it, i just use

forward_direction_vector = VECTOR( use_cam_rotMATRIX.m[2], use_cam_rotMATRIX.m[5], use_cam_rotMATRIX.m[8] );
which has worked flawlessly until now.
could this run into problems?

4

This is how I do it in my camera class and it works great.

Matrix4 camRot, cx, cy;

camRot.ToIdentity();
cx.ToIdentity();
cy.ToIdentity();
	
cx.RotateX( m_fPitchAngle * DEGTORAD );
cy.RotateY( m_fYawAngle   * DEGTORAD );

camRot = cy * cx;

vector_t dir = camRot * m_vVelocity;

if( dt > 0 )
	VectorMulFloat( &dir, dir, dt );

-SirKnight

5

Originally posted by zed:
[b]is it necessary to invert it, i just use

forward_direction_vector = VECTOR( use_cam_rotMATRIX.m[2], use_cam_rotMATRIX.m[5], use_cam_rotMATRIX.m[8] );
which has worked flawlessly until now.
could this run into problems?[/b]
unless you have scaling or sheering in your matrix (which shouldnt be the case) the inverted rotation part is the transposed. multiplying the "z"vector with that should result in the z axis of the matrix. ie. it should be pretty much what youre doing.

6

None of this is the viewing vector if the model has been orientated in world space using the modelview matrix. You’re extracting a vector that is the concatonation of the models transform and the viewing (camera) transform.
In d3d the 2 transforms are kept seperate, but in GL they’re fused into 1 transform.

7

Originally posted by knackered:
None of this is the viewing vector if the model has been orientated in world space using the modelview matrix.
hm, i guess we simply expected he is aware that he has to do it BEFORE any additional transformations are applied. but i admit i like the seperate matrices of d3d better.