Volumetric lighting in Houdini

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Volumetric lighting in Houdini

Postby Ferdinand13 » Sun Jan 16, 2022 5:43 pm

Ferdinand13 Sun Jan 16, 2022 5:43 pm
Hi there,

I've been doing some tests with Octane inside Houdini, mostly trying to emulate some of the functionality inside Cinema 4D. I'm mostly getting everything to work 1:1, but there is one thing that escapes me - Volumetric Lighting.

Essentially, what I'm trying to recreate a hazy room where a bright light is creating God rays. Like this:
https://www.youtube.com/watch?v=NWgwlib7svg

In Cinema 4D, you do that using an Octane Fog Volume, which gives you a 'box' inside which you have the medium already prepared, so inside that 'medium box', strong small lights will cause God rays.

How would I go about replicating this setup inside Houdini? I.e., where is the Octane Fog Volume option for Octane inside Houdini? If I'd take a guess, I'd say that for Houdini, you'd have to 'build' one yourself? But how would you do that? Or is there already a better/faster/more elegant way to achieve this look?

I did play around with the 'Spectron Light' set to 'Volumetric Spotlight', but to be honest, I'm not quite 'getting' that one. The manual says that you can't change its size, but 'sometimes' I'm able to do that inside Houdini, sometimes the scale option seems locked. There are all kinds of 'Scattering' and 'Barn Door' options, but none of them seem to allow me to create the aforementioned God rays.

It'd be great, if you could help me demystify volumetric lighting inside Houdini :)
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Re: Volumetric lighting in Houdini

Postby elsksa » Sun Jan 16, 2022 7:09 pm

elsksa Sun Jan 16, 2022 7:09 pm
Hi Ferdinand13,

It is not directly about Houdini but Octane "core". You should be able to apply it to Houdini:
https://www.elsksa.me/scientia/cgi-offline-rendering/octane-render-volumes

Let me know how it goes.
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Re: Volumetric lighting in Houdini

Postby Ferdinand13 » Sun Jan 16, 2022 7:12 pm

Ferdinand13 Sun Jan 16, 2022 7:12 pm
Thanks so much elsksa! I'll try that asap :)
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Re: Volumetric lighting in Houdini

Postby Ferdinand13 » Sun Jan 16, 2022 9:47 pm

Ferdinand13 Sun Jan 16, 2022 9:47 pm
Hey elsksa, it worked!

OctaneRender_VolumeMaterial.png

OctaneRender_VolumetricLighting.png


I wasn't aware that the Houdini plugin had a 'Null Material', but it does. With this, I can create the God rays as expected. Thanks so much again :)

I did a test with a Spectron Quad light and a standard Octane light casting the God rays, but there doesn't seem to be any difference in terms of noise or rendering times. Any idea if there's a way to speed this up (apart from using the Spectron Volumetric Spotlight?
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Re: Volumetric lighting in Houdini

Postby Ferdinand13 » Sun Jan 16, 2022 10:08 pm

Ferdinand13 Sun Jan 16, 2022 10:08 pm
Just one more question.

So this is what I'm getting using the 'fog medium' approach with a standard Octane light:
OctaneRender_VolumetricLighting.png

As you can see, the light distribution is even from left to right.

Now, when I'm using a Spectron Volumetric Spotlight at exactly the same distance from the object, I get a different result:
OctaneRender_SpectronLighting.png

The right side - which is closer to the light - is completely blown out. I've dragged sliders around for about half an hour now, but I can't seem to be able to reduce the power of the spotlight and make the distribution more even.

Any idea, how this could be achieved? Thanks for letting me know.
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Re: Volumetric lighting in Houdini

Postby elsksa » Mon Jan 17, 2022 10:41 am

elsksa Mon Jan 17, 2022 10:41 am
Ferdinand13 wrote:I did a test with a Spectron Quad light and a standard Octane light casting the God rays, but there doesn't seem to be any difference in terms of noise or rendering times. Any idea if there's a way to speed this up (apart from using the Spectron Volumetric Spotlight?

This could help: https://www.elsksa.me/scientia/cgi-offline-rendering/octane-render-settings
If not by much, feel free to comment again and I will have a closer look at your scene.

Ferdinand13 wrote:Now, when I'm using a Spectron Volumetric Spotlight at exactly the same distance from the object, I get a different result:
OctaneRender_SpectronLighting.png

The right side - which is closer to the light - is completely blown out. I've dragged sliders around for about half an hour now, but I can't seem to be able to reduce the power of the spotlight and make the distribution more even.

