[vtkusers] vtkLookUpTable transparency bug?

Jon Payne jpayne at hgl-dynamics.com
Fri Aug 12 05:56:22 EDT 2011

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
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.


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