[vtkusers] Opacity discrepancy with downscaled volume despite opacity unit distance == spacing

[David Gobbi]

On Sun, Feb 12, 2017 at 10:26 AM, Elvis Stansvik <
elvis.stansvik at orexplore.com> wrote:

> 2017-02-12 18:06 GMT+01:00 David Gobbi <david.gobbi at gmail.com>:
>
>>
>>
>> On Sun, Feb 12, 2017 at 10:01 AM, Elvis Stansvik <
>> elvis.stansvik at orexplore.com> wrote:
>>
>>> 2017-02-12 17:56 GMT+01:00 Elvis Stansvik <elvis.stansvik at orexplore.com>
>>> :
>>>
>>>> 2017-02-12 17:31 GMT+01:00 David Gobbi <david.gobbi at gmail.com>:
>>>>
>>>>> Hi Elvis,
>>>>>
>>>>> Here is something you can do as a check: set the sample distance for
>>>>> the ray casting to a very small value, so that it takes around 1000 steps
>>>>> for the ray to traverse the volume from front to back.  The thickness of
>>>>> your volume is 29*0.006 = 0.174, so you can try a sample spacing
>>>>> of 1.74e-04:
>>>>>
>>>>> mapper->AutoAdjustSampleDistancesOff();
>>>>> mapper->SetSampleDistance(1.74e-04);
>>>>>
>>>>> If the two volumes look the same when you use a small sample distance,
>>>>> then the difference you were seeing was probably just due to discretization.
>>>>>
>>>>
>>>> That indeed did the trick, thanks David!
>>>>
>>>
>>> And to clarify: The small difference I was seeing when I had the sample
>>> distance left to auto-adjusted isn't really a problem for my application. I
>>> was just surprised there was still a small difference even when both
>>> volumes were rendered with the same opacity unit distance, and I wanted to
>>> find the explanation for it. The discretization error coming into play
>>> makes sense.
>>>
>>
>> So everything should be fine if you leave AutoAdjustSampleDistances "On",
>> for the interactive rendering, but set the SampleDistance to half of your
>> high-res volume spacing for the high quality rendering (e.g. use 0.003 for
>> this data set).
>>
>
> Ah, I realize I may have misunderstood something about these two
> properties. I thought that setting a SampleDistance explicitly implied all
> auto-adjustment is turned off. But re-reading the docs I see that if
> auto-adjustment is left on, then the distance set with SetSampleDistance
> will only be used when the renderer has > 1 second to spend?
>
> BTW, regarding AutoAdjustSampleDistances, the docs says
>
>    "[...] the ImageSampleDistance will be varied to achieve the allocated
> render time of this prop"
>
> but doesn't it auto-adjust the SampleDistance as well? (the distance along
> the ray)
>

I'll have to leave this question for people who are more familiar with the
code.

 - David
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