First, Thank you Paul, answer my question, I am new to this newsletter so I
didn't write thank you at last letter.
huiliang yang wrote:
> Hi,
>
> Hello
>
> >huiliang yang wrote:
> > I have a question about computeLocalToWorldMatrix in camera class.
> > First, since the openscenegraph use row matrix instead column matrix
> >like opengl, I thought the default matrix multiply should be post
> > multiply so that :
> > V' = M*V (opengl premultiply) chanage to
> > V' = VT*MT ( osg post multiply)
> Ok
> > but looks like the camera still use pre_multiply as default, but the
> > _viewmatrix define in camera class is apparently is a row matrix MT.
> How do you get to this statement, which section of the code do you
> refer to?
>
> no any specific code for above statement. but I do trace the code and
> find _viewmatrix in camera class
> is a row matrix, which means the translation (x, y, z) is at last row.
Ok
> So, if the matrix is a row matrix, only way for multiply is
> by matrix = matrix*_viewmatrix. or matrix.postmultiply(_viewmatrix).
> is that correct?
No, why would that be the case?
All matrices in OSG are of type osg::Matrixd (or Matrixf), which as you
mentioned, has the translation part at the bottom row. In your multiply
statement above (matrix = matrix*_viewmatrix) you say that because
_viewmatrix is a row matrix it can only be post-multiplied. But what
about "matrix"? It's also a row matrix, but it's being PRE-multiplied.
As long as two matrices A and B have the same structure (translation at
the bottom row/right-most column) you can compute both A*B and B*A. The
results will be different, of course, but it's both "legal" in the
mathematical sense. The fact that _viewmatrix is a model-view matrix
doesn't matter, as that is simply also a form of linear transformation
expressed by a 4x4 matrix.
What I mean is: if both matrix are col matrix, then a transformation from local
to world should
matrix(world) = _viewmatrix*matrix(local);
Then back to osg's row matrix, which is the transpose of col matrix, then
matrix(world)T = (_viewmatrix*matrix(local))T ; T mean transpose.
= matrix(local)T*_viewmatrixT :
= matrix(local).postmult(_viewmatrix) ; should be post
multiply.
but look computeLocalToWorldMatrix: it is using
if( premultiply)
matrix(local).premult(_viewmatrix); which is
_viewmatrixT*matrix(local)T; since they are both
row matrix, transposed.
= matrix*_viewmatrix; here it totally change
the order of transformation.
I agree matrix multiplication can be done either way, but I if the order of
transformation shouldn't be changed. Thanks for answer my question.
> That is why I say why the default multiply in camera class is
> premultiply, in computeLocalToWorldMatrix,
> it use matrix = _viewmatrix*matrix or
> matrix.premultiply(_viewmatrix), if _viewmatrix is a row matrix, it
> looks weird.
>
> It also depends on your definition of 'pre-multiply'. If you say VT*MT
> is "post multiply" (as you do above), then this implies that you
> actually mean "MT post-multiplies VT". This is turn also means that "VT
> pre-multiplies MT". So it depends from which operand you look at it how
> you describe the operation.
> >
> > Second, even as the default pre_multiply, then let's say:
> > V' = M*V if (_transformOrder==PRE_MULTIPLY)
> > then its inverse should be
> > V = M-1* V' ,
> > why in function computeWorldToLocalMatrix
> Which class? You say "V", which implies a vector usually, but, for
> example, Camera::computeWorldToLocalMatrix works on a matrix as input.
> If you indeed refer to Camera::computeWorldToLocalMatrix() then I would
> say that that method does the inverse operation of
> computeLocalToWorldMatrix(). Assuming _referenceFrame==RELATIVE_RF and
> _transformOrder==PRE_MULTIPLY then the method computeLocalToWorldMatrix
> pre-multiplies its argument "matrix" with "_viewMatrix". I.e.
>
> matrix.preMult(_viewMatrix);
>
> actually does
>
> matrix = _viewMatrix * matrix;
>
> Method computeWorldToLocalMatrix() does the inverse of this operation,
> so instead of pre-multiplying with _viewMatrix it post-multiplies with
> the inverse of _viewMatrix (the comments also say this).
>
> That is I don't understand, if matrix' = _viewMatrix*matrix, then the
> inverse operation is
> matrix = _viewMatrix-1*matrix', right?
> why here it change the order of multiply?
local_to_world = _viewMatrix * matrix
=>
world_to_local = [local_to_world]^{-1} = [_viewMatrix * matrix]^{-1} =
matrix^{-1} * _viewMatrix^{-1}
(inverse_of_A*B = inverse_of_B * inverse_of_A)
Ok I get it, the function is to create a transformation matrix, I thought it is
using for vector transformation. Thanks.
Paul
Regards
Huiliang
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