[vtk-developers] My vtk class override not being used when an override already exists.

[David Thompson]

Hi all,

> I have been having some conversations about the factories ... One feedback I already received (from David Camp, LBNL) was that, due to string comparisons used, object factories have a significant impact on performance when creating large number of objects. ... We could potentially revisit the way factories work and optimize it a bit.

Are you thinking of something like this:

  http://www.altdevblogaday.com/2011/10/27/quasi-compile-time-string-hashing/

? I think it would be a useful utility in general (for more than just object overrides).

	David
> 
> On Wed, Feb 13, 2013 at 8:44 PM, Kathleen S. Biagas <biagas2 at llnl.gov> wrote:
>> On Wed, 2013-02-13 at 08:50 -0500, Marcus D. Hanwell wrote:
>>> On Tue, Feb 12, 2013 at 8:54 PM, Kathleen S. Biagas <biagas2 at llnl.gov> wrote:
>>>> Trying to come up to speed with current VTK master, and ran across
>>>> issues trying to override vtk classes.
>>>> 
>>>> One thing I noticed was that I could not override the classes I was
>>>> interested in without first building VTK with VTK_ALL_NEW_OBJECT_FACTORY
>>>> enabled.  So my first question is:  are more classes going to be opened
>>>> up for overriding within the new scheme of things without having to set
>>>> this flag?  Or will this flag be enabled by default in the future?  We
>>>> eventually want our code to compile with a user's preinstalled VTK, and
>>>> there wouldn't be a guarantee this flag would have been used.
>>> 
>>> I would be interested in knowing which classes you are interested in
>>> overriding that were not. There are several new macros now, and as you
>>> discovered the default one no longer uses the object factory without
>>> the build flag you mentioned. The plan was to leave that off by
>>> default, and add more classes where runtime overrides are needed.
>> 
>> One of the classes we override is vtkDataSetMapper, so that we can
>> instantiate specialized mappers for vtkRectilinearGrids and
>> vtkStructuredGrids.  Another is vtkCellDataToPointData, again with
>> specialized handling for vtkStructuredGrids and vtkRectilinearGrids.
>> We also override vtkRectilinearGrid and vtkStructuredGrid to provide
>> implementations with smaller memory footprints.
>> 
>> These are all handled correctly if we enable VTK_ALL_NEW_OBJECT_FACTORY.
>> 
>>>> 
>>>> The other thing I noticed was a change in behavior.  To illustrate, I
>>>> copied the TestObjectFactory.cxx from Common/Core/Testing/cxx, and
>>>> modified it to override vtkCamera instead of vtkPoints.
>>>> 
>>>> When built against an older version of VTK, say 5.8, the results show
>>>> that when vtkCamera is instantiated, the correct version (either
>>>> vtkTestCamera or vtkTestCamera2) is succesfully created. Running through
>>>> the vtkOverrideInformationCollection printing OverrideWithName, only two
>>>> are shown: vtkTestCamera and vtkTestCamera2.
>>>> 
>>>> When built against latest version of master, the results show that when
>>>> vtkCamera is instantiated, a vtkOpenGLCamera is created instead of
>>>> either of the registered overrides in the test (vtkTestCamera or
>>>> vtkTestCamera2). Running through the vtkOverrideInformationCollection,
>>>> and printing the OverrideWithName, three are shown: vtkOpenGLCamera,
>>>> vtkTestCamera and vtkTestCamera2.
>>>> 
>>> That is because the old VTK didn't use the object factory for the
>>> default overrides, and so you could come in and add an additional
>>> override. Now that we use the object factory internally that is
>>> generally the first. As far as I can tell the object factory mechanism
>>> has always returned the first match by default, and will only provide
>>> a list if you use that part of the API.
>> 
>> Aah, understand this aspect now, thanks.
>>>> 
>>>> In the new scheme of things, how would I go about ensuring my override
>>>> takes precedence over any default ones? eg vtkTestCamera over
>>>> vtkOpenGLCamera?
>>> 
>>> The object factory API would need to be modified in some way, this
>>> functionality has never been present. The old hardwired factory
>>> classes for particular kits would first check the object factory, and
>>> if nothing was returned construct the class that was set at compile
>>> time. This meant that effectively you could override the builtin
>>> class, but only once.
>>> 
>>> It would be great to know more about what you are doing, and what
>>> classes you are thinking of in VTK that you would like to override.
>>> 
>> One of the things we have is a Rendering module, and it overrides things
>> like vtkCamera.  Even enabling the VTK_ALL_NEW_OBJECT_FACTORY flag
>> doesn't allow this to work correctly, due to the OpenGL overrides
>> already existing.
>> 
>> I did a little tweaking of the build for this, making it a proper
>> 'Module' in the VTK sense, following instructions I found on
>> http://vtk.org/Wiki/VTK/Module_Development#External_modules
>> 
>> I ran into a problem in that vtkExternalModuleMacros.cmake was not
>> included in the VTK install tree.  Is there a specific flag to set
>> during build that allows this to be installed?  In the meantime, I
>> copied this file to the installation, but building this code as a
>> vtkModule still doesn't allow the overrides defined by the module to
>> take precedence.
>> 
>> I thought of using vtkObjectFactory::SetAllEnableFlags(0, "vtkCamera")
>> before registering the rendering module's factory, but wasn't sure if
>> that was overkill ...
>> 
>> Thanks for the feedback!
>> 
>> Kathleen
>> 
>>> Marcus
>> 
>> 
>> 
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