[vtkusers] vtkLookUpTable transparency bug?

[Artem Babayan]

Hi, Jon,

I did assign alpha 0.5 to the 'invisible' part -- it appears, that
transparency works only for some subsets of possible graphics
positions -- see attached pictures.

Thank you for trying to help, by the way.

Best wishes
Artem

> Hi Artem,
>
> I can't remember exactly but have you tried setting the alpha of the whole
> surface to 0.95? The reason is that somehow VTK/OpenGL needs to know to
> ignore the z-buffer value (I think) so that pixels behind other pixels which
> are now transparent are still drawn.
>
> Cheers,
> Jon

> On 12/08/2011 09:08, Artem Babayan wrote:
>>
>> Hi, Thomas
>>
>> If you look at the second picture, you will see, that some of the
>> 'colorful' graphic is covered by the white (or background color) field
>> -- this white bit is the part of the graphics which correspond to
>> 'undefined' values and have alpha=0. So it is not transparent -- I
>> cannot see remaining graphics through it.
>>
>> To make it more clear (or more confusing) -- with alpha=1  the first
>> picture would look like a colorful 'mountain' in the flat black
>> 'field' extended to the vertical boundaries of the domain. With
>> alpha=0 the black 'field' is of background color, so it is invisible
>> on the first picture, but if I tilt the graphics slightly backwards
>> (as on the second picture) part of the 'field' comes in front of the
>> 'mountain' and you can see, that it is not actually transparent.
>>
>> Best wishes
>> Artem
>>
>>
>>
>> On 12 August 2011 08:42, Kilgus, Thomas<t.kilgus at dkfz-heidelberg.de>
>>  wrote:
>>>
>>> Hi Artem,
>>>
>>> I don't if I get you right, but the pictures look correct. If you set the
>>> alpha value of something to 0, you will see the color of the stuff behind
>>> it. I guess in your case there is nothing behind it and that's why the
>>> background color can be seen?
>>>
>>> Best regards
>>> Thomas
>>>
>>> -----Ursprüngliche Nachricht-----
>>> Von: vtkusers-bounces at vtk.org [mailto:vtkusers-bounces at vtk.org] Im
>>> Auftrag von Artem Babayan
>>> Gesendet: Donnerstag, 11. August 2011 19:13
>>> An: vtkusers at vtk.org
>>> Betreff: [vtkusers] vtkLookUpTable transparency bug?
>>>
>>> Hi,
>>>
>>> I am trying to visualise a 2D scalar field as a 3D 'landscape'. Some of
>>> the numbers are just indicators of 'undefined' values, so I do not want to
>>> show them at all.
>>>
>>> I set these 'undefined' numbers to the values which correspond to the
>>> first row of vtkLookUpTable and then assign this row 'transparent'
>>> alpha=0:
>>>
>>>     lut->SetTableValue(0, 0, 0, 0, 0);
>>>
>>> However, there are 2 problems:
>>>
>>> 1) This results in 'background' color, not 'transparent' -- see second
>>> picture -- the transparent part actually closes some part of the graphics.
>>> 2) On one Unix system I haven't even got transparency -- the
>>> 'transparent' part was drawn in black color (as corresponds to the table
>>> value).
>>>
>>> So am I doing something wrong here or is it a bug in vtkLookUpTable?
>>>
>>> Also, may be there is a smarter way not to show 'undefined' values?
>>>
>>> Best wishes
>>> Artem
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