[vtkusers] Why data value range of volume affects its rendering --->I think it is a bug, please read my reply

[Weiguang Guan]

Hi James,

Thank you so much for your detailed insight into the problem. Can Kitware 
people confirm this is a bug?

Weiguang

On Mon, 29 Oct 2007, James Zhou wrote:

> Hi Weiguang,
>
> I am estimating the vtkVolumeTextureMapper3D these days. I found some
> problems (maybe) about this class. This problem does answer your question
> why the rendering is different from two different mappers.
>
> I am using vtk nightly source on Oct.16 2007. Here I present some cues on
> the problem:
>
> First please read function "UpdateColorLookup" in class
> vtkVolumeTextureMapper3D. We may assume our data type is unsigned char. We
> also assume we use rgba color TF. So the the size of color TF write into the
> ColorLookup is arraySizeNeeded as shown in the program. Line 1334 and line
> 1339 gives us that the pipeline how to get the TF table from the TF user
> interface with the scalar range of ( scalarRange[0], scalarRange[1]).
>
>      rgbFunc->GetTable( scalarRange[0], scalarRange[1],
>                         arraySizeNeeded, this->TempArray1 );
>
>  scalarOpacityFunc->GetTable( scalarRange[0], scalarRange[1],
>                               arraySizeNeeded, this->TempArray2 );
>
> Second  please read Lines between 1368 to 1401 (we use one component data),
> you can find that the program transfered TF table obtained before to the
> ColorLookup. The problems appear here:
>
> The TF table obtained in Line 1334 and 1339 is used for the scalar range of
> ( scalarRange[0], scalarRange[1]). But between lines 1368 to 1401
> transferring step, the TF table is set to the range of (0, arraySizeNeeded).
> Please be aware that:
> ( arraySizeNeeded = (int)(scalarRange[1] - scalarRange[0] + 1);)
>
> So the transfer function got from the TF user interface is applied to the
> wrong scalar range, that's why the rendering images created from two mappers
> are different.
>
> I hope to get your feedback about this. I also hope that VTK can correct
> this bug. Because the new graphics card supports non-power 2 texture size,
> the vtkVolumeTextureMapper3D should also be updated for this change.
>
> Good luck!
>
> James
>
>
>
>
>
>
> 2007/10/27, Weiguang Guan <guanw at rhpcs.mcmaster.ca>:
>>
>> Hi all,
>>
>> I recently notice an issue (maybe a bug) with volume rendering algorithms
>> (vtkVolumeTextureMapper3D and vtkVolumeRayCastMapper), and cannot find a
>> posting reporting the issue in the mailing list.
>>
>> I have a a scalar volume of vtkImagedata whose value ranges from 0 to
>> 7035. By thresholding it at 255, I generate another volume with value
>> range [0-255]. Then, I do volume renderings (both vtkVolumeTextureMapper3D
>> and vtkVolumeRayCastMapper) of them seperately with the same opacity
>> transfer function:
>>
>> vtkPiecewiseFunction opacityTransferFunction
>>      opacityTransferFunction AddPoint  50   0.0
>>      opacityTransferFunction AddPoint  255  0.2
>>      opacityTransferFunction ClampingOff
>>
>> Should I expect to have exactly the same renderings? Rendering with
>> vtkVolumeTextureMapper3D produces a great difference while rendering with
>> vtkVolumeRayCastMapper produces a slightly difference but still
>> noticeable. Can anybody explain why?
>>
>> I have a tcl script to reproduce the "bug", and can let you have it if
>> you're interested.
>>
>> Weiguang
>>
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>