Fog and opacity texture issue

[tombyrom]

Hello guys,
There is an issue when using a fog volume and there are assets that use opacity maps, such as leaves

I have tried loads of different settings, even found this super old post from 2015 which was solved apparently.
viewtopic.php?t=49455

Attaching a scene for debug.

Tried Std Medium Volume and a normal Volume Medium, makes no difference, tried every setting on the leaf material and checked and re-checked kernel settings.

Thanks

[tombyrom]

If you use the Std Medium Volume in compatibility mode, it is a lot harder to replicate this problem.

So the issue has to be tied to the fact that the ray march is no longer tied to an absolute length and rather a % based on voxel size.

If I change my voxel values, volume step %, density and transparency I can reduce this problem but I would not go as far as saying this is a fix as it requires very specific manipulation over multiple values to get the fog look you want and to reduce this problem.

See screenshots, it is still not perfect and can still make out some a slight border

[skientia]

The reproducibility of this situation has been unsuccessful with several approaches, but not all imaginable ones.

Asset used for testing: Fern 02 with Path tracing (optimized and default) on Octane 2025.2 stable.

It would be helpful to the case to:
- test with Fern 02
- share all Kernel settings
- share the Octane (core, not plugin) version
- asset file technical specifications (such as format, encoding, etc)
- test on a fresh new project file, to exclude any eventual retro-compatibility conflict

[tombyrom]

It's easily reproducible - I just did an orbx export file, which shows it in standalone.

The 3dsmax scene above shows it more easily as you can easily control the voxel size of the mesh volume.

The bitmap map is not the cause, just swap it for any procedural texture and it's the same issue.

I believe the issue is linked to the volume step % and how the ray calculates opacity through the volume, causing inaccuracies with a larger ray march relative to voxel size.

You need a really low step % relative to voxel size and then change the density and transparency to get your desired fog look and reduce this issue.

So the answer I'd like to know is whether this is a limitation of how raymarching works? Or is there something in Octane that can help deal with this.

Reducing your step % is all well and good, but it increases performance cost exponentially and it is only masking the underlying issue.

[skientia]

Thank you for the additional details. It confirmed a common cause of all sort of technical issues: the project file is out of scale.

The problem should be solved by correcting the project to an appropriate scale (the closest to true-to-size).

This Rendering Guidelines page has a check-list that could come handy to ensure (minimize) rendering defects, errors and alike.

Edit: for reference, the cube is the default 1x1x1m box primitive geometry (Info Kernel set to Object Layer ID):

[tombyrom]

This has nothing to do with scene scale.... 'Out of scale' based on what exactly?

You could have any size object that uses a texture with opacity.

Your comments make no sense and does not hint at a real solution based on what I am presenting

EDIT: And to prove it, here is a more 'appropriately' scaled debug scene. My original working scene was correctly scaled to real-world size. The debug scene was big yes, but has no impact on why this is happening.

[skientia]

Upon a second inspection, and while it's unarguably important for a project file to be set at a correct scale, unplugging the mesh volume and enabling the Medium volume from the Daylight system renders "correctly". In Standalone, adding a "Unit Volume" node results in an expectedly "correct" render as well.

However, changing the volume step length makes the transparent cut-out apparent again.

[tombyrom]

Let me be clear this is NOT an issue with the medium you attach to a daylight system. That medium is very different to an actual voxelised mesh volume, vdb or the likes.

That should be clear already based on the files I am presenting and the problem.

To re-summarise:

- Using a Mesh volume or any other volume that uses actual voxel data
- Std Medium Volume or a Volume Medium where you can control the volume step %
- Using a certain volume step % when you have materials with opacity maps such as plant leaves can cause ghosting/fog occlusion around geomatry with that material applied
- This can be reduced by tweaking the step %, density and transparency.

All I want to know is whether this is a bug or not...

[skientia]

It was very clear.

This is simply a trouble-solving exploration of all direct and indirect paths to gain as much information to help, and as a matter of fact, the issue persists with the Unit Volume (VDB-like, containing voxel data).

It's not a "new issue" but was due to a double-check in 2024 and 2025 in case an update may have improved it.

Additional attempts were made:

• 1. The "old and deprecated trick" of using a Cube (Box) primitive with a "Specular material (1.0 IOR)" wasn't viable.
• 2. Loading the texture file as Alpha type (instead of RGB or Greyscale, the common ones) was not a solution either.
  Worth mentioning the alpha had to be encoded properly as most "opacity maps" are simply a greyscale (pure white & black, "SDR" range) "mask".

  

• 3. Testing Fern_02 yields identical results (free 3d asset with an opacity map).
• 4. Nothing relevant in the Kernel sections.
• 5. The closest to an "outline-free" render output is a low-enough Volume Step Length, at the cost of higher computational resources.

  

• 6. A less attractive solution is an opacity-free asset, "baking" the opacity maps into the mesh (this has been a viable solutions for some environment-heavy projects in production, particularly in Houdini but applicable to any 3D DCC)

It's showing signs of the abiding core limitation.

[tombyrom]

5. This is my conclusion as well, the issue can be masked by low enough step volume % however, counter intuitavely to this, I found that in some sitations higher also worked depending on the fog look you are going for.

Hopefully a developer can confirm if this is just a core limition relating to volume step % (raymarching) and opacity maps or indeed a bug.