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BaVolume factory - weird results when using scattering

 
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Pitcher



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PostPosted: Fri Nov 15, 2013 3:48 pm    Post subject: BaVolume factory - weird results when using scattering Reply with quote

I am experiencing some undesirable results (not sure it's a bug) when increasing Max Distance for Scattering.

If I go from Max Distance 12 to 40, I get these results - see images. It looks like some sort of overcranking of values and subsequent clamping to black. There is alpha where the black parts are. I guess the weird coloring that makes it look sort of burnt is the complementary color of the blue I use for the scattering.






Morten
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schoenberger
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PostPosted: Sat Nov 16, 2013 12:10 am    Post subject: Reply with quote

How many SI units is the cloud in diameter?
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Pitcher



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PostPosted: Mon Nov 18, 2013 11:00 am    Post subject: Reply with quote

60 x 80x20 units.

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schoenberger
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PostPosted: Mon Nov 18, 2013 12:47 pm    Post subject: Reply with quote

Hmm, so the scatter range is really large.
It is always larger than the cloud itself.
+/- 40 units in a volume with only 80 units.

Does it help if you increase the scatter samplerate?
Otherwise I do not have a workaround.

I think the issue is a bug that that scatter tries to get illumination from outside the cloud.
But this is not possible and it either collects nothing or it is the MRay bug that if the volume tries to get illumination from outside the cloud, MRay thinks the whole scene is filled with the volume.

Anb other idea:
Instead if increasing scatter, I would try reduce the shadow density of the volume.
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Pitcher



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PostPosted: Mon Nov 18, 2013 2:56 pm    Post subject: Reply with quote

Increasing the samplerate does not help, no.

I am not quite sure how scattering works, ie. what the parameters are supposed to do. Scattering is important for rendering steam/humidity volumes imho, so I have been working with it for this project.

My workaround was in fact to reduce the shadow density, but it is not ideal.

I realize that I have more to learn about using scattering - ie. what exactly the scatter range is. Are you saying that a value of 40 searches 40 units outside the volume for samples?


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schoenberger
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PostPosted: Mon Nov 18, 2013 3:04 pm    Post subject: Reply with quote

It does not start at the edge of the volume.
The scatter range starts at the illumination cell that is currently rendered.

If you are not rendering the edge of the volume, then it is 40 units outside the volume in one direction and 40 units into the cloud.
If you render the center, then 40 units from the center to both directions inside the cloud.

In your case, if you render the center of your 60x80x20 volume, then it samples the whole cloud and even further to get +/-40 in all directions.
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PostPosted: Tue Nov 19, 2013 11:25 am    Post subject: Reply with quote

Thanks for elaborating Holger. I am not quite sure how it works though, sampling outside the illumination cells. I would have thought scatter range to be how far into the volume scattering occurs, a bit like SSS parameters, but maybe it actually does work that way.

For clouds and steam (water vapours) scattering is important for creating an interesting look imho and I find it hard to get light shining far enough into a volume without lowering shadow density, as I might still want the selfshadowed side of a cloud to be quite dark, while having strong scattered light from the lit side.

BTW do you think it might be possible to make scattering render faster? I would happily sacrifice detail precision, as long as it does not flicker.

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schoenberger
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PostPosted: Tue Nov 19, 2013 1:33 pm    Post subject: Reply with quote

No, Scatter does not get any faster.
Beside lowering the range.
For your cloud, I would not use a higher range than 10-20.

>as I might still want the selfshadowed side of a cloud to be quite dark,
>while having strong scattered light from the lit side.
Scatter will soften all shadows all over the cloud. If you have a high scatter range, you could instead just increase the ambience.

You can also try to use a illumination model. Enable the HE-GR shading.
Then the cloud is darker to the sides and lighter in scatter/light direction.
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