[vtk-developers] ImageSampleDistance not used in vtkOpenGLGPURayCastMapper?

[Elvis Stansvik]

2017-02-13 17:10 GMT+01:00 Sankhesh Jhaveri <sankhesh.jhaveri at kitware.com>:
> Hi Elvis,
>
> The ImageSampleDistance property was left out when coming up with the new
> OpenGL2 volume mapper to avoid an additional step of rendering the volume to
> an FBO and then using the texture to compute the scaling. This simplifies
> the code path and reduces the extra performance overhead. The documentation
> could have been better on this. We'll update it.

Ah, that explains it.

>
> That being said, do you have a specific use case for ImageSampleDistance? We
> could explore options for adding the feature back in.

I don't have a specific use case for it really. The reason I started
looking into how the auto-adjustment works is that I've found it to be
a little too aggressive in reducing the quality. I found that if I
turned off auto-adjustment and hardcode the sample distance, I could
get acceptable interactive frame rate at reasonable zoom level with a
quality that looked better than what happens when the auto-adjustment
kicks in. So I started looking at how the auto-adjustment works, and
if perhaps I could make my own heuristic for degrading the quality. I
was thus interested in what knobs for quality were available to me.

I think what I'm really looking for is some way of setting a lower
bound on the actual sample distance along the ray
(ActualSampleDistance internally), similar to
MinimumImageSampleDistance but for the sample distance.

Has anyone else found that the auto-adjustment is seemingly a bit too
aggressive? I'm keeping the desired frame rate of the render window at
the default (15 fps I think), but the quality during interaction
degrades to an unnecessary extent I think..

Elvis

>
> Thanks,
> Sankhesh
>
>
> On Mon, Feb 13, 2017 at 6:55 AM Elvis Stansvik
> <elvis.stansvik at orexplore.com> wrote:
>>
>> 2017-02-13 12:54 GMT+01:00 Elvis Stansvik <elvis.stansvik at orexplore.com>:
>> > I was digging through the code trying to understand how the
>> > auto-adjustment of sample distance and image sample distance takes
>> > place in the GPU volume ray cast mapper. From the looks of it, the
>> > ImageSampleDistance is not respected by this mapper, even in the
>> > non-automatic case, or am I missing something?
>>
>> Sorry, with "non-automatic case" I mean when AutoAdjustSampleDistance is
>> Off.
>>
>> Elvis
>>
>> >
>> > The only role ImageSampleDistance seems to play for this mapper is
>> > when the reduction factor is calculated (when auto-adjustment is
>> > turned off):
>> >
>> >   if ( !this->AutoAdjustSampleDistances )
>> >   {
>> >     this->ReductionFactor = 1.0 / this->ImageSampleDistance;
>> >     return;
>> >   }
>> >
>> > Elvis
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> --
>
> Sankhesh Jhaveri
>
> Sr. Research & Development Engineer | Kitware | (518) 881-4417