[vtkusers] Thresholding with Transparency

[David Gobbi]

Hi Ben,

If you want transparency, you need to use a filter that will add an 
alpha component to your data.  The vtkImageThreshold filter won't do that.

It might seem counterintuitive, but the best way to apply a transparency 
threshold to your data (assuming that it is greyscale data and not RGB) 
is with vtkImageMapToColors.  Just create a vtkLookupTable that keeps 
the same grey value for all pixels, but which sets the alpha value of 
all pixels that are below threshold to zero.  Color mapping is a very 
efficient operation so this should work well with real-time video.

 - David

Ben Wilde wrote:

> Hello,
>
> I am very, very new to VTK, and I'm trying to do some video processing 
> that I don't imagine should be too complicated.
>
> I have some video data that I would like to apply a threshold to, in 
> which only the image above the threshold appears, and the 
> non-thresholded sections are transparent.
>
> I am using am using video.getOutput to turn my video into an image.  I 
> have looked at using vtkImageThreshold, but I can't figure out how to 
> make the thresholded parts of the image transparent.  Am I missing 
> something easy?  If not, does anybody have suggestions on how to proceed?
>
> I apologize if this is too easy or if I misused any terminology.  
> Again, I'm am about as much of a beginner as you can be.
>
> Thank you,
> Ben
>
>
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