[vtkusers] VtkUnstructuredGrid SetOpacity

[David E DeMarle]

On Wed, Jan 25, 2012 at 5:52 PM, Bhanu Hariharan <bhanu at petrotel.com> wrote:

> David, Thanks for replying.
>
> When I flip, I see blue AGAIN. Is that what you expect with the external
> surface rendering logic you explained?
>
> What is not clear is even if there are 2 surfaces, shouldnt I be seeing a
> blend of blue and green?
> And shouldnt it be same whether I see from top or flipped?
>
>
That is what I was trying to figure out. I suspect you see only blue
because those are the first polygons rendered in the object. Correctly
rendering translucent geometry is order dependent and VTK doesn't do depth
sorting automatically (since it takes time). Try enabling depth peeling. If
your graphics card doesn't support depth peeling use vtkDepthSortPolyData.
You should then see a blueish green object and that should be bluer or
greener depending on what is nearer. However you still won't see anything
on the inside.

Bhanu
>
>
>
> On Wed, Jan 25, 2012 at 4:26 PM, David E DeMarle <dave.demarle at kitware.com
> > wrote:
>
>> When you flip the object around, do you see only green?
>>
>> In any case, the typical mapper people use for surface rendering,
>> vtkDataSetMapper, extracts the external surfaces. With that there is no
>> possibility to see the inner cells even with opacity.
>>
>> You could instead send the data through something like the shrink filter,
>>  to  break up the object so that all cells are unconnected and thus
>> external. Surface rendering with opacity then might to what you want.
>>
>> However, it is much better to use volume rendering. That will be much
>> faster and consume less memory.
>>
>> David E DeMarle
>> Kitware, Inc.
>> R&D Engineer
>> 21 Corporate Drive
>> Clifton Park, NY 12065-8662
>> Phone: 518-881-4909
>>
>>
>>
>> On Wed, Jan 25, 2012 at 4:58 PM, bhanu <bhanu at petrotel.com> wrote:
>>
>>> Hi,
>>>
>>> I am using VtkUnstructuredGrid to render a simple volume wherein each
>>> grid
>>> cell in the 3dvolume has 8 world coordinates. Each grid cell has a color
>>> associated with it and I am using a vtkLookUpTable for this. When I use
>>> SetOpacity(1) and see the top view, I expect to see the top layer alone
>>> since I have set an opacity of 1.0. And that works fine.
>>> Now When the opacity is changed to anything less than 1 (even 0.99), it
>>> seems the top layer vanishes and all I see from the top is the bottom
>>> most
>>> layer.
>>> For example, if top most layer is green and the bottom most layer is blue
>>> (with inbetween layers shades of lighter green), with opacity less than
>>> 1, I
>>> see all blue from the top. I was expecting mixed colors( green and blue).
>>> What am I missing.
>>>
>>> How do you see a transparent grid using VtkUnstructuredGrid and Opacity
>>> property. Should I not be using Scalar Opacity for such a problem. Is
>>> this
>>> where one uses Gradient Opacity.
>>> Can anyone please paste some code to direct me here.
>>>
>>> Thanks,
>>> Bhanu
>>>
>>> --
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>>
>
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