[vtk-developers] 3x3 rotation matrices

[Aashish Chaudhary]

David,

So the conversion depends upon what this 3x3 represents...if it is 3x3 with
homogeneous coordinate then it wont be like you described in 2). If this
matrix is  a part of 4x4 (so in general 3x3 is representing transformation
such as rotation and scaling)  then yes 2) would work. Also if this 3x3 has
w  != 1 then there may or may not be a need of dividing the 4x4 components
by this.

~ my 2 cents.


On Sun, Jan 10, 2010 at 1:36 PM, David Doria
<daviddoria+vtk at gmail.com<daviddoria%2Bvtk at gmail.com>
> wrote:

> On Sun, Jan 10, 2010 at 1:14 PM, Bill Lorensen <bill.lorensen at gmail.com>
> wrote:
> > Is this a hypothetical question or do you have a specific need? You
> > can always extract a 4x4 and create your  own 3x3 in your application.
> > vtk does not have a vtkMatrix3x3.
> >
> > Bill
> >
> > On Sun, Jan 10, 2010 at 1:08 PM, David Doria <daviddoria+vtk at gmail.com<daviddoria%2Bvtk at gmail.com>>
> wrote:
> >> On Sun, Jan 10, 2010 at 1:03 PM, Bill Lorensen <bill.lorensen at gmail.com>
> wrote:
> >>> vtk uses homogenous coordinates. Why extract a 3x3?
> >>>
> >>> Bill
> >>
> >> If you are implementing an algorithm that has a 3x3 multiplication and
> >> you want to create a rotation matrix. Or for creating a 2d rotation in
> >> homogeneous (now 3d) coordinates. If there is a Matrix3x3 class, you
> >> should be able to create common matrices in that type, no?
> >>
> >> Thanks,
> >>
> >> David
>
> I do have a specific application. I am dealing with camera projection
> matrices (3x4) and to create parts of them (the left 3x3) it seems
> easier to deal directly with 3x3's instead of having to keep
> extracting a 3x3 from a 4x4.
>
> This is the 3x3 matrix class I was referring to:
> http://www.vtk.org/doc/nightly/html/classvtkMatrix3x3.html
>
> I think we need 3 things:
> 1) Add RotateX(), RotateY() and RotateZ() to vtkMatrix3x3
> 2) Convert a 3x3 to a 4x4 (adding a 1 in the (4,4) position and zeros
> elsewhere) - where should this function live?
> 3) Convert a 4x4 to a 3x3 by simply extracting the upper left 3x3 -
> where should this function live?
>
> Thanks,
>
> David
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-- 
| Aashish Chaudhary
| R&D Engineer
| Kitware Inc.
| www.kitware.com
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