bypassing in vtkImageFilter's

[Kenneth Wang]

Hi,

I'm having a rather confusing problem with an imaging filter, and I
was wondering if anyone might be able to shed some light on my
situation.  I have a simple pipeline which looks like this (this is
using the official 2.4 release):


          vtkImageReader
                |
          vtkImageMathematics (do a 1-input operation)
                |
          vtkImageToStructuredPoints
                |
          vtkExtractVOI (extract slice)
                |
          vtkTexture
                |
         .... continue to texture map to plane


This pipeline has been working well for me for a while now.  Yesterday
I introduced vtkImageMathematics to do some simple stuff, and I want
to be able to toggle that processing on or off.  So I use the BypassOn
/ BypassOff methods in vtkImageMathematics which are inherited from
vtkImageFilter.

However, these really aren't working quite the way I expect.  If I
initialize with BypassOn, then the texture mapped plane is as
expected: not processed by my simple operation in vtkImageMathematics.
When I turn BypassOff, here again I see the expected result: the
processed image.  But, if I now call BypassOn once again, I don't get
the original image back.  Any methods based on vtkSetMacro should
update modification time, right?  So a change to the Bypass ivar
should cause the pipeline to update when a request comes along, right?
Calling Update directly on the image filter, preceding such a call
with an explicit call to Modified, and generating update requests from
the end of the pipeline with Render's and mouse manipulations do not
seem to cause the vtkImageMathematics filter to update (i.e. the
rendered image does not update).

In fact, I'm seeing something rather stranger, which is that each call
to BypassOn seems to lead to *further* processing of the image (as
opposed to a reversion to the original state).  I'm having trouble
understanding this, since the filter constants (ConstantC and
ConstantK) are unchanged, and the reader source is also unchanged.
Nor is the scalar range on my mappers changed, so the rendered
differences are not the result of differences in color mapping.

So while this question is superficially about something perhaps
relatively unimportant (i.e. bypassing in image filters, which could
always be dealt with by reconfiguring the pipeline directly I
suppose), I fear I may be suffering from some deeper, much more
fundamental problem with imaging pipelines in general.  Does anyone
have any advice?

Thanks,
Ken


------------------------------------------------------------------------
Kenneth C. Wang			E-mail: kenwang at stanfordalumni.org
Electrical Engineering Dept.	Voice:  (650) 725-0458
Stanford University		Fax:    (650) 725-7731
CIS 214, Stanford, CA 94305	Web:    http://www.stanford.edu/~kenwang
------------------------------------------------------------------------


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