[vtkusers] Generic vtkMatrix

[David Doria]

On Wed, Nov 25, 2009 at 12:10 PM, Aashish Chaudhary <
aashish.chaudhary at kitware.com> wrote:

> I think we can use existing data structures (like vtkArray) and have
> vtkVector or vtkMatrix. The reason which I think everyone agrees the
> expressiveness and semantics.
>
> For example I think it would make sense to have transpose and inverse on
> something like vtkMatrix but probably would not be very intuitive to use
> these operations on  vtkArray?
>
> Most of the open source SceneGraph libraries such as OpenSG, OpenSceneGraph
> provide vector, point and matrix classes so I think seeing them in VTK would
> not confuse new users of VTK probably?
>
> Like Marcus said as it is we cannot have locals (which would have been
> nice) but I think I am ok with having it on heap.
>
> ~Thanks,
> Aashish
>
>
>
> On Wed, Nov 25, 2009 at 11:28 AM, Marcus D. Hanwell <
> marcus.hanwell at kitware.com> wrote:
>
>> On Wednesday 25 November 2009 11:07:01 David Doria wrote:
>> > On Wed, Nov 25, 2009 at 10:44 AM, Aashish Chaudhary <
>> >
>> > aashish.chaudhary at kitware.com> wrote:
>> > > I think it will be really useful to have it. Last week I was looking
>> for
>> > > something similar...
>> > >
>> > > Also Passing array (pointers) is not the best thing without passing
>> the
>> > > size of the array which I think will be gone if we pass the vectors
>> (or
>> > > there references)
>> > >
>> > > Regards
>> >
>> > That is a very good point. Also, you could then even return objects
>> >
>> > vtkPoint GetPoint(int index);
>> >
>> > instead of
>> > void GetPoint(int index, double* point);
>>
>> With the current VTK this would not work, as vtkObjects are allocated from
>> the
>> heap. It would be great to have lightweight objects for points and colors
>> for
>> example. They could be returned by value, and the API would be simpler but
>> this API cannot be wrapped as far as I know.
>> >
>> > Another thing is that code readability is tremendously improved. From my
>> > example above, the math equations map directly to the c++ code:
>> >
>> > c = a-b;
>>
>> Wouldn't it be *c = (*a) - (*b); right now if they were vtkObject derived
>> classes following the current heap based semantics?
>> >
>> > rather than having to interpret what is going on when you see something
>> >  like this (or typically even much more complicated):
>> >
>> > for(unsigned int i = 0; i < 3; i++)
>> > {
>> > c[i] = a[i] - b[i];
>> > }
>> >
>> I agree that the code is more expressive. I spent a lot of time using
>> Eigen
>> inside the Avogadro project. Eigen allows for a very expressive syntax.
>> Because of the data structure they employ it also allowed for,
>>
>> std::vector<Eigen::Vector3f> points.
>> points.resize(3);
>> points[0] = Eigen::Vector3f(0.0, 1.0, 2.0);
>> ....
>>
>> You could then grab a pointer to the data, i.e. points[0].data(), and pass
>> that straight to glVertexPointer as in memory it was a float[] of length
>> n*3.
>> Currently I don't think any of this could be wrapped, and so would be of
>> less
>> use. It would also not fit well with the API currently in use in VTK.
>>
>> You can use vtkPoints (and now vtkPoints2D) in a similar way, but the
>> syntax
>> is not as expressive due to dealing with pointers and needing to check the
>> underlying type of the array.
>>
>> I will be pushing more of my work with 2D in VTK which will demonstrate
>> some
>> of the progress I have made whilst accommodating the VTK API.
>>
>> Marcus
>> --
>> Marcus D. Hanwell, Ph.D.
>> R&D Engineer, Kitware Inc.
>> (518) 881-4937
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>
>
>
> --
> Aashish Chaudhary
>
>
Jeff,

You mentioned we may be able to add some of these simple things to vtkMath.
These may be a few functions to kick things off:

void MultiplyScalar(double a[3], const double s)
   {
  for(unsigned int i = 0; i < 3; i++)
     {
     a[i] *= s;
     }
   }

void Add(double a[3], double b[3], double c[3])
   {
   //c = a+b
  for(unsigned int i = 0; i < 3; i++)
     {
     c[i] = a[i] + b[i];
     }
   }

void Subtract(double a[3], double b[3], double c[3])
   {
   //c = a-b
  for(unsigned int i = 0; i < 3; i++)
     {
     c[i] = a[i] - b[i];
     }
   }

Thanks,

David
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