semantics of DataReferenceImpl.setData(null) ??

[Curtis Rueden]

Hi Stuart,

Don is right that VisAD does not support setting a data reference to null.

In VisBio I also use a dummy variable initialized to "new Real(0)"
when I want to clear out a data reference. I also sometimes use
DataRenderer.toggle(false) to ensure that nothing gets depicted.

-Curtis

On 9/22/06, Don Murray <dmurray at unidata.ucar.edu> wrote:
> Hi Stuart-
>
> Stuart Maclean wrote:
> > I have an web based visad app which is to render as lines, vertices and
> > labels some 'Route' data structure posted by a browser based system.
> > Each 'waypoint' in a route has a lat,lon,name and pointer to 'next
> >  waypoint.  Lines are to be drawn from a waypoint to its next point.
> >
> > I have 3 DataReferenceImpls objects attached to a display (J2D).  I
> > re-use the display and dataRefs across multiple http calls.
> >
> > If the Route to be rendered contains a single point, there will be no
> > 'lines' (think Rivers.java from the visad examples). In this case, i use
> >
> > dataRefForLines.setData( null );
>
> Somewhere in the depths of my mind, I seem to remember that
> using null for setData is bad.  In the IDV, we don't add a
> DataReference to the display if the data is null.
>
> > This call appears to play havoc with the overall rendering of the image.
> > The image includes the initial vertex and label of previously composed
> > Route objects, i.e. it appears that the call affects the other two
> > DataRefs, one which plots vertices (mathtype is index -> LatLonTuple)
> > and one which draws labels (mathtype is LatLonTuple -> TextType.Generic)
>
> Do you have Cells listening for changes to the dataRefForLines?
> When you say "play havoc" what exactly are you seeing?
>
> > If I replace the above call with
> >
> > dataRefForLines.setData( new Real(0) );
> >
> > all is well.
>
> We do this in the IDV since setting null was considered bad.
> However, we were about to look at that again, so maybe you
> are prompting that investigation.
>
> > I have no idea what is going on.  I can post the code if necessary, it
> > may be easier than trying to explain in the abstract. (I did try the
> > display.reAutoScale as per the mail list archives but no change)
>
> I think posting the code would help.   You can send it to me
> offline if you want and I'll take a look when I get a chance
> (or post to the group and maybe someone else will chime in).
>
> Don
> *************************************************************
> Don Murray                               UCAR Unidata Program
> dmurray at unidata.ucar.edu                        P.O. Box 3000
> (303) 497-8628                              Boulder, CO 80307
> http://www.unidata.ucar.edu/staff/donm
> *************************************************************
>
>
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