[vtkusers] vtkColorTransferFunction and opacity

[Steve Joyce]

Lisa,

Thanks for your reply.

In my case I wanted to set the transparency for each dataset and
judging from previous postings other people have wanted to do the same.
Certainly this is the behaviour you would expect from
vtkActor.GetProperty().SetOpacity(...). This opacity eventually gets set
as the alpha value in vtkScalarsToColors.

I agree that having an opacity per scalar value would also be useful.

Thanks,
Steve

>>> Lisa Avila <lisa.avila at kitware.com> 28/01/2004 15:48:56 >>>
Hi Steve,

I think this Alpha value is not used because it is part of the
superclass 
vtkScalarsToColors and vtkColorTransferFunction was not originally a 
subclass of this class. It can be used - but I suspect you will need to

test that the alpha value is being set everywhere it needs to be set.
This 
also only allows you to have a single opacity for the whole image - and
I 
think what people want is an opacity per scalar value. What we real
need is 
to associate a vtkPiecewiseFunction with a vtkColorTransferFunction for

this - of course these are separately specified in the
vtkVolumeProperty 
for volume rendering so this could be confusing...

Lisa


At 10:03 AM 1/28/2004, Steve Joyce wrote:
>Hi,
>
>Setting the opacity of actors does not work when using a
>vtkColorTransferFunction as the colour lookup table. This is because
the
>alpha value for a colour is always returned as 255 by
>vtkColorTransferFunction.
>
>As a fix for this I replaced the places where this occurred in
>vtkColorTransferFunction  with
>(unsigned char)(this->GetAlpha() * 255.0)
>or
>(unsigned char)(self->GetAlpha() * 255.0)
>as appropriate.
>
>This seemed to work for me, so I was wondering why it was fixed at
255
>in the first place. Am I doing the wrong thing?
>
>Thanks,
>Steve
>
>
>
>
>
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