Volume step % - Medium Standard Volume

[vortexvfx]

Hey all.
The exact specification of what "Volume step %" actually does seems a bit contradictory between different versions and different documentation.
(Houdini plugin 2024 Beta tooltip currently: "Ray-marching step length, specified as a percentage of the voxel size.")

From memory, I think the older Houdini plugins' tooltip may have always referred to this as a percentage of voxel size, suggesting it was adaptive according to the volume being rendered. This was when the default was 4... it seems the default changed at some point recently to 1.
From what I can tell from the main Octane docs, at least when the default was 4, it actually referred to an absolute distance in meters, not a multiplier as it's referred to in Houdini.

Has anything changed, or is this still an absolute distance?
In tests with a volume of voxel size 0.02, it's very clear there's a substantial difference between a "Volume step %" of 1.0 and 0.5... and the volume only really starts to noticably resolve all detail correctly around 0.125. So I'm guessing it's not voxel-size-relative at all... there presumably shouldn't be a massive visual difference between stepping through a volume 0.5 voxels per step vs. 0.125, if that's what it was doing.

Assuming it is actually an absolute distance in meters, can anyone share any knowledge about what exactly the volume ray marching is doing? I presume internally it would be auto-offsetting the first step to be a random amount from zero to Step Length, so that each ray samples an even distribution of density points through the volume even though the marching on individual ray skips over multiple voxels at a time?

[ricky_otoy]

Hi vortexvfx, this is a change that was made in Standalone and thus the SDK.

"We now use percentages for volume step lengths for volume rendering. This means the absolute step length is removed. Step length is now therefore based on the volume's voxel dimension." ~Haze

[vortexvfx]

Ah I see.

Is this definitely tested working as expected then? I would assume with a percentage-based setup, the default of 1.0 would mean a marching step of 1x the individual voxel width, which would generally mean that a volume would have all its camera-z-axis detail resolved consistently without variability in perceived density, and would show very little difference stepping up to a 0.5 step length... but the difference between 1.0, 0.5, 0.25 are all quite visually pronounced (the lower you go, the greater the perceived density) - it appears that at the 1.0 default it must be stepping over multiple voxels at a time.

(Note, this is for volumes with thin streaks in, where rays that march over many voxels at a time will entirely miss certain details)

Could it be that the percentage is actually a percentage of the total bounding-box size of the volume, rather than voxel size?
...that or perhaps 1.0 is just an arbitrary multiplier that maps to something like 10x voxel width?