Is there a way to get an Arnold z-depth pass/AOV from an Arnold Volume VDB object?

[Anonymous]

I am rendering a scene with with an Arnold Volume VDB in it. I have z AOVs active, but I'm not getting any depth information for the VDB/Volume when compositing. Is there a way to get zdepth information in a pass for Volumes?

[peter.horvath6V6K3]

There's a separate depth AOV for volumes called volume_Z.

[Anonymous]

Thanks Peter
I'm getting a strange result though. I'm expecting the pass to have greyscale values, but instead I am getting a very stark black and white pass. Would you know why?

volume_Z .exr looks like this (expected more greyscale values):

[peter.horvath6V6K3]

The AOV contains the distance for the first volume contribution from the camera.

[Anonymous]

@Peter Horvath sorry I realised I needed to be in 32bpc in my compositing software.
The pass has some greyscale values now:

However now it looks like it isn't quite matching up with the volume:

Would anyone know why that is happening?

[Anonymous]

Thanks @Peter Horvath
That works to an extent, however the volumeZ pass doesnt quite render as expected.
For example in some areas the volumeZ has very sharp edges where my actual rendered volume/smoke beauty pass has soft edges.
Is there a way to more closely match it up?

Here are some images to illustrate the problem:
https://we.tl/t-MQh1K97MZJ

[peter.horvath6V6K3]

volume_Z is a float type AOV and uses the closest_filter by default. If you need an anti-aliased Z pass, then select the AOV in the object tree and change the filter to gaussian or just change to 'Use default filter' to match the render settings.

[Anonymous]

Thanks @Peter Horvath

Default and gaussian both produce better results and the edges in general look softer and more like the beauty pass.
There are however still a lot of things that don't come out as expected.

Is there a way to get the z volume pass edges to be very soft in places where the smoke is wispy and less thick?
There are also still strange edges around the volume and the background.
The background also comes out in the opposite luma value as expected.

Can't seem to upload any pictures to the website, here is a link to some images:https://we.tl/t-3xD2YPOWKw

[peter.horvath6V6K3]

Yeah, it looks odd. Hard to say from the images what's going on. Could you send me a scene I can take a look at?

[Anonymous]

Thanks @Peter Horvath
Scene in the wetransfer below:
https://we.tl/t-Arj3hoP1f0

I only included the vdb for frame 82 (as the whole thing is quite big), so if you could try render frame 82 please.
Thanks again.

[Anonymous]

Can't get the volume z pass to work properly. Is there maybe a way to cheat a z-depth pass somehow by using fog or something. Fog is usually a good way to get a z-depth pass by getting a grayscale render with luma values driven by distance from camera - but fog doesn't seem to work on the volumes in Arnold. Seems like volumes are just ignoring the fog. Wondering if there is another way?

[madsd]

I never realy use the volume z.
Since start I just create a local volume that is big enough to create a uniform space with no edges.
Stick my items in there, and it does not matter if they are volumes with special densities or regular geometry.
I started using this for being able to do post DOF through glass with abitrary bending refractive IOR values or curvature that bends rays around, before the transmit to AOV features were added and it has worked great so far in all my cases.

I set up some visuals from max, but that is not the point, the point is the technic. Where we exploint the uniform falloff inside a volume.
https://imgur.com/a/cKsv0w7

[Anonymous]

Thanks @Mads Drøschler
Your examples are exactly what I'm trying to achieve now.
That will definitely work, I'm just trying to figure out what sort of settings etc to use for C4D to A.
Not able to crack it yet, but I'll let you know if I figure it out.
For the longest time I couldnt figure out how to create a volume that surrounds the whole scene, but I finally realised I can just scale up an Arnold Volume (type: box) in the Coord tab. Almost there I think.

[madsd]

Right, thats the 3rd image showing the entire construct.
just make the bounding box volume big enough so you dont start to sample corners of the box.

[Anonymous]

I found some settings that worked for me in creating a z depth pass for volumes. It might differ depending on the scene.
Thanks again to @Mads Drøschler for the idea.

1. Create an Arnold Volume and scale it up in the Coord tab to fill up your whole scene.
2. Give it an Arnold Standard volume material and make this volume dark. I put the Emission mode for this on density, color on black and cranked the intensity.
3. Give the other volume/s in your scene Arnold standard volume materials and make them light. I put the Emission mode for these on density, color on white and cranked the intensity.
4. Put an Arnold sky in the scene for even lighting. I put the color on 50% grey and intensity on 1.