Volume displacement is not real displacement?

[Fran]

I wanted to create a volume with an animated displacement and I noticed some density appeared from nowhere. I searched further and here's what I've found. The volume displacement is not really displacing the volume but instead it's displacing the space at which the density is sample.

In this scene I demonstrate what I mean. If you render one frame of the Mantra node "const_disp" you will see a displaced volume -0.5 in X even though I've told the shader to displace 0.5 in X.

If you render the sequence "noise_disp" (or look at the attached video), you will see how the displacement evolves overtime while the noise amplitude is increasing. It's not displacing.

Is it normal?

François

volume_disp_v001.hip

volume_disp_noise_v001.mov

[tjeeds]

Yes, this is the conclusion I came to as well, you are warping the space around the volume, thus all the values you would intuitively use to displace a surface need to be negated. If you need to displace along the surface normal you can build an SDF from the fog volume and save it out as a bgeo file and use a Volume Gradient on it to get the approx normal. If you displace heavily from within your volume you'll leave holes inside which can be an issue if your volume is rendered with low density.