[vtk-developers] vtkMatrix4x4 and VisSimActor

[obara]

Hi Folks,

John Tourtellot's email on June 18,2001 stated :

>Issue 2: glMultMatrixd() is called by vtkOpenGLActor::Render(). In the 
>general case, this is needed to render the actor in the right place. In 
>our applications, however, the actors nearly always have an identity 
>matrix, making the glMultMatrixd() call unnecessary. (Mark Beall posted  
>this to the developers list on May 17, but there were no responses.) 
>
>Action: We added an "identity" flag to the vtkMatrix4x4 class to keep 
>track of its state. It's not ideal since the matrix elements in this 
>class are public (i.e., someone can change the matrix without the class, 
>or our identity flag, knowing about it). We also, of course, modified
>the 
>vtkOpenGLActor::Render() method to check the matrix identity flag before 
>calling glMultMatrixd(). 
>


This introduces a problem with the way that the matrix class is 
implemented.  The actual matrix of floats is public and therefore makes 
it impossible in general to track the state of the Matrix.  This is only 
a problem of course if the anyone directly manipulates the matrix and it 
seems (based on a quick grep of vtk's source) that some classes do 
directly manipulate the matrix's contents.

The main reason for testing if the matrix is an identity transform is to 
see if geometry needs to be transformed and in the case of an 
OpenGlActor, to see if a matrix needs to be pushed on the transformation 
stack.

There seems to be 2 possible approaches in getting around this efficiency 
issue:

1. If the actual matrix part is protected, as a result the user can 
"tell" the matrix it is identity and if the matrix object is modified via 
its member functions then it could set its state not to be identity. 

2. The alternative is to change Actor to have an member function that 
tells the actor it's coordinate system is the same as the world system 
(in this case it doesn't even create a transformation matrix at all) or 
that its defined in its own modeling coordinate system (internally it 
then can allocate a transformation matrix). 

The problem with 1 is that a bunch of code will not compile.  The problem 
with 2 is that a function asking for the transformation of the actor 
would need to see if they were given a null pointer which is not done 
(since currently it could never happen) - hence with option 2 we would 
have to deal with run-time errors.

Any ideas as to how we (the vtk community) should proceed ?

Bob


Robert M. O'Bara
Senior Software Engineer
Simmetrix Inc.
1223 Peoples Ave.
Troy, NY 12180

Lab: (518) 276-2867
Main Office: (518) 276-2729