[vtkusers] Data spacing affects volume rendering quality?

[Sankhesh Jhaveri]

Hi Elvis,

There are flags on the volume mapper that allow setting the distance
between samples along the ray. By default, the sample distance is computed
as 1/2 the average spacing throughout the volume. If you have
AutoAdjustSampleDistances enabled, the mapper computes the sample distance
each frame based on the time taken to render the last frame. For consistent
quality, you should disable AutoAdjustSampleDistances and set the sample
distance explicitly. That way, you can leave the volume spacing in the
units you desire.


*Sankhesh Jhaveri*
*Sr. Research & Development Engineer* | Kitware
<http://www.kitware.com> | (518)
881-4417




On Mon, Sep 12, 2016 at 8:56 AM, Elvis Stansvik <
elvis.stansvik at orexplore.com> wrote:

> 2016-09-12 14:01 GMT+02:00 Elvis Stansvik <elvis.stansvik at orexplore.com>:
>
>> Hi all,
>>
>> I've just noticed the there's a large difference in rendering quality if
>> I decrease the spacing of my data (using vtkGPUVolumeRayCastMapper).
>>
>> I found this very old post from 2007 about the same problem:
>>
>>     http://public.kitware.com/pipermail/vtkusers/2007-March/040559.html
>>
>> I was originally using a data spacing of 0.12, which is the voxel size in
>> centimeters for the particular volume I'm working with (stored in our data
>> files), but after I tried to divide it by 100.0 during loading, rendering
>> quality dropped considerably. The reason I wanted to do this is because
>> it's more convenient if VTK world coordinates are in meters in this
>> particular application, not centimeters.
>>
>> Anyone know why volume rendering quality seems dependant on the data
>> spacing?
>>
>
> I now found another more recent post from 2014 which describes in more
> detail what I seem to experience as well (seemingly also with no answer):
>
>     http://public.kitware.com/pipermail/vtkusers/2014-March/083311.html
>
> I get the expected rendering when the spacing is ~1.
>
> Must I make sure that my volumes have a spacing ~1? I'm willing to abandon
> the idea of having VTK world coordinates == meters and introduce another
> scale, but it's a bit cumbersome as I'm working with multiple volumes which
> I add to the renderer at once, and not all of them will have the same
> spacing. This means I must first analyse the volumes I'm going to load a
> priori, and calculate a factor common to them all that will bring all of
> their spacings as close to 1 as possible :(
>
> Would be great if someone in the know could chime in. Is there some
> workaround? Can I instruct the mapper to "expect" the small spacings I want
> to work with?
>
> Elvis
>
>
>> Thanks for any clarifications!
>>
>> Elvis
>>
>
>
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