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About two and a half weeks ago I sent out a message to this list asking
for general and specific information about 2D vis. and VTK.
I also asked for people's experiences. I wanted to followup up in
the message, let people know what I found out, what I have decided
to do, and ask a few more specific questions.
<p>I want to thank Hank Childs (LLNL) and Robert Stein (NCSA) for their
e-mails. Nothing was posted to this group but I would be happy to
forward their comments (with their permission, of course!) if there was
interest.
<p>Basically they both map their 2D stuff (sometimes using texture
maps) onto 3D objects and then restrict the interaction to 2D (i.e.
only rotations about the z axis) as necessary. This looks like the
approach I will be using. It will actually be a combination
of techniques. First I will produce a composite image from all the
different data grids/images and then I will texture map this onto a 3D rectangle.
All the overlays (grid bounding boxes, contours, etc.) will then be placed
"above" this in Z (with orthogonal projection turned on). I will
be padding the texture map to power of 2 dimensions and then mapping the
unpadded portion (the original data) to my rectangle.
<p>I know VTK will resample my texture to handle this but for this
application it isn't acceptable to resample the data - I need the original
representation and resolution. By the way, it would be nice to have
the option with texture maps in VTK to either resample (current setup)
or pad (and use the unpadded portion of the texture map) - of course, a
lot of things would be nice to have in VTK if you're simply asking
for them, :-).
<p>QUESTIONS (here are a few lingering, specific questions I have
about 2D stuff in VTK):
<ul>
<li>
In "vtkActor2D" there is a "SetLayerNumber" method. I have tried
to use this to set the order in which different actors will be drawn but
it hasn't had any effect. Specifically, I define two 2D actors and
I set one to have layer number 0 and one to have layer number 1.
I add each actor to a "vtkImager" (or "vtkRenderer") and all that seems
to matter is the order the 2D actors are added to the imager (i.e.
the first actor added is drawn first, the second actor added is drawn second).
The layer number doesn't seem to have any effect.</li>
<p><br>My guess is that I am misunderstanding what layer numbers are for
but I haven't been able to find any description of what this is supposed
to do. Several examples use "SetLayerNumber" but when I modified
the example to not use this (or changed the layer numbers around) nothing
changed in the example. The only change occurred when I changed
the order the actors were added to the imager or renderer.</ul>
<ul>
<li>
In the "vtkProperty2D" for an "vtkActor2D" there is a "SetOpacity" method.
It doesn't seem to matter what I set this to, the image I am displaying
(via "vtkImager" or "vtkRenderer") is always opaque. Is this correct
behavior? Is there any way to make an image translucent without changing
the original colormapping to have an opacity component (e.g. an alpha channel/value)?</li>
</ul>
I would still be very interested in any description of the image capabilities
of VTK which included descriptions of the use of the various classes
and the sematics of their methods.
<p>In any case, it has been helpful to have this forum to ask about these
(and other!) things and I thank everyone for their time, help, patience,
and guidance...
<p>
Terry J. (Ligocki, tjligocki@lbl.gov, (510)486-6140)</html>