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Rendering with Arnold in Houdini and Solaris using the HtoA plug-in.
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Volume Not Occluding Geometry

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Message 1 of 6
jamesearnest244
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Volume Not Occluding Geometry

Hi,

I have a simple density grid which I'm rendering with a Standard Volume, but I can't get it to occlude geometry in the scene, even when I drive the density very high.

It seems to be because my Transparent Color is set pretty high, but I need this to get the right look. When I set it to a low value, the volume does become occlusive, but then it looks murky and weird.

Isn't there a way to have a bright and opaque volume?


volume-transparency.jpg

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Message 2 of 6
madsd
in reply to: jamesearnest244

You need to use a second version as clip geo.

See here. The stick disapears when it gets inside the volume, the volume is more transparent than your candyfloss to illustrate it better.

qwe.gif

Message 3 of 6

Thanks Mads, that's a good solution, but it does seem kind of roundabout for a situation that isn't that rare, I would think. Isn't there a direct way to render occlusive volumes without them looking dark like the third image? Your solution might work in this example, but wouldn't be practical in a scene with more objects or with more varying densities where the objects are semi-hidden. Here's an example of what I'm trying to get at:


capture.png


Message 4 of 6
madsd
in reply to: jamesearnest244

In order to get some semi transparent edges going, you need to calibrate the density the transparency, and set the Arnold render volume depth to something higher than 0, I put it at 19 here, that will bounce light much more efficiently around inside it.

Further more you need an area light and a dome light, the dome light is your main light source and the area light creates the highlights in the top of the cannopy.

Last but not least, you need to set the Area lights shadow color to a deeper neon pink than the lighter over all pink on the volume material ( I did not show that on this image, so just remember )

This way you can control the deep side scatter color and differentiate it out from the main one, creating a more neon pink in the shadow area.

I used these rough settings to get a semi transparent edge going while on same time having branches disapear like in your refference.

Whats left from there is nursing final looks.

Pay attention to the fact that both density and transparency contributes to thickness, this is why you want to nurse both of them to get the right smoke density.
First work in a density and then transparency and then have a play with both to get the desired thickness, add some anisotropic amplifier to get a more contrasty look.

vvv.png

Message 5 of 6
madsd
in reply to: jamesearnest244

And here is the thing I didnt show on the previous image.
Shadow color.
It is a vital part of coloring volumes to work with the shadow area, especially when you fabricate real clouds, in that case it needs to be a dark grey to dark blue color, it will make the denser part of the cloud look spot on.

ddd.gif

Message 6 of 6

Thanks so much for the detailed answer! Got it working now.

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