Any idea, how this could be achieved? Thanks for letting me know.

I would strongly recommend to adopt a "display rendering transform", part of a proper digital imaging pipeline.
It is covered in this page: https://www.elsksa.me/scientia/cgi-offline-rendering/cgi-color-management-survival-kit
and this one is more specifically tailored to Octane:
https://www.elsksa.me/scientia/cgi-offline-rendering/octane-render-color-management

Feel free to also join the Octane Discord server, the link is in my forum signature. There are some Houdini users who could share their techniques, specifically related to Octane in Houdini as I only use Octane Standalone.

The aforementioned URLs (pages) I have linked should cover both the render settings optimization and the recommended digital imaging workflows.

Let me know how it goes.
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Re: Volumetric lighting in Houdini

Postby Ferdinand13 » Mon Jan 17, 2022 12:40 pm

Ferdinand13 Mon Jan 17, 2022 12:40 pm
Thanks so much elsksa, your page is a goldmine!

I'll check out the videos on the render settings in regards to the render time reduction when using Spectron lights with Volumes.

However, just to be sure we're on the same page regarding the overbright Spectron Volumetric Spotlight, I wanted to mention that my system in general and Houdini in particular are already set up to use ACES, which is why everything should be displaying properly (Octane Render doesn't even show any changes when I change the response curve, as the ACES/OCIO setting overrules everything else). So whatever I'm seeing is already taking into account that nothing should be 'clipped'.
Screenshot 2022-01-17 122441.png


There is something that doesn't seem to be working right, when using the Spectron Spotlight. The main issue is that you don't see the light ray unless you crank up the 'Throw Distance' enough so that it actually reaches the object and is visible from your respective camera angle. Whenever you increase the Throw Distance however, you loose power, so the light gets dimmer and dimmer. You therefore have to increase the light's "Power" to crazy high levels, what then again means that everything blows out. However, the falloff from left to right across the frame is really a tad extreme.
Volumetric_SpotLight.png


I checked this also as an EXR export and the issue inside an ACES comp pipeline is still the same. The way the image looks is as if the light was positioned just off frame on the right. In that case you would have an incredibly dramatic falloff in terms of brightness. However, the light is positioned around 5 metres to the side, so the falloff should really be a lot softer. The thing is, the further away I put the spotlight (to reduce the high contrast falloff), the more I have to crank up the Throw Distance, resulting in a high contrast yet again... :D
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Re: Volumetric lighting in Houdini

Postby elsksa » Mon Jan 17, 2022 1:53 pm

elsksa Mon Jan 17, 2022 1:53 pm
Ferdinand13 wrote:Octane Render doesn't even show any changes when I change the response curve, as the ACES/OCIO setting overrules everything else
Screenshot 2022-01-17 122441.png

That is to be expected. OCIO basically takes over the other color-management sections of Octane.
I remember replying to a similar topic in, if I'm not mistaken, this thread: https://render.otoy.com/forum/viewtopic.php?f=88&t=78025&p=403089#p403089

Ferdinand13 wrote:I checked this also as an EXR export and the issue inside an ACES comp pipeline is still the same.

Octane does export a linear "scene referred" OpenEXR (light data) file that has not yet been conformed for display (viewing) purposes. If there is an issue (which I will try to reproduce on my end, whenever I can) then it would come from the "scene" (light, in your case).
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Re: Volumetric lighting in Houdini

Postby elsksa » Mon Jan 17, 2022 2:09 pm

elsksa Mon Jan 17, 2022 2:09 pm
I am not able to reproduce it. Here is a screenshot with some scene information included:
octane_Bj4LRD0DVf.jpg
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Re: Volumetric lighting in Houdini

Postby Ferdinand13 » Mon Jan 17, 2022 9:23 pm

Ferdinand13 Mon Jan 17, 2022 9:23 pm
Hi elsksa,

I finally had some time to play around inside Standalone. I exported the scene from Houdini via ORBX, which meant I was greeted by a crazy spaghetti mess of unused nodes (the Houdini export is still a bit problematic). Either way, after cleaning everything up, I essentially ended up with the same problem, until I deducted that my issue was with the 'density' setting. My 'fog' was simply too dense, causing the volumetric spotlight to go nuclear nearer to the source. Have a look at the setup in Standalone, now with a far more even brightness across the God rays:
Volumetric_SpotLight_Standalone.png


I applied the same logic inside Houdini and now the volumetric spotlight behaves in there as well:
Volumetric_SpotLight.png
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