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

[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.

[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.

[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...

[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.