[vtkusers] Scalar data interpreted as 4 component color does not interprete the alpha value correctly

[Barth, Sebastian]

Hi Sebastian,

I've tried with and without peeling. The result is similar but changed a bit:
- The cluster of points (one Actor) seems to be darker than before
- The points in front of the plane seem to be in front of the plane not behind
- The points in front of the plane ignore the plane (not getting brighter)
- The points in front of the plane still get rendered correctly with its alpha value only if the plane behind them is NOT translucent

I've used this How-To:
http://www.itk.org/Wiki/VTK/Depth_Peeling

It really seems to be an ordering problem. Any idea?

VTK makes me getting angry because of its bad documentation (even the books).

Thanks for any help!

Sebastian

-----Ursprüngliche Nachricht-----
Von: Sebastien Jourdain [mailto:sebastien.jourdain at kitware.com] 
Gesendet: Montag, 21. Juni 2010 15:55
An: Barth, Sebastian
Cc: David Gobbi; vtkusers at vtk.org
Betreff: Re: [vtkusers] Scalar data interpreted as 4 component color does not interprete the alpha value correctly

Hi Sebastian,

Did you activate the Depth Peeling on the renderer ?

Seb

On Mon, Jun 21, 2010 at 9:17 AM, Barth, Sebastian
<sebastian.barth at iosb.fraunhofer.de> wrote:
> Hi again, David,
>
> I just noticed that I was wrong: With this work-around vtk does render cloud points only correctly if the white wall of my example I wrote in my first mail is not translucent! If it is, the points will be rendered behind the wall! I'm confused!
>
> Any idea?
>
> Sebastian
>
> -----Ursprüngliche Nachricht-----
> Von: vtkusers-bounces at vtk.org [mailto:vtkusers-bounces at vtk.org] Im Auftrag von Barth, Sebastian
> Gesendet: Montag, 21. Juni 2010 14:40
> An: David Gobbi
> Cc: vtkusers at vtk.org
> Betreff: Re: [vtkusers] Scalar data interpreted as 4 component color does not interprete the alpha value correctly
>
> Hi David,
>
> thanks a lot! That solved my problem.
>
> Does this mean, it's a bug?
>
> I set the opacity of the actor to 0.9999 and it's working now perfectly with the set alpha value in the scalar data. Is this a work around or the common style?
>
> Thanks,
>
> Sebastian
>
> -----Ursprüngliche Nachricht-----
> Von: David Gobbi [mailto:david.gobbi at gmail.com]
> Gesendet: Montag, 21. Juni 2010 13:36
> An: Barth, Sebastian
> Cc: vtkusers at vtk.org
> Betreff: Re: [vtkusers] Scalar data interpreted as 4 component color does not interprete the alpha value correctly
>
> Hi Sebastian,
>
> Could it be a sorting issue?  Translucent actors must be sorted
> back-to-front for alpha compositing to work properly.  Opaque actors
> do not have to be sorted, they are always rendered before any
> translucent actors are rendered.
>
> VTK will assume that any object with 4 color components is a
> translucent object, and it will assume that any object with an Opacity
> less than 1.0 is translucent.  Likewise, objects with 3 color
> components and with an Opacity of 1.0 are assumed to be opaque.
>
>   David
>
>
> On Mon, Jun 21, 2010 at 4:51 AM, Barth, Sebastian
> <sebastian.barth at iosb.fraunhofer.de> wrote:
>> Hello vtkusers,
>>
>>
>>
>> I display a cluster of points stored in a vtkPolyData with one vtkActor.
>> There is also scalar data for every point of the polydata. It's stored in a
>> vtkUnsignedCharArray which contains 4 components (r, g, b and alpha color
>> values) per point that are interpreted as color
>> (mapper->SetColorModeToDefault()).
>>
>>
>>
>> The problem is that the alpha value is intpreted only in respect to the
>> background color. Any other object is ignored!
>>
>> For example: You have a point cloud with only red points that have a alpha
>> value of 128 (the scalar range is 255), represented by the components {255,
>> 0, 0, 128}. The background color is black but between the background and the
>> point cloud is a while plane which hides the background. Normally this wall
>> and the alpha value 128 should color the points in front of the plane more
>> in a brighter red. The opposite happens: The points are getting darker
>> because of the black background behind the plane!
>>
>>
>>
>> Is this a bug or did I forget anything?
>>
>>
>>
>> Thanks a lot,
>>
>>
>>
>> Sebastian
>>
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