[vtkusers] Transparency

[Gomez, Alberto]

The vtkPiecewiseFunction class defines a function (that used for color 
transfer function, or opacity transfer function for instance). Add 
segment adds a line to that function. For example,

volumeScalarOpacity->AddSegment(-10,0.5,10,0.5);



creates a straight line in the function from x=-10 to x=+10. In this 
case the line is horizontal (constant ) and the value is 0.5. This means 
that if your image has in one pixel a value of 2, as 2€[-10,10] it will 
be assigned an opacity of 0.5. If you remove a point, that point is 
mapped to 0.

I am not sure how but I believe that you can do the same thing if you 
have a polydata... Maybe someone else is more helpful in that case.


Banks, Robert wrote:
> Will this work with vtkPolyData?
>
> Looking at the class, I'm not sure what the AddSegment() routine is doing. Can you elaborate?
>
> Thanks
> Robbie
>
> Robbie Banks  
>
>
> -----Original Message-----
> From: Gomez, Alberto [mailto:alberto.gomez at kcl.ac.uk] 
> Sent: Tuesday, February 23, 2010 9:49 AM
> To: Banks, Robert
> Cc: 'vtkusers at vtk.org'
> Subject: Re: [vtkusers] Transparency
>
> Hi,
>
> You can set a point of your opacity transfer function (i.e. one scalar 
> value) to transparent without changing the rest. For example, if you are 
> using vtkPiecewiseFunction, then you set it up as you like and you 
> remove the 0 value with the method RemovePoint(0):
>
> vtkSmartPointer<vtkPiecewiseFunction> volumeScalarOpacity =
> vtkSmartPointer<vtkPiecewiseFunction>::New();
>
> volumeScalarOpacity->AddSegment(-10,0.5,10,0.5);
> volumeScalarOpacity->RemovePoint(0);
>
>
>
> Banks, Robert wrote:
>   
>> Hi all.
>>
>> Two questions, although the answer for one, may be the same for the other.
>>
>> I have a dataset that is the cross sectional displacement of a model. However, that model is in void, so there is no displacement in the void. When I plot the info, I get a large band of color around the data relating to the 0 value of the displacement in void. Is it possible to hide or make this data transparent? Or should I window the data?
>>
>> I'm thinking, I could set this color value to transparent, but am not sure how this would affect the rest of the data, having a transparent color data value in the middle of the plotting range.
>>
>> The second question is on a similar vein. Using the same example, if I have a model of, say 5 different materials, which I have defined each material as a different color. Can I just set that color value to fully transparent to hide it in the rendered picture?
>>
>> Or is there a better way to achieve the described problems?
>>
>> Thanks
>> Robbie
>>
>>
>> Robbie Banks
>>
>>
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-- 

Alberto Gómez

/Division of Imaging Sciences
The Rayne Institute
4th Floor, Lambeth Wing
St Thomas' Hospital
London SE1 7EH /

phone: +44 (0) 20 718 88364
email: alberto.gomez at kcl.ac.uk <mailto:alberto.gomez at kcl.ac.uk>