[vtkusers] How does VTK OSPRay-based volume rendering produce smooth images?

[Alexander Devaykin]

Hello,

sorry for late reply and thank you for the answers!
I actually managed to achieve nice smooth surface of the volume thanks to
your help! And I didn't see any significant performance change at least on
the datasets at hand.

Best regards,
Alexander

On Thu, Jul 6, 2017 at 7:52 PM, Brownlee, Carson <carson.brownlee at intel.com>
wrote:

> The smoothness of the resulting image is based on several factors,
> notably: number of accumulation passes, number of samples per pixel,
> samplingStep, and adaptive sampling settings.  Please see the OSRPay
> documentation for an explanation of each.
>
> The volume renderer in VTK appears smoother because the samplingStep and
> adaptive sampling settings were set for taking more samples than the
> viewers that come with OSPRay, which were designed  speed in mind and rely
> on progressive rendering to smooth the image after the first frame.  We
> assumed that with VTK and ParaView that most users would not be using
> progressive refinement, which unfortunately is likely a mostly unknown
> feature in ParaView.
> Carson
> ------------------------------
> *From:* David E DeMarle [dave.demarle at kitware.com]
> *Sent:* Thursday, July 06, 2017 7:15 AM
> *To:* Alexander Devaykin; Brownlee, Carson
> *Cc:* vtkusers at vtk.org
> *Subject:* Re: [vtkusers] How does VTK OSPRay-based volume rendering
> produce smooth images?
>
> Most likely the only difference is that VTK sets up OSPRay's volume
> sampling differently than the OSPRay demos do.
> You will find how it is setup here:
>   https://gitlab.kitware.com/vtk/vtk/blob/master/Rendering/OSPRay/
> vtkOSPRayVolumeMapperNode.cxx#L255
>
> Note also: starting with ParaView 5.3, if you start ParaView with
> --enable-streaming, and set "Progressive Passes" on the Properties panel to
> something higher than one, ParaView will do progressive refinement like you
> see in the OSPRay demos.
>
>
>
> David E DeMarle
> Kitware, Inc.
> Principal Engineer
> 21 Corporate Drive
> Clifton Park, NY 12065-8662
> Phone: 518-881-4909
>
> On Mon, Jul 3, 2017 at 5:46 PM, Alexander Devaykin <xaxellx at gmail.com>
> wrote:
>
>> Hello everyone,
>>
>> I wonder how does OSPRay-based volume renderer in VTK, used in ParaView,
>> is able to produce volume images with smooth surface from the first pass
>> (without buffer accumulation).
>>
>> OSPRay raw volume rendering e.g. using OSPRay demos, results in very
>> densly sampled volumes. It looks like only one sample per voxel is taken
>> which results in one pixel per voxel colored in the first pass at some
>> regions (image 1). Only after several accumulation passes the rendering
>> produces smooth image (image 3).
>>
>> In contrast to that, volume rendering in ParaView via OSPRay shows smooth
>> image after first pass (image 4) with a little bit of densly sampled
>> artifacts. It actually looks like no frames accumulation is active at all,
>> because I no difference is visible between moving and resting camera.
>> Zooming in also keeps the image smooth (image 5).
>>
>> Any articles or hints about what is going on behind the scenes? I'm
>> digging into the VTK source code but no understanding so far. OSPRay seems
>> to be configured just like I do in my application, but VTK seems to do the
>> magic somewhere else.
>>
>> Screenshots or standalone OSPRay and ParaView-OSPRay renderings are
>> attachend.
>>
>> Best Regards,
>> Alexander Devaykin
>>
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